Pathways and Integrative Functions 519

OUTLINE

17.1 General Characteristics of Nervous System Pathways 519 17 17.2 Sensory Pathways 519 17.2a Functional Anatomy of Sensory Pathways 520 17.3 Motor Pathways 523 17.3a Functional Anatomy of Motor Pathways 523 Pathways and 17.3b Levels of Processing and Motor Control 528 17.4 Higher-Order Processing and Integrative Functions 529 17.4a Development and Maturation of Higher-Order Processing 529 Integrative 17.4b Hemispheric Lateralization 529 17.4c Language 530 17.4d Cognition 531 17.4e Memory 532 Functions 17.4f Consciousness 532 17.5 Aging and the Nervous System 534

MODULE 7: NERVOUS SYSTEM

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onald Reagan, the fortieth president of the United States, died in June is called somatotopy (sō -mă-tot ́ō -pē; soma = body, R 2004 after a long bout with Alzheimer disease. More than topos = place). For example, recall the homunculus map in a decade earlier, Mr. Reagan had publicly revealed the onset of chapter 15 (see figure 15.12), which depicted the surface his illness by saying, “At the moment, I feel just fine.” Alzheimer of the precentral gyrus and showed the parts of the disease is a progressive dementia that debilitates the functioning primary motor cortex that control specific body regions. of the central nervous system (CNS) and usually affects people in The pathways that connect these parts of the primary their 60s or over. This neurodegenerative disease causes progressive motor cortex to a specific body part exhibit somatotopy. decline in memory, judgment, and reasoning, as well as disrup- Somatotopy is also seen in the sensory homunculus on the tion of neurologic function within the brain. The cerebral cortex primary somatosensory cortex of the postcentral gyrus. atrophies, and abnormal protein deposits accumulate in the brain. ■ All pathways are composed of paired tracts. A pathway on Mr. Reagan’s intellectual capacity declined over the ensuing years. the left side of the CNS has a matching tract on the right As one anonymous individual put it, “His mind just faded away.” side of the CNS. Because each tract innervates structures This chapter focuses on the brain’s higher-order activities—such as on only one side of the body, both left and right tracts are memory and learning—which depend on the proper functioning of needed to innervate both the left and right sides of the body. sensory and motor pathways in the nervous system. ■ Most pathways are composed of a series of two or three neurons that work together. Sensory pathways have primary neurons, secondary neurons, and sometimes 17.1 General Characteristics tertiary neurons that facilitate the pathway’s functioning. In contrast, motor pathways use an upper motor neuron of Nervous System Pathways and a lower motor neuron. The cell bodies are located in Learning Objective: the nuclei associated with each pathway. We discuss the 1. Identify and describe the characteristics of sensory and specific neurons in greater detail later in this chapter. motor pathways in the spinal cord. WHATW DID YOU LEARN? The CNS communicates with peripheral body structures through pathways. These pathways conduct either sensory or ●1 What is meant by somatotopy? motor information; processing and integration occur continuously along them. These pathways travel through the white matter of the Study Tip! brainstem and/or spinal cord as they connect various CNS regions with cranial and spinal nerves. Tracts and pathways are named according to their origin and ter- A pathway consists of a tract and nucleus. Tracts are groups mination. Each has a composite name: The prefix, or first half of the or bundles of axons that travel together in the CNS. Each tract may name, indicates its origin, and the suffix, or second half of the name, work with multiple nuclei groups in the CNS. A nucleus is a collec- indicates its destination. For example, sensory pathways usually begin tion of neuron cell bodies located within the CNS (see table 15.2). with the prefix spino-, indicating that they originate in the spinal cord. Nervous system pathways are sensory or motor. Sensory So the tract that originates in the spinal cord and terminates in the pathways are also called ascending pathways because the sensory cerebellum is called the spinocerebellar tract. Motor pathways begin with information gathered by sensory receptors ascends through the either cortico-, indicating an origin in the cerebral cortex, or the name spinal cord to the brain, while motor pathways are also called of a brainstem nucleus, such as rubro-, indicating an origin within the descending pathways because they transmit motor information red nucleus of the mesencephalon. Tracts that terminate in the spinal that descends from the brain through the spinal cord to muscles cord have the suf f ix -spinal as part of their name. Thus, both corticospinal or glands. Most of the nervous system pathways we discuss in this and rubrospinal denote motor tracts. chapter share several general characteristics: ■ Most pathways decussate (dē ́kŭ-sāt; decusseo = to make in the form of an X) (cross over) from one side of the body to 17.2 Sensory Pathways the other side at some point in their travels. This crossover Learning Objectives: process, called decussation, means that the left side of the 1. Identify the locations and describe the relationships of brain processes information from the right side of the body, primary, secondary, and tertiary neurons. and vice versa. For example, when you write with your right 2. Describe and compare the three major somatosensory hand, the left side of your brain is controlling those right-sided pathways. muscles. The term contralateral is used to mean the opposite side, whereas the term ipsilateral means the same side. Over Sensory pathways are ascending pathways that conduct 90% of all pathways decussate, although the point at which information about limb position and the sensations of touch, tem- decussation occurs can vary slightly from pathway to pathway. perature, pressure, and pain to the brain. Somatosensory pathways process stimuli received from receptors within the skin, muscles, WHAT DO YOU THINK? and joints, while viscerosensory pathways process stimuli received from the viscera. ●1 Can you think of a reason why most pathways decussate (cross over) from one side of the body to the other? The multiple types of body sensations detected by the somatosensory system are grouped into three spinal cord pathways, each ■ In most pathways, there is a precise correspondence of with a different brain destination: (1) Discriminative touch permits receptors in body regions, through axons, to specific us to describe textures and shapes of unseen objects and includes functional areas in the cerebral cortex. This correspondence pressure, touch, and vibration perception. (2) Temperature and pain

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Table 17.1 Sensory Pathway Neurons Neuron Functional Classiﬁ cation Cell Body Origin Projects To: Primary Sensory neuron Posterior root ganglia of spinal nerves; sensory ganglia of cranial nerves Secondary neuron Secondary Interneuron Posterior horn of brainstem nucleus Thalamus or cerebellum Tertiary Interneuron Thalamus Cerebral cortex

Posterior

Fasciculus gracilis Posterior funiculus− Figure 17.1 medial lemniscal pathway Fasciculus cuneatus Sensory Pathways in the Spinal Cord. The major sensory (ascending) pathways, shown in various shades of blue, and bilaterally symmetrical tracts. The major motor tracts are indicated in pale red. Posterior Note: These colors are used to denote the spinocerebellar tract different sensory pathways only. Spinocerebellar pathway Anterior spinocerebellar tract

Lateral spinothalamic tract Anterolateral pathway Anterior spinothalamic tract

allow us to detect those sensations, as well as the sensation of an proprioception. The axon of the secondary neuron arriving in the itch. (3) Proprioception allows us to detect the position of joints, thalamus synapses with the tertiary neuron, the third neuron in stretch in muscles, and tension in tendons. (Note: Visceral pain the chain. The tertiary neuron (or third-order neuron) is an inter- pathways will be discussed in chapter 19.) neuron whose cell body resides within the thalamus. Recall that Sensory receptors detect stimuli and then conduct nerve the thalamus is the central processing and coding center for almost impulses to the central nervous system. Sensory pathway centers all sensory information; thus, it makes sense (pun intended!) that within either the spinal cord or the brainstem process and filter the the last neuron in a sensory pathway chain resides in the thalamus. incoming sensory information. These centers determine whether The three major types of somatosensory pathways are the the incoming sensory stimulus should be transmitted to the cere- posterior funiculus–medial lemniscal pathway, the anterolateral brum or terminated. Consequently, not all incoming impulses pathway, and the spinocerebellar pathway (figure 17.1). reach the cerebral cortex and our conscious awareness. Posterior Funiculus–Medial Lemniscal Pathway 17.2a Functional Anatomy of Sensory Pathways The posterior funiculus–medial lemniscal pathway (or poste- Sensory pathways utilize a series of two or three neurons to trans- rior column pathway) projects through the spinal cord, brainstem, mit stimulus information from the body periphery to the brain and diencephalon before terminating within the cerebral cortex (table 17.1). The first neuron in this chain is the primary neuron (figure 17.2). Its name derives from two components: the tracts (or first-order neuron). The dendrites of this sensory neuron are within the spinal cord, collectively called the posterior funiculus part of the receptor that detects a specific stimulus. The cell bodies (fū -nik ́ū -lŭs; funis = cord); and the tracts within the brainstem, of primary neurons reside in the posterior root ganglia of spinal collectively called the medial lemniscus (lem-nis ́kŭs; ribbon). nerves or the sensory ganglia of cranial nerves. The axon of the This pathway conducts sensory stimuli concerned with proprio- primary neuron projects to a secondary neuron within the CNS. ceptive information about limb position and discriminative touch, The secondary neuron (or second-order neuron), the second neuron precise pressure, and vibration sensations. in this chain, is an interneuron. The cell body of this neuron resides The posterior funiculus–medial lemniscal pathway uses a within either the posterior horn of the spinal cord or a brainstem chain of three neurons to signal the brain about a specific stimulus. nucleus. The axon of a secondary neuron projects either to the thal- Axons of the primary neurons traveling in spinal nerves reach the amus for conscious sensations or to the cerebellum for unconscious CNS through the posterior roots of spinal nerves. Upon entering the

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Right side of body Left side of body Right side of body Left side of body Primary somatosensory Cerebrum Primary somatosensory Cerebrum cortex (postcentral gyrus) cortex (postcentral gyrus)

Tertiary neuron

Tertiary neuron Thalamus Thalamus

Secondary neuron Mesencephalon Medial lemniscus Mesencephalon

Nucleus gracilis Secondary neuron Medulla oblongata Nucleus cuneatus Pons Medial lemniscus Decussation prior Receptors for to entry into the discriminative touch, medial lemniscus proprioception, precise pressure, and vibration (from neck, Primary neuron trunk, limbs) Medulla oblongata Fasciculus gracilis Posterior Fasciculus cuneatus funiculus Anterior root Posterior root Receptors for pain, temperature, crude Anterior spinothalamic tract touch, pressure Spinal cord Lateral spinothalamic tract Pathway direction Primary neuron

Figure 17.2 Posterior horn Posterior Funiculus–Medial Lemniscal Pathway. This pathway conducts sensory information about limb position, fine touch, precise Spinal cord pressure, and vibration. This pathway is bilaterally symmetrical; to Pathway direction avoid confusion, only sensory input from the right side of the body is shown here. Decussation of axons occurs in the medulla oblongata. Figure 17.3 The primary neuron is purple, the secondary neuron is dark blue, This pathway conducts crude touch, and the tertiary neuron is light green. Anterolateral Pathway. pressure, pain, and temperature sensations toward the brain. Decussation of axons occurs at the level where the primary neuron spinal cord, these axons ascend within a specific posterior funicu- axon enters the spinal cord. The primary neuron is purple, the ū ́ ē ā ŭ cuneus = lus, either the fasciculus cuneatus (k n - -t s; wedge) or secondary neuron is dark blue, and the tertiary neuron is light green. the fasciculus gracilis (gras ́i-lis). The fasciculus cuneatus houses axons from sensory neurons originating in the upper limbs, superior trunk, neck, and posterior region of the head, whereas the secondary neuron axons exit their specific nuclei and before they fasciculus gracilis carries axons from sensory neurons originating enter the medial lemniscus. As the sensory information travels in the lower limbs and inferior trunk. The sensory input into both toward the thalamus, the same classes of sensory input (touch, posterior funiculi is organized somatotopically—that is, there is a pressure, and vibration) that have been collected by cranial nerves correspondence between a receptor’s location in a body part and CN V (trigeminal), CN VII (facial), CN IX (glossopharyngeal), and a particular location in the CNS. Thus, the sensory information CN X (vagus) are integrated and incorporated into the ascending originating from inferior regions is medially located within the pathways, collectively called the trigeminothalamic tract. fasciculus, and the sensory information originating at progressively The axons of the secondary neurons synapse on cell bodies more superior regions is located more laterally. of the tertiary neurons within the thalamus. Within the thalamus, Sensory axons ascending within the posterior funiculi syn- the ascending sensory information is sorted according to the apse on secondary neuron cell bodies housed within a posterior region of the body involved (somatotopically). Axons from these funiculus nucleus in the medulla oblongata. These nuclei are either tertiary neurons conduct sensory information to a specific location the nucleus cuneatus or the nucleus gracilis, and they correspond of the primary somatosensory cortex. to the fasciculus cuneatus and fasciculus gracilis, respectively. These secondary neurons then project axons to relay the incom- Anterolateral Pathway ing sensory information to the thalamus on the opposite side of The anterolateral pathway (or spinothalamic pathway) is located in the brain through the medial lemniscus. Decussation occurs after the anterior and lateral white funiculi of the spinal cord (figure 17.3).

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It is composed of the anterior spinothalamic tract and the lateral Right side of body Left side of body spinothalamic tract. Axons projecting from primary neurons enter the spinal cord and synapse on secondary neurons within the posterior horns. Axons entering these pathways conduct stimuli related to crude touch and pressure as well as pain and temperature. Axons of the secondary neurons in the anterolateral pathway cross over to the opposite side of the spinal cord before Cerebellum ascending toward the brain. This decussation occurs through the anterior white commissure, located anterior to the gray commis- Pons sure. The anterior and lateral spinothalamic pathways, like the posterior funiculus–medial lemniscal pathway, are somatotopi- Secondary neuron cally organized: Axons transmitting sensory information from Posterior spinocerebellar tract more inferior segments of the body are located lateral to those Anterior spinocerebellar tract from more superior segments. Secondary neuron axons synapse Spinocerebellar pathway on tertiary neurons located within the thalamus. Axons from the Medulla oblongata tertiary neurons then conduct stimulus information to the appropriate region of the primary somatosensory cortex. Proprioceptive input from joints, muscles, and tendons Spinocerebellar Pathway The spinocerebellar pathway conducts proprioceptive information Primary neuron to the cerebellum for processing to coordinate body movements. The Spinal cord spinocerebellar pathway is composed of anterior and posterior spinocerebellar tracts; these are the major routes for transmitting postural input to the cerebellum (figure 17.4). Sensory input arriving at the Pathway direction cerebellum through these tracts is critical for regulating posture and Figure 17.4 balance and for coordinating skilled movements. Note that these spino- Spinocerebellar Pathway. This pathway conducts proprioceptive cerebellar tracts are different from the other sensory pathways in that information to the cerebellum through both the anterior and posterior they do not use tertiary neurons; rather, they only have primary and spinocerebellar tracts. Only some of the axons destined to enter the secondary neurons. Information conducted in spinocerebellar path- anterior spinocerebellar pathway decussate at the level where the ways is integrated and acted on at a subconscious level. primary neuron axon enters the spinal cord. Only primary (purple) Anterior spinocerebellar tracts conduct impulses from the and secondary (dark blue) neurons are found in this type of pathway. inferior regions of the trunk and the lower limbs. Their axons enter the cerebellum through the superior cerebellar peduncle. Posterior spinocerebellar tracts conduct impulses from the lower limbs, the trunk, and the upper limbs. Their axons enter the cerebellum WHAT DO YOU THINK? through the inferior cerebellar peduncle. ●2 You have learned that most sensory impulses never reach our Specific characteristics and details of these pathways are conscious awareness. Why? What would be the drawback to being summarized in table 17.2. consciously aware of almost all sensory impulses?

Table 17.2 Principal Sensory Spinal Cord Pathways, Locations, Functions, and Decussation Sites Pathway Origin of Location of Neuron Cell Bodies and Termination Function Pathway Neuron Decussation Site Primary Secondary Tertiary Neuron Neuron Neuron POSTERIOR FUNICULUS–MEDIAL LEMNISCAL PATHWAY Fasciculus Upper limb Posterior root Nucleus cuneatus Thalamus Nucleus cuneatus Conduct sensory impulses cuneatus Superior trunk, ganglion Axons decussate of medulla for proprioceptive neck prior to entry into oblongata information about limb Posterior head medial lemniscus position and discriminative touch, precise pressure, and vibration sensation

Fasciculus gracilis Lower limb Posterior root Nucleus gracilis Thalamus Nucleus gracilis of Same as fasciculus cuneatus Inferior trunk ganglion Axons decussate medulla oblongata prior to entry into medial lemniscus

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Table 17.2 Principal Sensory Spinal Cord Pathways, Locations, Functions, and Decussation Sites (continued) Pathway Origin of Location of Neuron Cell Bodies and Termination Function Pathway Neuron Decussation Site Primary Secondary Tertiary Neuron Neuron Neuron ANTEROLATERAL PATHWAY Anterior Posterior horn Posterior root Posterior horn Thalamus Thalamus: Conducts sensory impulses spinothalamic interneurons ganglion interneurons Tertiary neurons for crude touch and Axons decussate project to primary pressure within spinal cord somatosensory at level of entry cortex

Lateral Posterior horn Posterior root Posterior horn Thalamus Thalamus: Conducts sensory impulses spinothalamic interneurons ganglion interneurons Tertiary neurons for pain and temperature Axons decussate project to primary within spinal cord somatosensory at level of entry cortex

SPINOCEREBELLAR PATHWAY Anterior Posterior horn Posterior root Posterior horn None Cerebellum Conducts proprioceptive spinocerebellar interneurons ganglion interneurons impulses from inferior Some axons regions of trunk and lower decussate in limbs spinal cord and pons, while others do not decussate

Posterior Posterior horn Posterior root Posterior horn None Cerebellum Conducts proprioceptive spinocerebellar interneurons ganglion interneurons impulses from lower limbs, Axons do not regions of trunk and upper decussate limbs

WHATW DID YOU LEARN? Posterior ●2 What information is conducted by sensory pathways? ●3 Compare primary and secondary neurons in the sensory pathways. Lateral 4 ● Which type of sensory pathway conducts proprioceptive corticospinal tract information? Rubrospinal tract 17.3 Motor Pathways

Learning Objectives: Anterior corticospinal tract 1. Identify and describe the key features and regional anatomy of motor pathways. Reticulospinal tract 2. Compare the characteristics of direct and indirect motor Vestibulospinal pathways. tract 3. Describe how cerebral nuclei and the cerebellum function Tectospinal tract in motor activities. Motor pathways are descending pathways in the brain and spinal cord that control the activities of skeletal muscle. Anterior 17.3a Functional Anatomy of Motor Pathways Motor pathways are formed from the cerebral nuclei, the cer- Figure 17.5 ebellum, descending projection tracts, and motor neurons. Descending Projection Tracts. Bilaterally symmetrical motor Descending projection tracts are motor pathways that originate pathways descend from both the cortex and the brainstem into the from the cerebral cortex and brainstem (figure 17.5). Motor spinal cord. Tract names indicate their point of origin and the spinal neurons within these tracts either synapse directly on motor cord as their destination. Descending motor pathways are shown in neurons in the CNS or on interneurons that, in turn, synapse on shades of red and orange; ascending sensory pathways are shown in motor neurons. pale blue.

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Table 17.3 Types of Motor Pathway Neurons Neuron Cell Body Origin Projects To: Activity Upper motor neuron Cerebral cortex or brainstem nucleus Lower motor neuron May be either excitatory or inhibitory Lower motor neuron Brainstem nucleus or anterior horn of spinal cord Skeletal muscle Excitatory only

There are at least two motor neurons in the somatic motor Pathway direction pathway: an upper motor neuron and a lower motor neuron Right side of body Left side of body (table 17.3). These neurons are involved in voluntary movements. The cell body of an upper motor neuron is housed within either Cerebrum Primary motor cortex the cerebral cortex or a nucleus within the brainstem. Axons of (precentral gyrus) the upper motor neuron synapse either directly on lower motor neurons or on interneurons that synapse directly on lower motor Internal capsule neurons. The cell body of a lower motor neuron is housed either Thalamus within the anterior horn of the spinal cord or within a brainstem Upper motor cranial nerve nucleus. Axons of the lower motor neurons exit the neurons CNS and project to the skeletal muscle to be innervated. The two types of motor neurons perform different activities: The activity of the upper motor neuron either excites or inhibits Corticospinal tracts (combined anterior the activity of the lower motor neuron, but the activity of the lower Mesencephalon and lateral tracts) motor neuron is always excitatory because its axon connects directly to the skeletal muscle fibers. The cell bodies of motor neurons and Cerebral peduncle most interneurons involved in the innervation and control of limb and trunk muscles reside in the spinal cord anterior horn and the gray matter zone between the anterior horn and the posterior horn. Fourth ventricle The neurons that innervate the head and neck are located in the Medulla oblongata motor nuclei of cranial nerves and in the reticular formation (intro- duced in chapter 15 and discussed in this chapter on page 532). Anterior corticospinal tract Motor neuron axons form two types of somatic motor pathways: direct pathways and indirect pathways. The direct pathways To skeletal Decussation in pyramids of medulla oblongata are responsible for conscious control of skeletal muscle activity, muscles and the indirect pathways are responsible for unconscious control Lateral corticospinal tract Lower of skeletal muscle activity. motor neurons

Direct Pathway Spinal cord Decussation in spinal cord The direct pathway, also called the pyramidal (pi-ram ́i-dal) pathway or corticospinal pathway, originates in the pyramidal cells of the Figure 17.6 primary motor cortex. The name pyramidal is derived from the shape Corticospinal Tracts. Corticospinal tracts originate as collections of of the upper motor neuron cell bodies, which have a tetrahedral, or motor neurons within the motor cortex of the cerebrum and synapse pyramid-like, shape. Their axons project either into the brainstem on motor neurons within the anterior horns of the spinal cord to or into the spinal cord to synapse directly on lower motor neurons. control voluntary motor activity. The upper motor neuron is dark The axons from pyramidal cell upper motor neurons descend green, and the lower motor neuron is lavender. through the internal capsule, enter the cerebral peduncles, and ultimately form two descending motor tracts of the direct pathway: corticobulbar tracts and corticospinal tracts. Corticospinal Tracts The corticospinal (kor ti-k ́ ō-sp ı̄ n ́ ăl; spinalis = backbone) tracts descend from the cerebral cortex through the Corticobulbar Tracts The corticobulbar (kō r ti-ḱ ō -bŭl bar)́ tracts brainstem and form a pair of thick anterior bulges in the medulla originate from the facial region of the motor homunculus within the oblongata called the pyramids. Then they continue into the spinal primary motor cortex. Axons of these upper motor neurons extend cord to synapse on lower motor neurons in the anterior horn of the to the brainstem, where they synapse with lower motor neuron spinal cord (figure 17.6). The corticospinal tracts are composed of cell bodies that are housed within brainstem cranial nerve nuclei. two components: lateral and anterior corticospinal tracts. (Note: The term bulbar means resembling a bulb and is used to indi- The lateral corticospinal tracts include about 85% of the cate the rhombencephalon in the brainstem.) Axons of these lower axons of the upper motor neurons that extend through the medulla motor neurons help form the cranial nerves. The corticobulbar tracts oblongata. They decussate within the pyramids of the medulla transmit motor information to control the following movements: oblongata and then form the lateral corticospinal tracts in the ■ Eye movements (via CN III, IV, and VI) lateral funiculi of the spinal cord. These tracts contain axons that ■ Cranial, facial, pharyngeal, and laryngeal muscles (via innervate both lower motor neurons of the anterior horn of the CN V, VII, IX, and X) spinal cord and interneurons within the spinal cord. Axons of the ■ Some superficial muscles of the back and neck (via CN XI) lower motor neurons innervate skeletal muscles that control skilled ■ Intrinsic and extrinsic tongue muscles (via CN XII) movements in the limbs. Some examples of skilled movements

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include playing a guitar, dribbling a soccer ball, or typing on your Pathway direction computer keyboard. Right side of body Left side of body The anterior corticospinal tracts represent the remaining 15% of the axons of upper motor neurons that extend through the Cerebrum medulla oblongata. The axons of these neurons do not decussate at the level of the medulla oblongata. Instead, they remain on their origi- nal side of the CNS and descend ipsilaterally, meaning “on the same side,” to form the anterior corticospinal tracts in both anterior white Thalamus funiculi. At each spinal cord segment, some of these axons decussate Lentiform nucleus through the median plane in the anterior white commissure. After crossing to the opposite side, they synapse either with interneurons or lower motor neurons in the anterior horn of the spinal cord. Axons of the lower motor neurons innervate axial skeletal muscle.

Red nucleus Indirect Pathway Mesencephalon Substantia nigra Several nuclei within the mesencephalon initiate motor commands for activities that occur at an unconscious level. These nuclei Decussation in and their associated tracts constitute the indirect pathway, so mesencephalon named because upper motor neurons originate within brainstem Pons nuclei (that is, they are not pyramidal cells in the cerebral cortex). Reticular formation The axons of the indirect pathway take a complex, circuitous route before finally conducting the motor impulse into the spinal cord. Upper motor neurons Motor impulses conducted by axons of the upper motor neurons in the indirect pathway descend from specific brainstem nuclei into major tracts of the spinal cord and terminate on either interneu- Medulla oblongata rons or lower motor neurons (figure 17.7). The indirect pathway modifies or helps control the pattern of Rubrospinal tract Reticulospinal tract somatic motor activity. This is accomplished by (1) altering motor Posterior root neuron sensitivity to incoming impulses to control muscles indi- Spinal cord vidually or in groups, and (2) activating feedback loops that project Lower motor neuron to the primary motor cortex. This pathway controls some muscular Anterior root activity localized within the head, limbs, and trunk of the body. It is multisynaptic and exhibits a high degree of complexity: Nerve Figure 17.7 impulses travel through diverse circuits that involve the primary Indirect Motor Pathways in the Spinal Cord. These motor pathways motor cortex, premotor cortex, cerebral nuclei, thalamus, limbic originate from neurons housed within the brainstem. The upper motor system, reticular formation, cerebellum, and brainstem nuclei. neuron is dark green, and the lower motor neuron is lavender. Motor signals within the indirect pathway can alter or help regulate the contraction of skeletal muscles by exciting or inhibiting the lower motor neurons that innervate the muscles. Interaction ■ The vestibulospinal (ves-tib ́ū -lō -spı̄ n ́ ăl) tracts originate among components of these motor pathways occurs both within the within vestibular nuclei of the brainstem. Impulses brain and at the level of the motor neurons. conducted within these tracts regulate muscular activity The different tracts of the indirect pathway are grouped that helps maintain balance during sitting, standing, and according to their primary functions. The lateral pathway regu- walking. lates and controls precise, discrete movements and tone in flexor muscles of the limbs—for example, the type of movement required Table 17.4 summarizes the characteristics of the principal to gently lay a baby in her crib. (See discussion in chapter 16.) This types of motor pathways. pathway consists of the rubrospinal (roo ́brō -spı̄ ń ăl; rubro = red) tracts that originate in the red nucleus of the mesencephalon. The Role of the Cerebral Nuclei medial pathway regulates muscle tone and gross movements of the muscles of the head, neck, proximal limb, and trunk. The medial Cerebral nuclei, discussed in chapter 15, are described again pathway consists of three groups of tracts: reticulospinal tracts, here because they interact with motor pathways in important tectospinal tracts, and vestibulospinal tracts. ways. The cerebral nuclei receive impulses from the entire cerebral cortex, including the motor, sensory, and association corti- ■ The reticulospinal (re-tik-ū -lō -spı̄ n ́ ăl) tracts originate from cal areas, as well as input from the limbic system (figure 17.8). the reticular formation in the mesencephalon. They help Most of the output from cerebral nuclei goes to the primary motor control more unskilled automatic movements related to cortex; cerebral nuclei do not exert direct control over lower posture and maintaining balance. motor neurons. Cerebral nuclei provide the patterned background ■ The tectospinal (tek-tō -spı̄ n ́ ăl) tracts conduct motor movements needed for conscious motor activities by adjusting commands away from the superior and inferior colliculi in the motor commands issued in other nuclei. For example, when the tectum of the mesencephalon to help regulate positional you start walking, you voluntarily initiate the movement, and changes of the arms, eyes, head, and neck as a consequence the cerebral nuclei then control the continuous motor commands of visual and auditory stimuli. until you decide to stop walking.

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Table 17.4 Principal Motor Spinal Cord Pathways Origin of Tract Manner of Decussation Destination of Upper Termination Site Function Motor Neurons DIRECT PATHWAY Corticobulbar tracts All cranial nerve motor nuclei Brainstem only Cranial nerve nuclei; reticular Voluntary movement of cranial receive bilateral (both ipsilateral formation muscles and contralateral) input except CN VI, VII to the lower face, and XII. These receive only contralateral input Lateral corticospinal All decussate at the pyramids Lateral funiculus Gray matter region between Voluntary movement of limb tracts posterior and anterior horns; muscles anterior horn; all levels of spinal cord Anterior corticospinal Decussation occurs in spinal cord Anterior funiculus Gray matter region between Voluntary movement of axial tracts at level of lower motor neuron posterior and anterior horns; muscles cell body anterior horn; cervical part of spinal cord

INDIRECT PATHWAY Lateral pathway Rubrospinal tract Decussate at ventral tegmentum Lateral funiculus Lateral region between posterior Regulates and controls precise, and anterior horns; anterior discrete movements and tone in horn; cervical part of spinal cord ﬂ exor muscles of the limbs Medial pathway Reticulospinal tract No decussation (ipsilateral) Anterior funiculus Medial region between posterior Controls more unskilled and anterior horns; anterior automatic movements related to horn; all parts of spinal cord posture and maintaining balance Tectospinal tract Decussate at dorsal tegmentum Anterior funiculus Medial region between posterior Regulates positional changes of and anterior horns; anterior the upper limbs, eyes, head, and horn; cervical part of spinal cord neck due to visual and auditory stimuli Vestibulospinal Some decussate (contralateral) Anterior funiculus Medial region between posterior Regulates muscular activity that tract and some do not (ipsilateral) and anterior horns; anterior helps maintain balance during horn; medial tracts to cervical sitting, standing, and walking and superior thoracic parts of spinal cord; lateral tracts to all parts of spinal cord

Figure 17.8 Cerebral Nuclei and Selected Indirect Motor System Components. A partially Primary motor cortex cut-away brain diagram shows the general physical location of the cerebral nuclei and some structures of the indirect motor system. Thalamus Tectum

Mesencephalon Superior colliculus Inferior colliculus Red nucleus Cerebral nuclei Caudate nucleus Putamen Cerebellar nuclei Globus pallidus Pons Vestibular nucleus Reticular formation Medulla oblongata

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CLINICAL VIEW Cerebrovascular Accident gyrus (see chapter 15) in the region of the motor homunculus upper limb and the motor speech area. If the obstruction lasts longer than about A cerebrovascular accident (CVA, or stroke) is caused by reduced blood 10 minutes, tissue in the brain may die. A massive stroke can leave a supply to a part of the brain due to a blocked or damaged arterial blood person completely paralyzed and without sensation over as much as vessel. If a thrombus (a blood clot within the blood vessel) forms at a half the body. narrowed region of a cerebral artery, it can completely block the lumen of Additionally, elderly people sometimes experience brief episodes of lost the artery. On occasion, a CVA also results from an embolus (a blood clot sensation or motor ability or “tingling” in the limbs. Such a short-lived that formed someplace else) that breaks free, travels through the vascular episode, called a transient ischemic attack (TIA) or “mini-stroke,” system, and becomes lodged in a cerebral blood vessel. An especially results from a temporary plug in a blood vessel that dissolves in a serious form of stroke results when a weakened blood vessel in the brain matter of minutes. However, TIAs can indicate substantial risk for ruptures and hemorrhages, quickly leading to unconsciousness and death. a more serious vessel blockage in the future. Symptoms of a CVA include loss or blurring of vision, weakness or slight The risk for a CVA increases with age, and is also influenced by family numbness, headache, dizziness, and walking difficulties. Depending history, race, and gender. People can lower their risk of stroke by making on the location of the blockage, the person may experience regional lifestyle changes and getting treatment for existing heart conditions, high sensory loss, motor loss, or both. For example, a patient who suddenly cholesterol levels, and high blood pressure. People with a history of TIAs exhibits speech difficulties and loss of motor control of the right arm should take an agent that inhibits platelet aggregation, such as aspirin. may be experiencing a CVA that affects the left hemisphere precentral

Role of the Cerebellum the body, receives the plan for movement, and then follows the The cerebellum plays a key role in movement by regulating the activity to see if it was carried out correctly. When the cerebel- functions of the motor pathways. The cerebellum continuously lum detects a disparity between the intended and actual move- receives convergent input from the various sensory pathways ment, it may generate an error-correcting signal. This signal is and from the motor pathways themselves (figure 17.9). In transmitted to both the premotor and primary motor cortices this way, the cerebellum unconsciously perceives the state of via the thalamus and the brainstem. Descending pathways

Voluntary movements The primary motor cortex and the basal nuclei Cerebral hemisphere in the forebrain send impulses through the nuclei of the pons to the cerebellum. Assessment of voluntary movements Proprioceptors in skeletal muscles and joints report degree of movement to the cerebellum.

Integration and analysis Corpus The cerebellum compares the callosum planned movements (motor signals) Thalamus against the results of the actual movements (sensory signals). Corrective feedback The cerebellum sends impulses through the thalamus to the Cerebellar cortex primary motor cortex and to motor nuclei in the brainstem. Pontine nucleus

Pons

Figure 17.9 Cerebellar Pathways. Input to the cerebellum Direct (pyramidal) pathway originates from the motor cortex of the cerebrum and the pons (blue arrows), and the spinocerebellar tracts (purple arrow). Within the cerebellum, the integration and analysis of input information occurs (green arrows). Output from the cerebellum (red arrows) extends through the cerebellar peduncles (not shown). Sagittal section

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Cerebral Cerebral hemisphere hemisphere Motor association Primary motor areas Motor association cortex areas

Decision in frontal lobes Cerebral Cerebral nuclei nuclei

Cerebellum Cerebellum

Pontine nuclei of the indirect system

Direct pathway

Lower Motor activity motor neurons

(a) Motor programming (b) Process of movement Figure 17.10 Somatic Motor Control. Several regions of the brain participate in the control and modification of motor programming to produce somatic motor activities. (a) Motor programs require conscious directions from the frontal lobes to cerebral nuclei and the cerebellum. (b) The process of movement is initiated when commands are received by the primary motor cortex from the motor association areas.

then transmit these error-correcting signals to the motor neu- these carefully patterned motor activities include riding a bicycle rons. Thus, the cerebellum influences and controls movement (cerebellum), swinging the arms while walking (cerebral nuclei), by indirectly affecting the excitability of motor neurons. The and sudden startled movements due to visual or auditory stimuli cerebellum is critically important in coordinating movements (mesencephalon). because it specifies the exact timing of control signals to differ- Highly variable and complex voluntary motor patterns are ent muscles. For example, when you randomly run your fingers controlled by the cerebral cortex and occupy the highest level of up and down the strings of a guitar, you are only making noise, processing and motor control. Motor commands may be conduct- not music. It is the cerebellum that directs the precise, exquisite ed to specific motor neurons directly, or they may be conveyed finger movements necessary to produce a recognizable instru- indirectly by altering the activity of a reflex control center. mental piece. Figure 17.10b diagrammatically illustrates some steps involved in the interactions between the cerebral nuclei, motor associa- 17.3b Levels of Processing tion areas, and cerebellum, and the primary motor cortex, which then issues commands for the programming and execution of a and Motor Control voluntary movement. Simple reflexes that stimulate motor neurons represent the lowest le vel of motor control. The nuclei controlling these reflexes are WHATW DID YOU LEARN? located in the spinal cord and the brainstem. Brainstem nuclei also participate in more complex reflexes. Upon receipt of sensory ●5 Identify the CNS components that form the somatic motor pathways. impulses, they initiate motor responses to control motor neurons directly or oversee the regulation of reflex centers elsewhere in ●6 Compare and contrast the upper and lower motor neurons with the brain. The pattern of feedback, control, and modification respect to their cell body origin, what structure(s) they project to, and whether they are excitatory or inhibitory. between brain regions establishes the motor programming that ultimately produces somatic motor control over the process of ●7 What is the primary difference between direct and indirect motor movement as illustrated in figure 17.10. The most complex pathways? unconscious motor patterns are controlled by neurons in the ●8 Compare and contrast the influence of cerebral nuclei and the cerebellum, cerebral nuclei, and mesencephalon. Examples of cerebellum on skeletal muscle.

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17.4 Higher-Order Processing CLINICAL VIEW and Integrative Functions Learning Objectives: Hemispherectomies and 1. Identify the locations and describe the functions of the Hemispheric Lateralization integrative areas of the cerebral cortex. Epilepsy is a disorder in which neurons emit nerve impulses 2. Describe hemispheric lateralization and functional too frequently and rapidly, causing seizures that detrimentally differences between the hemispheres. affect motor and sensory function. The seizure activity almost 3. Identify the cerebral centers involved in written and always originates on one side of the brain. Most seizures may be spoken language. controlled by anticonvulsant medications, but if medications are 4. Identify and describe the processes and brain locations ineffective, surgery may be the next therapy. Surgical removal of related to cognition. the brain part that is the source of the seizures often eliminates 5. Describe the brain regions and structures involved in seizure episodes. Because the most common source of seizures memory storage and recall. is the temporal lobe, most patients undergo a temporal lobec- Higher-order mental functions encompass learning, tomy. If the seizure source is in a different part of the brain, memory, reasoning, and consciousness. These functions occur either an extratemporal lobectomy or a corpus callosotomy may within the cortex of the cerebrum and involve multiple brain be attempted. In severe cases, a drastic form of therapy is a regions connected by complicated networks and arrays of axons. cerebral hemispherectomy (hem e ́¯-sfe¯r-ek tó ¯-me¯) in which the Both conscious and unconscious processing of information are side of the brain responsible for the seizure activity is surgi- involved in higher-order mental functions, and they may be con- cally removed. Physicians only pursue a procedure of this mag- tinually adjusted or modified. nitude when studies have unequivocally shown which cerebral hemisphere is the source of the seizures. When a hemisphere is 17.4a Development and Maturation removed, additional cerebrospinal fluid fills the space it previ- of Higher-Order Processing ously occupied. About 90–95% of epilepsy patients experience From infancy on, our motor control and processing capabili- long-term seizure control following hemispherectomy. Of that ties become increasingly complex as we grow and mature. The group, 70–85% remain seizure-free, while 10–20% experience maturation of the control and processing pathways is reflected in at least an 80% reduction in seizure frequency. increased structural and functional complexity within the CNS. Although brain function does not return to complete normalcy fol- As previously discussed (see chapter 16), the spinal reflex is the lowing a hemispherectomy, amazingly the remaining hemisphere most basic level of CNS control. As the CNS continues to develop, takes over some of the functions of the missing hemisphere. The many neurons expand their number of connections, providing the younger the individual, the better the chances that the other increased number of synaptic junctions required for increasingly hemisphere can take over functions previously performed by complex reflex activities and processing. However, even though the missing hemisphere. Hemispherectomy is not without risk. we are born with a large number of already formed synapses and Death from the surgery alone occurs in about 2% of individuals. many more form during childhood and adolescence as our ner- Long-term complications include displacement of the remaining vous system matures, numerous synapses will degenerate unless cerebral hemisphere and problems with CSF flow. But despite we “activate and exercise” our brain to stimulate their use and these possible adverse developments, most patients’ conditions retention. are improved by the surgery. During the first year of life, the number of cortical neurons continues to increase. The myelination of most CNS axons continues throughout the first 2 years. The brain grows rapidly in size and complexity so that by the age of 5, brain growth is 95% complete. (The rest of the body doesn’t reach its adult size until of their functions. Each hemisphere tends to be specialized for puberty.) Some CNS axons remain unmyelinated until the teenage certain tasks, a phenomenon called hemispheric lateralization years (e.g., some of the axons in the prefrontal cortex). In general, (lat er-al- ́ ı̄ -ză sh ́ ŭn). Higher-order centers in both hemispheres the axons of PNS neurons continue to myelinate past puberty. A tend to have different but complementary functions. person’s ability to carry out higher-order mental functions is a In most people, the left hemisphere is the categorical hemi- direct result of the level of nervous system maturation. sphere. It usually contains the Wernicke area and the motor speech area. It is specialized for language abilities, and is also 17.4b Hemispheric Lateralization important in performing sequential and analytical reasoning Anatomically, the left and right cerebral hemispheres appear tasks, such as those required in science and mathematics. This identical, but careful examination reveals a number of differ- hemisphere appears to direct or partition information into smaller ences. Humans tend to have shape asymmetry of the frontal fragments for analysis. The term “categorical hemisphere” reflects and occipital lobes of the brain, called petalias. Right-handed this hemisphere’s function in categorization and symbolization. individuals tend to have right frontal petalias, meaning that the The other hemisphere (the right in most people) is called the right frontal lobe projects farther than the left frontal lobe, and representational hemisphere, because it is concerned with visuo- left occipital petalias, meaning that the left occipital lobe projects spatial relationships and analyses. It is the seat of imagination farther than the right occipital lobe. Conversely, left-handed indi- and insight, musical and artistic skill, perception of patterns and viduals tend to have the reverse pattern (left frontal–right occipi- spatial relationships, and comparison of sights, sounds, smells, tal petalias). The hemispheres also differ with respect to some and tastes.

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Left eye Right eye Left Right Left Right visual visual visual visual field field field field

Left hand Right hand

Verbal memory Memory for shapes (limited language comprehension)

Speech (motor Corpus callosum speech area) Left hemisphere Right hemisphere (categorical hemisphere) (representational hemisphere) Right hand Left hand motor control motor control

Feeling shapes Feeling shapes with left hand with right hand

Musical ability Precognition of faces and Superior language and spatial relationships mathematic comprehension (Wernicke area)

Right visual field Left visual field

Primary visual cortex Figure 17.11 Hemispheric Lateralization. The cerebral hemispheres exhibit functional differences as a result of specialization.

Please note that the terms categorical hemisphere and rep- of the population, the left hemisphere is the categorical hemi- resentational hemisphere reflect cognitive localizations. This ter- sphere, thus correlating with the 90% incidence of right-handed minology is not anatomic in nature. In fact, the hemisphere terms individuals in the population. However, the correlation is not are psychology terms. Additionally, it must be recognized that the nearly as strict among left-handed people, who may have either relative size of the dominant versus the nondominant hemisphere hemisphere as their categorical hemisphere. Interestingly, a thicker is not appreciated at the gross anatomic level. corpus callosum in left-handers suggests that more signals may be Both cerebral hemispheres remain in constant communica- relayed between their hemispheres. Finally, the left hemisphere is tion through commissures, especially the corpus callosum, which the speech-dominant hemisphere; it controls speech in almost all contains hundreds of millions of axons that project between the right-handed people as well as in many left-handed ones. hemispheres (figure 17.11). Lateralization of the cerebral hemispheres develops early in 17.4c Language life (prior to 5–6 years of age). In a young child, the functions of The higher-order processes involved in language include read- a damaged or removed hemisphere are often taken over by the ing, writing, speaking, and understanding words. You may recall other hemisphere before lateralization is complete. Some aspects from chapter 15 that two important cortical areas involved in of lateralization differ between the sexes. Women have a thicker integration are the Wernicke area and the motor speech area posterior part of the corpus callosum due to additional commis- (Broca area) (figure 17.12). The Wernicke area is involved in sural axons in this region. Adult males tend to exhibit more later- interpreting what we read or hear, while the motor speech area alization than females and suffer more functional loss when one receives axons originating from the Wernicke area and then hemisphere is damaged. helps regulate the respiratory patterns and precise motor activi- Hemispheric lateralization is highly correlated with handed- ties required to enable us to speak. Thus, the Wernicke area is ness. Right-handed individuals tend to have a slightly different lat- central to our ability to recognize written and spoken language. eralization pattern than those who are left-handed. Neuroanatomic Immediately posterior to the Wernicke area is the angular gyrus, research indicates that the petalia patterns just described differ a region that processes the words we read into a form that we between left-handed and right-handed individuals. In about 95% can speak (figure 17.12a). First, the Wernicke area sends a speech

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Motor speech area Wernicke area Figure 17.12 Functional Areas Within the Cerebral Cortex. (a) In most people, the left cerebral hemisphere houses the Wernicke area, the motor speech area, and the prefrontal cortex. (b) A PET scan shows the areas of the brain that are most active during speech. Angular gyrus Prefrontal cortex

(a) Lateral view

1 Auditory information about a sentence travels 2 Information from the Wernicke area travels to 3 Information travels from the motor speech area to the primary auditory cortex. The Wernicke the motor speech area. to the primary motor cortex, where motor area then interprets the sentence. commands involving muscles used for speech (b) PET scans are given.

CLINICAL VIEW plan to the motor speech area, which initiates a specific patterned motor program that is transmitted to the primary motor cortex. Next, upper motor neurons in the primary motor cortex (pyrami- Dyslexia dal cells) signal the lower motor neurons, which then innervate Dyslexia (dis-lek se ́ ¯-a˘; dys = bad, lexis = word) is an inherited the muscles of the cheeks, larynx, lips, and tongue to produce learning disability characterized by problems with single-word speech. decoding. Affected individuals not only have trouble reading, but In most people, the Wernicke area is in the categorical may also have problems writing and spelling accurately. They may hemisphere (the left). In the representational hemisphere, a corti- be able to recognize letters normally, but their level of reading cal region opposite the Wernicke area recognizes the emotional competence is far below that expected for their level of intel- content of speech. A lesion in this area of the cerebrum can make ligence. Their writing may be disorganized and uneven, with the a person unable to understand emotional nuances, such as bitter- letters of words in incorrect order or even completely reversed. ness or happiness, in spoken words. A lesion in the cortical region Occasionally, the ability to recognize and interpret the meaning of of the representational hemisphere opposite the motor speech pictures and objects is also impaired. Interestingly, many people area results in aprosody, which causes dull, emotionless speech. seemingly “outgrow” this condition, or at least develop improved (Do not immediately assume that some of your instructors have reading ability over time. This improvement may reflect neural aprosody, if you frequently experience boring lectures.) maturation or retraining of parts of the brain to better decode words and symbols. 17.4d Cognition Mental processes such as awareness, knowledge, memory, percep- Some researchers have postulated that dyslexia is a form of dis- tion, problem solving, decision making, information processing, and connect syndrome, in which transfer of information between the thinking are collectively called cognition (kog-ni sh ́ ŭn; cognitio = cerebral hemispheres through the corpus callosum is impaired. to become acquainted with). The term cognition is often used to Genetic studies of a Finnish family identified a defective gene mean the act of knowing, and it may be interpreted in a social or on chromosome 15 that appeared related to the transmission of cultural sense to describe knowledge development culminating in dyslexia from a father to three of his children. Further research thought and action. The association areas of the cerebrum, which is ongoing to determine the environmental and genetic factors form about 70% of the nervous tissue in the brain, are responsible involved in dyslexia. for both cognition and the processing and integration of information between sensory input and motor output areas.

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Figure 17.13 Practice Model of Information Processing. Sensory Cognitive psychologists have proposed input from Deliver environment Sensory Deliver Short-term Long-term a model to show the relationships memory memory memory between sensory memory and short- Regain, return term memory. Long-term memory develops later. Fail to remember Fail to remember

Various studies of individuals suffering from brain lesions on the driveway, and at the same time decide the most efficient (caused by cancer, infection, stroke, and trauma) have helped us route to follow. determine the functions of the association areas of the cerebrum. Once information is placed into long-term memory (LTM), it For example, the frontal association area (prefrontal cortex) inte- may exist for limitless periods of time. Continuing with the same grates information from the sensory, motor, and association areas example, if over the weekend you read and recopy your lecture to enable the individual to think, plan, and execute appropriate notes, review the text and figures in the book, and prepare note behavior. Thus, an individual with a frontal lobe lesion exhibits cards, you may have stored the information about the cerebral personality abnormalities. If a person loses the ability to detect lobes as LTM. Not only will you be well prepared for your next and identify stimuli (termed loss of awareness) on one side of the examination, but you may even remember this information for body or in the limbs on that side, the primary somatosensory years to come. (However, information in LTM needs to be retrieved area in the hemisphere opposite the affected side of the body has occasionally or it can be “lost,” and our ability to store and retrieve been damaged. An individual who has agnosia (ag-nō z ́ ē -ă; a = information declines with age.) without, gnosis = knowledge) displays an inability to either recog- It appears that our brain must organize complex informa- nize or understand the meaning of various stimuli. For example, tion in short-term memory prior to storing it in long-term memory a lesion in the temporal lobe may result in the inability to either (figure 17.13). Conversion from STM to LTM is called encoding, recognize or understand the meanings of sounds or words. or memory consolidation. Encoding requires the proper function- Specific symptoms of agnosia vary, depending on the location of ing of two components of the limbic system: the amygdaloid body the lesion within the cerebrum. and the hippocampus (see chapter 15). When a sensory perception forms in the primary somatosensory cortex, cortical neurons con- 17.4e Memory vey impulses along two parallel tracts extending to the amygdaloid Memory is a versatile element of human cognition involving different body and the hippocampus. Connections from the amygdaloid lengths of time and different storage capacities. Storing and retriev- body to the hypothalamus may link memories to specific emotions, ing information requires higher-order mental functions and depends while normal functioning of the hippocampus is required for the on complex interactions among different brain regions. These brain formation of STM. Long-term memories are stored in the asso- regions include components of the limbic system, such as the amyg- ciation areas of the cerebral cortex. For example, voluntary motor daloid body and hippocampus, the insula lobe, and the frontal cortex. activity is housed in the premotor cortex, and memory of sounds On a broader scale, in addition to memory, information management is stored in the auditory association area. Because STM and LTM by the brain entails both learning (acquiring new information) and involve different anatomic structures, loss of the ability to form forgetting (eliminating trivial or nonuseful information). STM does not affect the maintenance or accessibility of LTM. Neuroscientists and psychologists classify memory in various ways. For example, sensory memory occurs when we form WHAT DO YOU THINK? important associations based on sensory input from the environ- 3 ment. Sensory memory holds an exact copy of what is heard or ● What types of study habits best convert short-term memories into long-term memories? Do you practice these habits when you study seen (auditory and visual). It lasts for fractions of a second (a few for your exams? seconds at the most) and has unlimited capacity. Short-term memory (STM) follows sensory memory. It is generally characterized by limited capacity (approximately seven 17.4f Consciousness small segments of information) and brief duration (ranging from Consciousness includes an awareness of sensation, voluntary con- seconds to hours). Suppose that, in a Friday morning anatomy trol of motor activities, and activities necessary for higher mental lecture, your instructor lists the general functions of the cerebral processing. Levels of consciousness exist on a conti nuum. The lobes on the board. Unless you study this information over the highest state of consciousness and cortical activity is alertness, in weekend, you will probably not recall it by Monday’s lecture which the individual is responsive, aware of self, and well-oriented because it was just a small bit of information in STM. to person, place, and time. Normal people alternate between Some psychologists believe that short-term memory repre- periods of alertness and sleep, which is the natural, temporary sents a group of systems used to temporarily store and manipulate absence of consciousness from which a person can be aroused by information that together comprise working memory. This type of normal stimulation. Cortical activity is depressed during sleep, but memory is required to perform several different mental activities functions continue in the vital centers in the brainstem. simultaneously. For example, a newly hired newspaper delivery Consciousness involves the simultaneous activity of large person must glance at an address list for customer home delivery, areas of the cerebral cortex. Projecting vertically through the core drive through the neighborhood, and make judgments as to where of the midbrain, pons, and medulla is a loosely organized core of to place the delivered paper, such as on the porch, in the mailbox, gray matter called the reticular formation (figure 17.14). The

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CLINICAL VIEW Amnesia alcohol, temporarily disrupt memory processing, resulting in temporary anterograde amnesia immediately following their use. Amnesia (am-ne¯ ze ́ ¯-a˘ ; forgetfulness) refers to complete or partial loss of In retrograde (ret ro ́ ¯-gra¯d; behind) amnesia, the person loses memo- memory. Most often, amnesia is temporary and affects only a portion of a ries of past events. Short-term retrograde amnesia may follow a blow person’s experiences. Causes of amnesia range from psychological trauma to the head, as might occur in an auto accident or a football game. to direct brain injury, such as a severe blow to the head or even a CVA. These patients often experience a memory gap spanning as much as Because memory processing and storage involve numerous regions 20 minutes immediately preceding the event. of the brain, the type of memory loss that occurs in an episode of Posttraumatic amnesia follows a head injury. The duration and extent amnesia depends on the area of the brain damaged. For example, of the amnesia depend on the severity and location of the brain damage. damage to or loss of sensory association areas in the cerebral cortex Posttraumatic amnesia commonly involves features of both retrograde can prevent sensory signals from arriving at the primary somato- and anterograde amnesia, with the patient exhibiting decreased ability sensory cortex adjacent to the area of injury. The most serious to recall past events as well as to process current events. forms of amnesia result from damage to the thalamus and limbic structures, especially the hippocampus. If one or more of these Hysterical amnesia covers episodes of memory loss linked to psycho- structures is damaged, memory storage and consolidation may be logical trauma. The condition is usually temporary, with memories of seriously disrupted or completely lacking. The nature of the under- the traumatic event typically returning slowly several days afterward. lying problem determines whether amnesia is complete or partial, For some people, however, recall is never complete. and to what degree recovery, if any, is possible. There are several Korsakoff psychosis is a form of impaired memor y processing that fol- common types of amnesia. lows years of alcohol abuse, possibly coupled with thiamine (vitamin B1) Anterograde (an ter-ó ¯-gra¯d; moving ahead) amnesia is a form in which deficiency. Although short-term memory seems intact, the person has a person f inds it hard or even impossible to process and/or store ongo- serious difficulty recalling simple stories or lists of unrelated words, ing events, although his or her memories from the past are intact and relating events, identifying common symbols, and even recognizing retrievable. Because day-to-day events are forgotten so quickly, the the faces of friends. This progressive condition is often accompanied world is always new to the person with anterograde amnesia. Some by neurologic problems, such as uncoordinated movements and sensory medications, including general anesthetics, illicit drugs, and even loss in the limbs.

Posterior RAS output to cerebral cortex

Cerebral aqueduct

Reticular formation

Red nucleus

Substantia nigra

Visual impulses Reticular formation Anterior

Auditory (b) Cross section of mesencephalon Motor tracts impulses to spinal cord Figure 17.14 General sensory tracts The Reticular Formation. (a) The reticular formation is distributed (touch, pain, temperature) throughout the brainstem. The reticular formation receives and processes various types of stimuli (blue arrow). It participates in cyclic Sensory input to RAS activities such as arousing the cortex to consciousness (purple arrow) Motor output from RAS and controlling the sleep-wake cycle. Some outputs from the reticular RAS output to cerebrum formation influence muscle activity (red arrow). (b) A cross section (a) Reticular formation through the mesencephalon shows the position of the reticular formation in the brainstem.

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CLINICAL VIEW Pathologic States of of periodic breathing (Cheyne-Stokes respiration), can help determine Unconsciousness the location and nature of the problem. A persistent vegetative state (PSV) is a condition in which the person The level of consciousness of a healthy person varies greatly during has lost his or her thinking ability and awareness of the environment, but a 24-hour period, ranging from wide awake and alert to deep sleep. noncognitive brain functions continue, such as the brainstem’s monitoring When a person is asleep, he or she is technically unconscious, but of heart rate, breathing, and the sleep-wake cycle. Some people in this not pathologically so. Prior to entering the sleep state, an individual state exhibit spontaneous movements, such as moving their eyes, grimac- becomes lethargic (leth-ar ́jik), a normal level of reduced alertness ing, crying, and even laughing. A persistent vegetative state may follow and awareness associated with an inclination to sleep. a coma, and its underlying causes are similar to those producing comas. Other unconscious conditions are pathologic. A brief loss of conscious- People in a persistent vegetative state may outwardly appear somewhat ness, termed fainting, or syncope (sin ́ko˘-pe¯), often signals inadequate normal, but they are unable to speak and do not respond to commands. cerebral blood flow due to low blood pressure, as might follow hemorrhage We were vividly reminded of the impreciseness of the medical assess- or sudden emotional stress. Stupor (stoo per; ́ stupeo = to be stunned) is ment and description of consciousness with the dramatic case of Terri a moderately deep level of unconsciousness from which the person can Schiavo. Ms. Schiavo experienced respiratory and cardiac arrest in be aroused only by extreme repeated or painful stimuli. A stupor may be 1990 at age 26. In 1993, she was diagnosed as being in a persistent associated with metabolic disorders such as low blood sugar, diseases of vegetative state. In 1998, her husband (guardian) petitioned the courts the liver or kidney, CVA or other brain trauma, or drug use. to remove her feeding tube. A legal, political, and social battle then A coma is a deep and profound state of unconsciousness from which began, and lasted until March 2005, when the feeding tube was removed the person cannot be aroused, even by repeated or painful stimuli. A and Ms. Schiavo passed away. Many judges, lawyers, politicians, and person in a coma is alive, but unable to respond to the environment. laypeople weighed in on both sides of this issue; even federal and state A coma may result from severe head injury or CVA, marked metabolic legislatures became embroiled in the battle. But although many people failure (as occurs in advanced liver and kidney disease), very low blood had maintained that Ms. Schiavo’s condition was capable of improving, sugar, or drug use. To gauge the depth of a coma, and possibly identify autopsy eventually revealed extensive and “irreversible” damage to the cause, the physician performs a detailed neurologic exam. The all regions of her brain, according to the coroner. The meaning of PSV presence or absence of certain reflexes, coupled with a particular type continues to be a controversial issue.

reticular formation extends slightly into the diencephalon and the spinal cord as well. This functional brain system has both motor 17.5 Aging and the Nervous System and sensory components. Learning Objective: The motor component of the reticular formation communicates with the spinal cord and is responsible for regulating muscle 1. Identify and describe the effects of aging on the nervous tone (especially when the muscles are at rest). The motor component system. of the reticular formation also assists in autonomic motor functions, Noticeable effects of aging on the brain and nervous system such as respiration, blood pressure, and heart rate, by working with commence at about 30 years of age. Structural changes occur in the autonomic centers in the medulla and pons. nervous tissue, in the blood vessels of the brain, and in the brain’s The sensory component of the reticular formation is respon- gross appearance. However, it must be emphasized that although sible for alerting the cerebrum to incoming sensory information. This aging may result in some brain atrophy, recognizable changes in component, also known as the reticular activating system (RAS), brain size may not necessarily occur. Functional changes in the contains sensory axons that project to the cerebral cortex. The RAS brain affect its performance and the body’s ability to maintain processes visual, auditory, and touch stimuli and uses this informa- homeostasis. Structural and functional changes overlap somewhat tion to keep us in a state of mental alertness. Additionally, the RAS because, for example, a decrease in a blood vessel’s carrying arouses us from sleep. The sound of an alarm clock can awaken us capacity (a structural change) often results in diminished meta- because the RAS receives various sensory stimuli and sends it to the bolic activity in a particular brain region (a functional change). cerebrum, thereby arousing it. Conversely, little or no stimuli (e.g., Structurally, at the tissue level the number of neurons in when you are in bed with the lights out, and no sounds are disturb- the brain diminishes. Neurons that die are not replaced; thus, the ing you) allow you to sleep, because the RAS is not stimulated to act. amount of gray matter decreases. Superficial observation confirms The Clinical View examines other levels of consciousness. that overall weight is reduced in older individuals. Loss of neurons causes a reduction in the number of synaptic connections, which WHATW DID YOU LEARN? adversely affects brain function due to diminished communication ●9 Distinguish between the activities controlled by the categorical between neurons. Reduction in the number of sensory neurons and representational hemispheres. leads to diminished ability to detect and discriminate among ●10 What is the function of the Wernicke area? external stimuli such as pain, light touch, pressure, and postural changes. Because the affected individual cannot appropriately ●11 What is cognition? assess the immediate environment, he or she may exhibit reduced ●12 What is meant by encoding with respect to memory formation? control and coordination of movement. Physical injury is also

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CLINICAL VIEW: In Depth Alzheimer Disease: The underlying cause of AD remains a mystery, although both genetics The “Long Goodbye” and environment seem to play a role. Postmortem examinations of the brains of AD patients show marked and generalized cerebral atrophy. Alzheimer disease (AD) has become the leading cause of dementia Microscopic examinations of brain tissue reveal a profound decrease in the developed world. (Dementia refers to a general loss of cognitive in the number of cerebral cortical neurons, especially those within the abilities, including memory, language, and decision-making skills.) temporal and frontal lobes. Surviving neurons contain abnormal aggre- gations of protein fibers, termed a neurofibrillary tangle. In addition, Typically, AD does not become clinically apparent until after the age of 65; an abnormal protein, amyloid (am ́i-loyd) precursor protein (APP), its diagnosis is often delayed due to confusion with other forms of cognitive appears in the brain as well as in the walls of the cerebral arterioles. impairment. Symptoms include slow, progressive loss of higher intellectual These histologic changes are most evident in the hippocampus, the functions and changes in mood and behavior. AD gradually causes language region of the brain vital to memory processing. Biochemical alterations deterioration, impaired visuospatial skills, indifferent attitude, and poor also occur, most significantly a decreased level of the neurotransmitter judgment, while leaving motor function intact. Patients become confused acetylcholine in the cerebrum. and restless, often asking the same question repeatedly. AD progresses relentlessly over months and years, and thus has come to be known as “the At present, there is no cure for AD, although some medications help long goodbye.” Eventually, it robs its victims of their memory, their former alleviate the symptoms and seem to slow the progress of the dis- personality, and even the capacity to speak. ease. In the meantime, researchers are trying to develop diagnostic tests that can better predict who may be at risk for AD. Recent We have all heard that mental activity helps us stay sharp. As people research has suggested that difficulty or loss in identifying com- age, mental decline often appears to be related to altered or decreased mon smells (like lemon, cinnamon, etc.) is linked with an increased numbers of synapses between neurons. Many different avenues of risk in developing AD. In fact, this loss of smell may be one of the research suggest that the key to brain vitality is brain activity. It first signs of developing the disease, presumably because the brain is not necessary to initiate extreme changes to obtain vital brain bene- regions involved with smell are among the first regions to develop fits. Ways to stimulate an active brain include taking a daily walk, play- the neurofibrillary tangles of AD. So, in the near future researchers ing games, attending plays or lectures, gardening, working crosswords may develop a “scratch and sniff” test to help predict an individual’s or other puzzles, reading and writing daily, or participating in com- risk of developing AD. munity groups.

Enlarged ventricles

Cortical atrophy

(a) Normal brain (b) Alzheimer brain MRIs show coronal sections of (a) a normal brain and (b) an Alzheimer brain. (Note the large ventricles and wide spaces between gyri in the AD brain.)

possible, due to the inability to assess and avoid harmful stimuli, resulting diminished nutrient supply affects nervous tissue such as a hot object or a noxious odor. performance. In addition, blood flow to specific brain regions decreases Ultimately, any decrease in the number of neurons, the because deposited lipids and atherosclerotic plaques often number of interneuronal synapses, or the supply of nutrients and cause narrowing of the internal walls of blood vessels. The removal of wastes impairs a person’s cognitive capacity.

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Clinical Terms

ataxia (ă-tak ś ē -ă; a = not, taxis = order) Inability to perform delirium (dē -lir ́ē -ŭm; deliro = to be crazy) Altered state of coordinated body movements. consciousness consisting of confusion, distractibility, confusion Condition of reduced awareness in which a person disorientation, disordered thinking and memory, defective is easily distracted and easily startled by sensory stimuli; perception, hyperactivity, agitation, and autonomic nervous alternates between drowsiness and excitability; resembles system overactivity. a state of minor delirium. hemiballismus (hem-ē -bal-iz ḿ ŭs; ballismos = jumping about) Jerking, involuntary movement involving proximal limb musculature on only one side of the body.

Chapter Summary

17.1 General ■ Sensory and motor pathways share the following characteristics: decussation, somatotopy, pairing, and usually two or Characteristics of three neurons within the CNS. Nervous System Pathways 519

17.2 Sensory ■ Pathways of the somatosensory system mediate limb position, touch, temperature, and pain. Pathways 519 17.2a Functional Anatomy of Sensory Pathways 520 ■ Primary neurons conduct stimuli to the CNS; secondary neurons are interneurons that synapse either onto a tertiary neuron within the thalamus or in the cerebellum. Decussation occurs by axons of either the primary or secondary neurons. ■ The posterior funiculus–medial lemniscal pathway conducts stimuli of fine touch, precise pressure, and proprioception (posture). ■ The anterolateral pathway (anterior and posterior spinothalamic tracts) carries stimuli related to pain, pressure, temperature, and touch. Decussation of secondary neuron axons occurs in the spinal cord at the level of entry. ■ The spinocerebellar pathway (anterior and posterior spinocerebellar tracts) conducts stimuli to the cerebellum related to tendons, joints, and muscle posture.

17.3 Motor ■ The motor pathways of the brain and spinal cord work together to control skeletal muscle. Pathways 523 17.3a Functional Anatomy of Motor Pathways 523 ■ Somatic motor pathways involve an upper motor neuron and a lower motor neuron. ■ Somatic motor commands travel through either the direct system (conscious control) or the indirect system (unconscious control). ■ Pyramidal cells are primary motor cortex neurons; the pyramidal pathway provides a rapid, direct mechanism for conscious skeletal muscle control. It consists of two pairs of descending motor tracts: the corticobulbar tracts, the lateral corticospinal tracts, and the anterior corticospinal tracts. ■ The indirect pathway is composed of centers that issue motor commands at an unconscious level: the rubrospinal, reticulospinal, tectospinal, or vestibulospinal tracts. ■ The cerebral nuclei are processing centers for patterned background movement (such as arm swinging while walking). They adjust motor commands issued in other processing centers. ■ The cerebellum helps regulate the functions of the descending somatic motor pathways by influencing and controlling movement. 17.3b Levels of Processing and Motor Control 528 ■ The most complex motor patterns are controlled by neurons in the cerebellum, cerebral nuclei, and mesencephalon.

17.4 Higher- ■ Higher-order mental functions encompass learning, memory, reasoning, and consciousness. Order Processing and Integrative 17.4a Development and Maturation of Higher-Order Processing 529 Functions 529 ■ Higher-order functions mature and increase in complexity as development proceeds. 17.4b Hemispheric Lateralization 529 ■ In most individuals, the left hemisphere is the categorical hemisphere, and the right is the representational hemisphere. 17.4c Language 530 ■ The Wernicke area is responsible for recognition of spoken and written language. The motor speech area initiates a specific motor program for the muscles of the cheeks, larynx, lips, and tongue to produce speech. 17.4d Cognition 531 ■ Mental processes such as awareness, perception, thinking, knowledge, and memory are collectively called cognition.

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17.4 Higher- 17.4e Memory 532 Order Processing ■ Memory is a higher-order mental function involving the storage and retrieval of information gathered through previous and Integrative activities. Memory classifications include sensory memory, short-term memory (STM), working memory, and long-term Functions memory (LTM). (continued) 17.4f Consciousness 532 ■ The highest state of consciousness and cortical activity is alertness. Sleep is a natural, temporary absence of consciousness.

17.5 Aging and ■ Both structural and functional changes accompany the aging of the brain. Some of these changes are reduced neuron the Nervous population, including sensory neurons; reduced blood flow through the brain; and reduced cognition. System 534

Challenge Yourself

Matching ______3. Higher-order mental functions encompass each of Match each numbered item with the most closely related lettered the following except item. a. memory. b. learning. ______1. aprosody a. contains no tertiary neurons c. reasoning. ______2. spinothalamic tract b. exits the CNS d. coughing. ______3. tertiary neuron c. axon crossover ______4. Which of these are not part of an indirect motor pathway? ______4. decussation d. information storage and a. rubrospinal tracts retrieval ______5. direct pathway b. tectospinal tracts e. secondary neuron in an c. corticobulbar tracts ______6. lower motor neuron ascending pathway d. reticulospinal tracts ______7. memory f. originates in the thalamus ______5. Pyramidal cell axons project through corticospinal ______8. indirect pathway tracts and synapse at g. unconscious control of a. motor nuclei of cranial nerves. ______9. spinocerebellar skeletal muscle b. motor neurons in the anterior horns of the spinal ______10. interneuron h. absence of emotional speech cord. c. motor neurons in the posterior horns of the spinal i. pyramidal cell cord. j. detects crude touch, pain, d. motor neurons in the lateral horns of the spinal pressure, temperature cord. ______6. The right hemisphere tends to be dominant for Multiple Choice which functions? Select the best answer from the four choices provided. a. mathematical calculations b. motor commands involved with speech ______1. The fasciculus cuneatus and fasciculus gracilis c. musical and artistic skill compose the ______d. analytic reasoning a. spinocerebellar tracts. b. posterior funiculi. ______7. Somatotopy is the c. spinothalamic tracts. a. relationship between sensory receptors and motor d. anterior white commissure. units. b. positioning of motor neurons in the cerebellar ______2. The motor tracts that conduct impulses to regulate cortex. the skilled movements of the upper and lower limbs c. precise correspondence between specific body are the and CNS areas. a. reticulospinal tracts. d. relationship between upper and lower motor b. corticospinal tracts. neurons. c. rubrospinal tracts. d. tectospinal tracts. ______8. A loss of consciousness due to fainting is called a. lethargy. b. syncope. c. coma. d. sleep.

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______9. Which of these is the least likely to affect 5. Identify and describe the distribution of the motor pathways information transfer from STM (short-term memory) that conduct conscious, voluntary motor impulses through to LTM (long-term memory)? the spinal cord. a. emotional state 6. Describe the relationship between the cerebral nuclei and b. repetition or rehearsal the cerebellum in motor activities. c. auditory association cortex d. cerebral nuclei 7. What is meant by the term hemispheric lateralization? 8. Distinguish between sensory memory, short-term memory, ______10. Where are tertiary neurons found? and long-term memory. a. extending between the posterior horn and the anterior horn 9. Describe the activities of the reticular activating system. b. extending between the posterior horn and the 10. What is the consequence of reduction in the number of brainstem sensory neurons during aging? c. extending between the thalamus horn and the primary somatosensory cortex Developing Critical Reasoning d. extending between the primary motor cortex and 1. Melissa had a horrible headache, restricted movement the brainstem in her right arm, and slight slurring of her speech. After an MRI was performed, the ER physician suggested that Content Review Melissa had suffered a CVA (cerebrovascular accident). 1. Discuss the concept of somatotopy as it relates to the motor What structures in the brain were affected by this cortex. incident? What might be the cause of the problem and the 2. Describe the function of primary neurons, secondary expected outcome? neurons, and tertiary neurons in the sensory pathways of 2. Randolph, a college professor, suffered a severe blow to the the nervous system. head while being robbed at an ATM. This trauma caused 3. Describe the pathway by which the pressure applied to the him to be unable to impart any emotion into his lectures, right hand during a handshake is transmitted and perceived although he could still speak. Identify Randolph’s condition, in the left primary somatosensory cortex. and locate the area of damage to his brain. 4. A young gymnast has suffered a spinal cord injury and can no longer detect pain sensations in her leg. What spinal tract has been affected?

Answers To “What Do You Think?”

1. No one is quite sure why most pathways decussate. Some There would be too many sensations to effectively process researchers have speculated that in more primitive brain and interpret, and we would not be able to concentrate on systems, most pathways were bilateral (a combination of the task at hand. For example, being able to “tune out” decussating and undecussating pathways), and as brains many sensory stimuli allows you to study in a crowded evolved, only one of the two pathways remained (typically, student lounge with the music blaring. the decussating pathway). However, this hypothesis has 3. Study habits that repeat and reinforce the material are not been proven and does not explain why the decussating best for converting short-term memories into long-term pathways would remain. memories. Rewriting your notes, making flashcards, reading 2. If our brains were consciously aware of every single the text, and reviewing the material on a regular basis (not sensory stimulus, we would go on “sensory overload.” just once or twice) all help form long-term memory.

www.mhhe.com/mckinley3 Enhance your study with practice tests and activities to assess your understanding. Your instructor may also recommend the interactive eBook, individualized learning tools, and more.

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