NERVOUS SYSTEM 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 mck78097_ch17_518-538.indd 518 2/14/11 3:37 PM Chapter Seventeen Pathways and Integrative Functions 519 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 somato- sensory 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 mck78097_ch17_518-538.indd 519 2/14/11 3:37 PM 520 Chapter Seventeen Pathways and Integrative Functions 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 Anterior 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).