Sensory Pathways and the Somatic Nervous System

Fundamentals of Anatomy & Physiology Eleventh Edition

Chapter 15 Sensory Pathways and the Somatic Nervous System

Lecture Presentation by Lori Garrett, Parkland College

15-1 Specify the components of the afferent and efferent divisions of the nervous system, and explain what is meant by the somatic nervous system. 15-2 Explain why receptors respond to specific stimuli, and how the organization of a receptor affects its sensitivity. 15-3 Identify the receptors for the general senses, and describe how they function. 15-4 Identify the major sensory pathways, and explain how it is possible to distinguish among sensations that originate in different areas of the body. 15-5 Describe the components, processes, and functions of the somatic motor pathways, and the levels of information processing involved in motor control.

▪ Focus for this chapter – Sensory pathways • General senses – Motor pathways • Somatic nervous system (SNS) – Controls contractions of skeletal muscles

▪ Sensory pathways – Series of neurons that relays sensory information from receptors to CNS ▪ Sensory receptors – Specialized cells or cell processes that monitor specific conditions • In the body or external environment – When stimulated, a receptor generates action potentials that are sent along sensory pathways

▪ Afferent division of nervous system – Somatic and visceral sensory pathways ▪ Efferent division of nervous system – Somatic motor portion • Carries out somatic motor commands that control peripheral effectors • Commands travel from motor centers in brain along somatic motor pathways

Sensory Pathway

Depolarization of Sensory Action Potential CNS Arriving Propagation stimulus Receptor Generation Processing

A stimulus produces a If the stimulus depolarizes Axons of sensory neurons Information processing graded change in the the receptor cell to carry information about occurs at every relay membrane potential of threshold, action the type of stimulus synapse. Sensory informa- a receptor cell. potentials develop in the (touch, pressure, tion may be distributed to initial segment. temperature) as action multiple nuclei and centers potentials to the CNS. in the spinal cord and brain.

Immediate Involuntary Response Motor Pathway (involuntary) Processing centers in the spinal cord or brainstem may direct an immediate reflex response even before sensations reach the cerebral cortex.

Voluntary Response Perception Motor Pathway The voluntary response, which is not (voluntary) Only about 1 percent of immediate, can moderate, enhance, arriving sensations are or supplement the relatively simple relayed to the primary involuntary reflexive response. somatosensory cortex.

6 15-2 Sensory Receptors

▪ Sensory receptors – Processes of specialized sensory neurons – Or cells monitored by sensory neurons ▪ Sensation – Arriving information ▪ Perception – Conscious awareness of a sensation

▪ General senses – Describe our sensitivity to • Temperature • Pain • Touch • Pressure • Vibration • Proprioception (body position)

▪ Special senses – Olfaction (smell) – Gustation (taste) – Vision (sight) – Equilibrium (balance) – Hearing ▪ Special sensory receptors – Provide sensations of special senses – Located in sense organs such as the eye or ear – Protected by surrounding tissues

▪ Detection of stimuli – Receptor specificity • Each receptor has a characteristic sensitivity – Receptive field • Area monitored by a single receptor cell • The larger the receptive field, the more difficult it is to localize a stimulus – Transduction • Conversion of an arriving stimulus into an action potential by a sensory receptor

Receptive Receptive field 1 field 2

Epidermis

Free nerve endings

11 15-2 Sensory Receptors

▪ Taste, hearing, equilibrium, and vision – Provided by specialized receptor cells – Communicate with sensory neurons across chemical synapses

▪ Interpretation of sensory information – Stimulus reaches cortical neurons via labeled line – Each labeled line carries information about one modality, or type of stimulus (e.g., touch or light) – Frequency and pattern of action potentials contain information • About strength, duration, and variation of stimulus – Your perception of the nature of a stimulus • Depends on the path it takes in CNS

▪ Adaptation – Reduction of receptor sensitivity in the presence of a constant stimulus – Peripheral adaptation in PNS – Central adaptation in CNS – Nervous system quickly adapts to painless, constant stimuli

▪ Tonic receptors – Always active – Slow-adapting receptors • Show little peripheral adaptation – Pain receptors • Remind you of an injury long after damage has taken place

Increased Normal Normal Stimulus

Frequency of action potentials Time a Tonic receptors are always active. Action potentials are generated at a frequency that reflects the background level of stimulation. When the stimulus increases or decreases, the rate of action potential generation changes accordingly.

16 15-2 Sensory Receptors

▪ Phasic receptors – Normally inactive – Provide information about intensity and rate of change of a stimulus – Fast-adapting receptors • Respond strongly at first but then activity decreases

Increased Stimulus Normal Normal

Frequency of action potentials

Time b Phasic receptors are normally inactive. Action potentials are generated only for a short time in response to a change in the conditions they are monitoring.

18 15-3 General Sensory Receptors

▪ Classifying sensory receptors – Exteroceptors • Provide information about external environment – Proprioceptors • Report positions of skeletal muscles and joints – Interoceptors • Monitor visceral organs and functions

▪ General sensory receptors – Divided into four types by nature of stimulus • Nociceptors (pain) • Thermoreceptors (temperature) • Mechanoreceptors (physical distortion) • Chemoreceptors (chemical concentration)

▪ Nociceptors (pain receptors) – Free nerve endings with large receptive fields – Are common • In superficial portions of skin • In joint capsules and within periostea of bones • Around walls of blood vessels – May be sensitive to • Temperature extremes • Mechanical damage • Dissolved chemicals (as released by injured cells)

▪ Nociceptors – Myelinated Type A fibers • Carry sensations of fast pain (prickling pain) such as that caused by injection or deep cut • Sensations reach CNS quickly and often trigger somatic reflexes • Relayed to primary somatosensory cortex and thus receive conscious attention

▪ Nociceptors – Unmyelinated Type C fibers • Carry sensations of slow pain (burning and aching pain) • Sensations cause generalized activation of reticular formation and thalamus • You become aware of the pain but only have a general idea of the area affected

▪ Thermoreceptors (temperature receptors) – Free nerve endings located in • Dermis • Skeletal muscles • Liver • Hypothalamus – Sensations are conducted along same pathways that carry pain sensations • Sent to reticular formation, thalamus, and (to a lesser extent) the primary somatosensory cortex

▪ Mechanoreceptors – Sensitive to physical stimuli that distort their plasma membranes – Membranes contain mechanically gated ion channels that open or close in response to • Stretching • Compression • Twisting • Other distortions of the membrane

▪ Three classes of mechanoreceptors – Tactile receptors provide sensations of • Touch (shape or texture) • Pressure (degree of mechanical distortion) • Vibration (pulsing pressure) – Baroreceptors • Detect pressure changes in blood vessels and in digestive, respiratory, and urinary tracts – Proprioceptors • Monitor positions of joints and skeletal muscles

▪ Tactile receptors – Fine touch and pressure receptors • Extremely sensitive • Narrow receptive fields • Provide detailed information about source of stimulation – Crude touch and pressure receptors • Large receptive fields • Provide poor localization • Give little information about stimulus

▪ Six types of tactile receptors in skin 1. Free nerve endings • Sensitive to touch and pressure • Situated between epidermal cells • Tonic receptors with small receptive fields provide touch sensations

a Free nerve endings

Free nerve endings are branching tips of sensory neurons that respond to touch, pressure, pain, and temperature.

29 15-3 General Sensory Receptors

▪ Six types of tactile receptors in skin 2. Root hair plexus nerve endings • Monitor distortions and movements across body surface wherever hairs are located • Adapt rapidly, so are best at detecting initial contact and subsequent movements

b Root hair plexus

A root hair plexus is made up of free nerve endings stimulated by hair movement.

31 15-3 General Sensory Receptors

▪ Six types of tactile receptors in skin 3. Tactile discs • Fine-touch and pressure receptors • Sensitive to shape and texture • Extremely sensitive tonic receptors • Very small receptive fields

c Tactile discs

Tactile disc Merkel cell Nerve terminal (dendrite)

Afferent nerve fiber

Tactile discs are fine touch and pressure receptors sensitive to shape and texture.

33 15-3 General Sensory Receptors

▪ Six types of tactile receptors in skin 4. Bulbous corpuscles (Ruffini corpuscles) • Sensitive to pressure and distortion of skin • Located in reticular (deep) dermis • Tonic receptors that show little if any adaptation

d Bulbous corpuscle

Collagen fibers Capsule Dendrites

Sensory nerve fiber

Bulbous (Ruffini) corpuscles are sensitive to pressure and distortion of the deep dermis.

35 15-3 General Sensory Receptors

▪ Six types of tactile receptors in skin 5. Lamellar corpuscles (pacinian corpuscles) • Sensitive to deep pressure • Fast-adapting receptors • Most sensitive to pulsing or high-frequency vibrating stimuli • A single dendrite lies within a series of concentric layers of collagen fibers

e Lamellar corpuscle

Dermis

Dendrite

Concentric layers (lamellae) of collagen fibers separated by fluid

Lamellar (pacinian) corpuscles are sensitive to deep pressure Cross section LM × 125 and high-frequency vibration.

37 15-3 General Sensory Receptors

▪ Six types of tactile receptors in skin 6. Tactile corpuscles (Meissner corpuscles) • Perceive sensations of fine touch, pressure, and low-frequency vibration • Adapt to stimulation within 1 second after contact • Fairly large structures • Most abundant in eyelids, lips, fingertips, nipples, and external genitalia

f Tactile corpuscle

Tactile corpuscle

Epidermis

Capsule

Dendrites Dermis Sensory nerve fiber

Tactile (Meissner) corpuscles are sensitive to fine touch, pressure, and low-frequency vibration. LM × 330

39 15-3 General Sensory Receptors

▪ Baroreceptors – Monitor changes in pressure in an organ – Free nerve endings that branch within elastic tissues • In wall of distensible organ (e.g., a blood vessel) – Respond immediately to change in pressure, but adapt rapidly

Baroreceptors in the Body Baroreceptors of Carotid Sinus and Aortic Sinus Provide information on blood pressure to cardiovascular and respiratory control centers Baroreceptors of Lung Provide information on lung expansion to respiratory rhythmicity centers for control of respiratory rate Baroreceptors of Digestive Tract Provide information on volume of tract segments, trigger reflex movement of materials along tract Baroreceptors of Colon Provide information on volume of fecal material in colon, trigger defecation reflex

Baroreceptors of Bladder Wall Provide information on volume of urinary bladder, trigger urination reflex 41 15-3 General Sensory Receptors

▪ Proprioception – A somatic sensation – No proprioceptors in visceral organs of thoracic and abdominopelvic cavities • You cannot tell where your spleen, appendix, or pancreas is at the moment – Proprioceptors are found in • Joints • Tendons and ligaments • Muscles

▪ Three major groups of proprioceptors – Muscle spindles • Monitor skeletal muscle length • Trigger stretch reflexes – Golgi tendon organs • At junction between skeletal muscle and its tendon • Monitor tension during muscle contraction – Receptors in joint capsules • Free nerve endings that detect pressure, tension, and movement at the joint

▪ Chemoreceptors – Respond to water- and lipid-soluble substances that are dissolved in body fluids – Exhibit peripheral adaptation in seconds – Monitor pH, carbon dioxide, and oxygen levels in arterial blood at • Carotid bodies – Near origin of internal carotid arteries • Aortic bodies – Between major branches of aortic arch

Chemoreceptors In and Near Trigger reflexive Respiratory Centers of Medulla adjustments in Oblongata depth and rate of respiration Sensitive to changes in pH

and CO2 in cerebrospinal fluid

Cranial nerve IX Chemoreceptors of Carotid Bodies Trigger reflexive Sensitive to changes in pH, adjustments in respiratory and CO2, and O2 in blood cardiovascular Cranial nerve X activity Chemoreceptors of Aortic Bodies Sensitive to changes in pH,

CO2, and O2 in blood

45 15-4 Sensory Pathways

▪ First-order neuron – Sensory neuron that delivers sensations to CNS ▪ Second-order neuron – Interneuron in spinal cord or brainstem that receives information from first-order neuron – Crosses to opposite side of CNS (decussation) ▪ Third-order neuron – Neuron in thalamus that must receive information from second-order neuron • For the sensation to reach our awareness

▪ Somatic sensory pathways – Carry sensory information from skin and muscles of body wall, head, neck, and limbs to CNS – Major somatic sensory pathways • Spinothalamic pathway • Posterior column pathway • Spinocerebellar pathway

Posterior Column Pathway Spinal Posterior Gracile fasciculus ganglion root Cuneate fasciculus

Spinocerebellar Pathway

Posterior spinocerebellar tract Anterior spinocerebellar tract

Anterior root Spinothalamic Pathway

Lateral spinothalamic tract Anterior spinothalamic tract

48 15-4 Sensory Pathways

▪ Spinothalamic pathway – Carries sensations of crude touch, pressure, pain, and temperature – First-order neurons enter spinal cord and synapse within posterior horns – Second-order neurons cross to opposite side of spinal cord before ascending – Third-order neurons in ventral nuclei of thalamus • After sorting and processing, sensations are sent to primary somatosensory cortex

▪ Spinothalamic pathway – Anterior spinothalamic tract • Crude touch and pressure – Lateral spinothalamic tract • Pain and temperature

KEY Torso Axon of first- Upper limb order neuron Lower Second-order limb neuron

Third-order Face and neuron Genitalia lips

Jaw Tongue

Midbrain

The anterior spinothalamic tracts of the spinothalamic Medulla pathway carry crude oblongata touch and pressure sensations.

Anterior spinothalamic tract

Spinal cord

Crude touch and pressure sensations from right side of body 51 Figure 15–7 Sensory Pathways and Ascending Tracts in the Spinal Cord (Part 1 of 2).

Spinal Posterior ganglion root

Anterior root

52 15-4 Sensory Pathways

▪ Results of abnormality in spinothalamic pathway – Painful sensations that are not produced where they are perceived to originate • E.g., phantom limb syndrome (continued feeling of pain in amputated limb) – Referred pain • Feeling pain in an uninjured part of body when pain originates at another location • Visceral pain can manifest as pain in body surface • E.g., a heart attack is frequently felt in the left arm

KEY Axon of first- order neuron Second-order neuron Third-order neuron

Midbrain

The lateral spinothalamic tracts of the spinothalamic pathway carry pain Medulla and temperature sensa- oblongata tions.

Lateral spinothalamic tract

Spinal cord

Pain and temperature sensations from right side of body 54 Figure 15–9 Referred Pain.

Heart

Liver and gallbladder

Stomach Small intestine Appendix Ureters Colon

55 15-4 Sensory Pathways

▪ Posterior column pathway – Carries sensations of fine touch, vibration, pressure, and proprioception – Spinal tracts involved • Left and right gracile fasciculus • Left and right cuneate fasciculus – After second-order neurons of gracile and cuneate nuclei decussate, • Their axons enter the medial lemniscus (tract)

▪ Posterior column pathway – Second-order neurons synapse on third-order neurons in ventral nuclei of thalamus • Nuclei sort arriving information according to – Nature of stimulus – Region of body involved – Processing in thalamus • Determines how a sensation is perceived – Localization of sensation depends on where it arrives in primary somatosensory cortex

▪ Posterior column pathway – Sensory homunculus • Functional map of primary somatosensory cortex • Area devoted to a particular body region is – Proportional to density of sensory neurons – Not proportional to region’s size

b POSTERIOR COLUMN PATHWAY The posterior column pathway carries sensations of highly localized (“fine”) touch, pressure, vibration, and proprioception. This pathway is also known as the posterior column–medial lemniscus pathway. It begins at a peripheral receptor and ends at the primary somatosensory cortex of the cerebral hemispheres.

KEY Axon of first- order neuron Second-order neuron Third-order neuron

Ventral nuclei in thalamus

Midbrain

Medial lemniscus Gracile nucleus and cuneate Medulla nucleus oblongata

Gracile fasciculus and cuneate fasciculus

Spinal ganglion

Spinal cord

Fine touch, vibration, pressure, and proprioception sensations from right side of body 59 15-4 Sensory Pathways

▪ Spinocerebellar pathway – Conveys information about positions of muscles, tendons, and joints from spinal cord to cerebellum – This information does not reach our awareness

▪ Spinocerebellar tracts – Posterior spinocerebellar tracts • Axons do not cross to opposite side of spinal cord • Travel through inferior cerebellar peduncle – Anterior spinocerebellar tracts • Sensations reach cerebellar cortex via superior cerebellar peduncle • Many axons cross over twice – Once in spinal cord – Once in cerebellum

c SPINOCEREBELLAR PATHWAY The cerebellum receives proprioceptive information about the position of skeletal muscles, tendons, and joints along the spinocerebellar pathway. The posterior spinocerebellar tracts contain axons that do not cross over to the opposite side of the spinal cord. These axons reach the cerebellar cortex by the inferior cerebellar peduncle of that side. The anterior spinocerebellar tracts are dominated by axons that have crossed over to the opposite side of the spinal cord. KEY Axon of first- order neuron Second-order neuron Third-order neuron

PONS

Cerebellum

Medulla oblongata Spinocerebellar pathway Posterior spinocerebellar tract

Anterior spinocerebellar Spinal cord tract

Proprioceptive input from Golgi tendon organs, muscle spindles, and joint capsule receptors 62 15-4 Sensory Pathways

▪ Visceral sensory pathways – Visceral sensory information is collected by interoceptors monitoring visceral tissues and organs • Primarily within thoracic and abdominopelvic cavities – Interoceptors include nociceptors, baroreceptors, thermoreceptors, tactile receptors, chemoreceptors • Not as numerous as in somatic tissues

▪ Visceral sensory pathways – Cranial nerves V, VII, IX, and X • Carry sensory information from mouth, palate, pharynx, larynx, trachea, esophagus, etc. – Solitary nucleus • Large nucleus on each side of medulla oblongata • Major processing and sorting center for visceral sensory information • Extensive connections with cardiovascular and respiratory centers and reticular formation

▪ Somatic nervous system (SNS) – Controls contractions of skeletal muscles ▪ Somatic motor pathways – Always involve at least two motor neurons 1. Upper motor neuron 2. Lower motor neuron ▪ Upper motor neuron – Cell body lies in a CNS processing center – Synapses on lower motor neuron – Activity may facilitate or inhibit lower motor neuron

▪ Lower motor neuron – Cell body lies in a nucleus of brainstem or spinal cord • Only the axon extends outside CNS – Innervates a single motor unit in a skeletal muscle – Activation triggers a contraction in innervated muscle – Damage eliminates voluntary and reflex control over innervated motor unit

▪ Conscious and subconscious motor commands – Control skeletal muscles by traveling over three integrated motor pathways • Corticospinal pathway • Medial pathway • Lateral pathway

Corticospinal Pathway

Lateral Anterior corticospinal corticospinal Lateral Pathway tract tract Rubrospinal tract

Medial Pathway

Medial and lateral reticulospinal tracts

Tectospinal tract

Vestibulospinal tract

68 15-5 Somatic Motor Pathways

▪ Corticospinal pathway (pyramidal system) – Provides voluntary control over skeletal muscles – Begins at pyramidal cells of primary motor cortex • Axons descend into brainstem and spinal cord • Synapse on lower motor neurons that control skeletal muscles – Contains three pairs of descending tracts 1. Corticobulbar tracts 2. Lateral corticospinal tracts 3. Anterior corticospinal tracts

▪ Corticobulbar tracts – Allow conscious movement of eyes, jaw, face, and some muscles of neck and pharynx – Innervate motor centers of medial and lateral pathways

▪ Corticospinal tracts – Axons synapse on lower motor neurons in anterior horns of spinal cord – Visible along anterior surface of medulla oblongata as a pair of thick bands, the pyramids – Lateral corticospinal tracts • Contain axons that decussate at pyramids – Anterior corticospinal tracts • Contain axons that cross over at targeted spinal segment in anterior white commissure

▪ Corticospinal pathway – Motor homunculus • Functional map of primary motor cortex • Corresponds with specific regions of the body • Indicates degree of fine motor control available – Hands, face, and tongue appear large – Trunk is relatively small • Proportions are similar to those of sensory homunculus

Corticobulbar tract

To skeletal muscles Midbrain Cerebral peduncle Motor nuclei of cranial nerves The descending axons To form the corticospinal skeletal tracts that descend within muscles the brainstem.

The axons in the Medulla oblongata corticospinal tracts cross to the opposite side either in the brainstem or within the spinal cord. Lateral corticospinal tract Anterior corticospinal tract To Spinal cord skeletal muscles Motor neuron in anterior horn 73 15-5 Somatic Motor Pathways

▪ Centers in cerebrum, diencephalon, and brainstem – May issue somatic motor commands in response to subconscious processing – Medial pathway • Helps control gross movements of trunk and proximal limb muscles – Lateral pathway • Helps control distal limb muscles that perform precise movements

▪ Medial pathway – Controls muscle tone and gross movements of neck, trunk, and proximal limb muscles – Upper motor neurons are located in • Vestibular nuclei • Superior and inferior colliculi • Reticular formation

▪ Medial pathway – Vestibular nuclei • Receive information from vestibulocochlear nerve (VIII) regarding position and movement of head • Primary goal is to maintain posture and balance • Descending fibers form vestibulospinal tracts

▪ Medial pathway – Superior and inferior colliculi • Located in tectum (roof of midbrain) • Superior colliculi receive visual sensations • Inferior colliculi receive auditory sensations • Axons of upper motor neurons descend in tectospinal tracts – Decussate immediately, before descending to synapse on lower motor neurons

▪ Medial pathway – Reticular formation • Loosely organized network of neurons that extends throughout brainstem • Axons of upper motor neurons descend into medial and lateral reticulospinal tracts – Without crossing to opposite side

▪ Lateral pathway – Controls muscle tone and precise movements of distal parts of limbs – Axons of upper motor neurons in red nuclei decussate in brain • Descend into spinal cord in rubrospinal tracts

▪ Basal nuclei and cerebellum – Responsible for coordination and feedback control over muscle contractions • Consciously or subconsciously directed

▪ Basal nuclei – Provide background patterns of movement involved in voluntary motor activities – Some axons extend to premotor cortex, which directs activities of primary motor cortex • Alters instructions carried by corticospinal tracts – Other axons alter excitatory or inhibitory output of reticulospinal tracts

▪ Cerebellum monitors – Proprioceptive (position) sensations – Visual information from eyes – Vestibular (balance) sensations from internal ear ▪ Patterns of cerebellar activity are learned by trial and error over many repetitions – Fine-tuning of complex movements improves with practice