Fundamentals of Anatomy & Physiology Eleventh Edition Chapter 15 Sensory Pathways and the Somatic Nervous System Lecture Presentation by Lori Garrett, Parkland College

Learning Outcomes 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.

Sensory Pathways and Somatic Nervous System 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

15-1 Sensory and Motor Pathways 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

Figure 15–1 An Overview of Events Occurring Along the Sensory and Motor Pathways. Sensory Pathway Depolarization of Sensory Receptor Arriving stimulus Action Potential Generation CNS Propagation Processing A stimulus produces a graded change in the membrane potential of a receptor cell. If the stimulus depolarizes the receptor cell to threshold, action potentials develop in the initial segment. Axons of sensory neurons carry information about the type of stimulus (touch, pressure, temperature) as action potentials to the CNS. Information processing occurs at every relay synapse. Sensory informa- tion may be distributed to multiple nuclei and centers 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 (voluntary) The voluntary response, which is not immediate, can moderate, enhance, or supplement the relatively simple involuntary reflexive response. Only about 1 percent of arriving sensations are relayed to the primary somatosensory cortex.

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

15-2 Sensory Receptors General senses Describe our sensitivity to Temperature Pain Touch Pressure Vibration Proprioception (body position)

15-2 Sensory Receptors 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 Taste, hearing, equilibrium, and vision Provided by specialized receptor cells Communicate with sensory neurons across chemical synapses

15-2 Sensory Receptors 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

Figure 15–2 Receptors and Receptive Fields. Epidermis Free nerve endings

15-2 Sensory Receptors 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

15-2 Sensory Receptors 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

Figure 15–3a Tonic and Phasic Sensory Receptors. 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.

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

Figure 15–3b Tonic and Phasic Sensory Receptors. Increased Normal Normal Stimulus 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.

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)

15-3 General Sensory Receptors 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)

15-3 General Sensory Receptors 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 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

15-3 General Sensory Receptors 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

15-3 General Sensory Receptors 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

15-3 General Sensory Receptors 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

15-3 General Sensory Receptors 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

15-3 General Sensory Receptors Six types of tactile receptors in skin Free nerve endings Sensitive to touch and pressure Situated between epidermal cells Tonic receptors with small receptive fields provide touch sensations Free nerve endings Free nerve endings are branching tips of sensory neurons that respond to touch, pressure, pain, and temperature. a

15-3 General Sensory Receptors Six types of tactile receptors in skin ​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 Root hair plexus A root hair plexus is made up of free nerve endings stimulated by hair movement. b

15-3 General Sensory Receptors Six types of tactile receptors in skin Tactile discs Fine-touch and pressure receptors Sensitive to shape and texture Extremely sensitive tonic receptors Very small receptive fields 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. c

15-3 General Sensory Receptors Six types of tactile receptors in skin Bulbous corpuscles (Ruffini corpuscles) Sensitive to pressure and distortion of skin Located in reticular (deep) dermis Tonic receptors that show little if any adaptation Bulbous corpuscle Collagen fibers Capsule Dendrites Sensory nerve fiber Bulbous (Ruffini) corpuscles are sensitive to pressure and distortion of the deep dermis. d

15-3 General Sensory Receptors Six types of tactile receptors in skin 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 Lamellar corpuscle Dermis Dendrite Concentric layers (lamellae) of collagen fibers separated by fluid Lamellar (pacinian) corpuscles are sensitive to deep pressure and high-frequency vibration. Cross section LM × 125 e

15-3 General Sensory Receptors Six types of tactile receptors in skin 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 Tactile corpuscle Epidermis Capsule Dendrites Dermis Sensory nerve fiber Tactile (Meissner) corpuscles are sensitive to fine touch, pressure, and low-frequency vibration. LM × 330 f

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

Figure 15–5 Baroreceptors and the Regulation of Autonomic Functions. 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

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

15-3 General Sensory Receptors 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

15-3 General Sensory Receptors 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

Figure 15–6 Locations and Functions of Chemoreceptors. Chemoreceptors In and Near Respiratory Centers of Medulla Oblongata Trigger reflexive adjustments in depth and rate of respiration Sensitive to changes in pH and CO2 in cerebrospinal fluid Cranial nerve IX Chemoreceptors of Carotid Bodies Trigger reflexive adjustments in respiratory and cardiovascular activity Sensitive to changes in pH, CO2, and O2 in blood Cranial nerve X Chemoreceptors of Aortic Bodies Sensitive to changes in pH, CO2, and O2 in blood

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

15-4 Sensory Pathways Somatic sensory pathways Carry sensory information from skin and muscles of body wall, head, neck, and limbs to CNS Major somatic sensory pathways Posterior Column Pathway Spinal ganglion Gracile fasciculus Cuneate fasciculus Spinocerebellar Pathway Posterior spinocerebellar tract Anterior Anterior root Spinothalamic Pathway Lateral spinothalamic tract Anterior spinothalamic tract root Spinothalamic pathway Posterior column pathway Spinocerebellar pathway

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 Anterior spinothalamic tract Crude touch and pressure Lateral spinothalamic tract Pain and temperature

and temperature sensa- KEY Axon of first- order neuron Second-order neuron Third-order Torso Upper limb Genitalia Face and lips Jaw Tongue Midbrain The anterior spinothalamic tracts of the pathway carry crude touch and pressure sensations. Medulla oblongata Anterior tract Spinal cord Crude touch and pressure sensations from right side of body Lower limb KEY Axon of first- order neuron Second-order neuron Third-order Midbrain Medulla oblongata The lateral spinothalamic tracts of the spinothalamic pathway carry pain and temperature sensa- tions. Lateral spinothalamic tract Spinal cord Pain and temperature sensations from right side of body

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

Heart Liver and gallbladder Stomach Small intestine Ureters Appendix Figure 15–9 Referred Pain. Heart Liver and gallbladder Stomach Small intestine Appendix Ureters Colon

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) 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

15-4 Sensory Pathways Posterior column pathway Processing in thalamus Determines how a sensation is perceived Localization of sensation depends on where it arrives in primary somatosensory cortex 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

Figure 15–8b Somatic Sensory Pathways. 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 nucleus Medulla oblongata Gracile fasciculus and cuneate fasciculus Spinal ganglion Spinal cord Fine touch, vibration, pressure, and proprioception sensations from right side of body

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

Figure 15–8c Somatic Sensory Pathways. 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 tract Spinal cord Proprioceptive input from Golgi tendon organs, muscle spindles, and joint capsule receptors

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

15-4 Sensory Pathways 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

15-5 Somatic Motor Pathways Somatic nervous system (SNS) Controls contractions of skeletal muscles Somatic motor pathways Always involve at least two motor neurons Upper motor neuron Lower motor neuron Cell body lies in a CNS processing center Synapses on lower motor neuron Activity may facilitate or inhibit lower motor neuron

15-5 Somatic Motor Pathways 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

Figure 15–10 Descending (Motor) Tracts in the Spinal Cord. Corticospinal Pathway Lateral corticospinal tract Anterior corticospinal tract Lateral Pathway Rubrospinal tract Medial Pathway Medial and lateral reticulospinal tracts Tectospinal tract Vestibulospinal tract

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 Corticobulbar tracts Lateral corticospinal tracts Anterior corticospinal tracts

15-5 Somatic Motor Pathways 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

15-5 Somatic Motor Pathways 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

Figure 15–11 The Corticospinal Pathway. Motor homunculus on primary motor cortex of left cerebral hemisphere KEY Upper motor neuron Lower motor neuron Corticobulbar tract To skeletal muscles Midbrain Cerebral peduncle Motor nuclei of cranial nerves The descending axons form the corticospinal tracts that descend within the brainstem. To skeletal muscles The axons in the corticospinal tracts cross to the opposite side either in the brainstem or within the spinal cord. Medulla oblongata Lateral corticospinal tract Anterior corticospinal tract To skeletal muscles Spinal cord Motor neuron in anterior horn

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

15-5 Somatic Motor Pathways 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

15-5 Somatic Motor Pathways 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