Chapter 13: The Peripheral Nervous System and Reflex Activity

1. Chapter 13: The Peripheral Nervous System and Reflex Activity

2. Peripheral Nervous System (PNS) All neural structures outside the brain Sensory receptors Peripheral nerves and associated ganglia Motor endings

3. Sensory Receptors Specialized to respond to changes in their environment (stimuli) Activation results in graded potentials that trigger nerve impulses Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain

4. Classification of Receptors Based on: Stimulus type Location Structural complexity

5. Classification by Stimulus Type Mechanoreceptors�respond to touch, pressure, vibration, stretch, and itch Thermoreceptors�sensitive to changes in temperature Photoreceptors�respond to light energy (e.g., retina) Chemoreceptors�respond to chemicals (e.g., smell, taste, changes in blood chemistry) Nociceptors�sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)

6. Classification by Location Exteroceptors Respond to stimuli arising outside the body Receptors in the skin for touch, pressure, pain, and temperature Most special sense organs

7. Classification by Location Interoceptors (visceroceptors) Respond to stimuli arising in internal viscera and blood vessels Sensitive to chemical changes, tissue stretch, and temperature changes

8. Classification by Location Proprioceptors Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles Inform the brain of one�s movements

9. Classification by Structural Complexity Complex receptors (special sense organs) Vision, hearing, equilibrium, smell, and taste (Chapter 15) Simple receptors for general senses: Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense Unencapsulated (free) or encapsulated dendritic endings

10. Unencapsulated Dendritic Endings Thermoreceptors Cold receptors (10�40�C); in superficial dermis Heat receptors (32�48�C); in deeper dermis

11. Unencapsulated Dendritic Endings Nociceptors Respond to: Pinching Chemicals from damaged tissue Temperatures outside the range of thermoreceptors Capsaicin

12. Unencapsulated Dendritic Endings Light touch receptors Tactile (Merkel) discs Hair follicle receptors

14. Encapsulated Dendritic Endings All are mechanoreceptors Meissner�s (tactile) corpuscles�discriminative touch Pacinian (lamellated) corpuscles�deep pressure and vibration Ruffini endings�deep continuous pressure Muscle spindles�muscle stretch Golgi tendon organs�stretch in tendons Joint kinesthetic receptors�stretch in articular capsules

16. From Sensation to Perception Survival depends upon sensation and perception Sensation: the awareness of changes in the internal and external environment Perception: the conscious interpretation of those stimuli

17. Sensory Integration Input comes from exteroceptors, proprioceptors, and interoceptors Input is relayed toward the head, but is processed along the way

18. Sensory Integration Levels of neural integration in sensory systems: Receptor level�the sensor receptors Circuit level�ascending pathways Perceptual level�neuronal circuits in the cerebral cortex

20. Processing at the Receptor Level Receptors have specificity for stimulus energy Stimulus must be applied in a receptive field Transduction occurs Stimulus energy is converted into a graded potential called a receptor potential

21. Processing at the Receptor Level In general sense receptors, the receptor potential and generator potential are the same thing stimulus ? receptor/generator potential in afferent neuron ? action potential at first node of Ranvier

22. Processing at the Receptor Level In special sense organs: stimulus ? receptor potential in receptor cell ? release of neurotransmitter ? generator potential in first-order sensory neuron ? action potentials (if threshold is reached)

23. Adaptation of Sensory Receptors Adaptation is a change in sensitivity in the presence of a constant stimulus Receptor membranes become less responsive Receptor potentials decline in frequency or stop

24. Adaptation of Sensory Receptors Phasic (fast-adapting) receptors signal the beginning or end of a stimulus Examples: receptors for pressure, touch, and smell Tonic receptors adapt slowly or not at all Examples: nociceptors and most proprioceptors

25. Processing at the Circuit Level Pathways of three neurons conduct sensory impulses upward to the appropriate brain regions First-order neurons Conduct impulses from the receptor level to the second-order neurons in the CNS Second-order neurons Transmit impulses to the thalamus or cerebellum Third-order neurons Conduct impulses from the thalamus to the somatosensory cortex (perceptual level)

26. Processing at the Perceptual Level Identification of the sensation depends on the specific location of the target neurons in the sensory cortex Aspects of sensory perception: Perceptual detection�ability to detect a stimulus (requires summation of impulses) Magnitude estimation�intensity is coded in the frequency of impulses Spatial discrimination�identifying the site or pattern of the stimulus (studied by the two-point discrimination test)

27. Main Aspects of Sensory Perception Feature abstraction�identification of more complex aspects and several stimulus properties Quality discrimination�the ability to identify submodalities of a sensation (e.g., sweet or sour tastes) Pattern recognition�recognition of familiar or significant patterns in stimuli (e.g., the melody in a piece of music)

29. Perception of Pain Warns of actual or impending tissue damage Stimuli include extreme pressure and temperature, histamine, K+, ATP, acids, and bradykinin Impulses travel on fibers that release neurotransmitters glutamate and substance P Some pain impulses are blocked by inhibitory endogenous opioids

30. Structure of a Nerve Cordlike organ of the PNS Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue

31. Structure of a Nerve Connective tissue coverings include: Endoneurium�loose connective tissue that encloses axons and their myelin sheaths Perineurium�coarse connective tissue that bundles fibers into fascicles Epineurium�tough fibrous sheath around a nerve

33. Classification of Nerves Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers Pure sensory (afferent) or motor (efferent) nerves are rare Types of fibers in mixed nerves: Somatic afferent and somatic efferent Visceral afferent and visceral efferent Peripheral nerves classified as cranial or spinal nerves

34. Ganglia Contain neuron cell bodies associated with nerves Dorsal root ganglia (sensory, somatic) (Chapter�12) Autonomic ganglia (motor, visceral) (Chapter�14)

35. Regeneration of Nerve Fibers Mature neurons are amitotic If the soma of a damaged nerve is intact, axon will regenerate Involves coordinated activity among: Macrophages�remove debris Schwann cells�form regeneration tube and secrete growth factors Axons�regenerate damaged part CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration

40. Cranial Nerves Twelve pairs of nerves associated with the brain Most are mixed in function; two pairs are purely sensory Each nerve is identified by a number (I�through XII) and a name �On occasion, our trusty truck acts funny�very good vehicle anyhow�

43. I: The Olfactory Nerves Arise from the olfactory receptor cells of nasal cavity Pass through the cribriform plate of the ethmoid bone Fibers synapse in the olfactory bulbs Pathway terminates in the primary olfactory cortex Purely sensory (olfactory) function

45. II: The Optic Nerves Arise from the retinas Pass through the optic canals, converge and partially cross over at the optic chiasma Optic tracts continue to the thalamus, where they synapse Optic radiation fibers run to the occipital (visual) cortex Purely sensory (visual) function

47. III: The Oculomotor Nerves Fibers extend from the ventral midbrain through the superior orbital fissures to the extrinsic eye muscles Functions in raising the eyelid, directing the eyeball, constricting the iris (parasympathetic), and controlling lens shape

49. IV: The Trochlear Nerves Fibers from the dorsal midbrain enter the orbits via the superior orbital fissures to innervate the superior oblique muscle Primarily a motor nerve that directs the eyeball

51. V: The Trigeminal Nerves Largest cranial nerves; fibers extend from pons to face Three divisions Ophthalmic (V1) passes through the superior orbital fissure Maxillary (V2) passes through the foramen rotundum Mandibular (V3) passes through the foramen ovale Convey sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication

54. VI: The Abducens Nerves Fibers from the inferior pons enter the orbits via the superior orbital fissures Primarily a motor, innervating the lateral rectus muscle

56. VII: The Facial Nerves Fibers from the pons travel through the internal acoustic meatuses, and emerge through the stylomastoid foramina to the lateral aspect of the face Chief motor nerves of the face with 5 major branches Motor functions include facial expression, parasympathetic impulses to lacrimal and salivary glands Sensory function (taste) from the anterior two-thirds of the tongue

59. VIII: The Vestibulocochlear Nerves Afferent fibers from the hearing receptors (cochlear division) and equilibrium receptors (vestibular division) pass from the inner ear through the internal acoustic meatuses, and enter the brain stem at the pons-medulla border Mostly sensory function; small motor component for adjustment of sensitivity of receptors

61. IX: The Glossopharyngeal Nerves Fibers from the medulla leave the skull via the jugular foramen and run to the throat Motor functions: innervate part of the tongue and pharynx for swallowing, and provide parasympathetic fibers to the parotid salivary glands Sensory functions: fibers conduct taste and general sensory impulses from the pharynx and posterior tongue, and impulses from carotid chemoreceptors and baroreceptors

63. X: The Vagus Nerves The only cranial nerves that extend beyond the head and neck region Fibers from the medulla exit the skull via the jugular foramen Most motor fibers are parasympathetic fibers that help regulate the activities of the heart, lungs, and abdominal viscera Sensory fibers carry impulses from thoracic and abdominal viscera, baroreceptors, chemoreceptors, and taste buds of posterior tongue and pharynx

65. XI: The Accessory Nerves Formed from ventral rootlets from the C1�C5 region of the spinal cord (not the brain) Rootlets pass into the cranium via each foramen magnum Accessory nerves exit the skull via the jugular foramina to innervate the trapezius and sternocleidomastoid muscles

67. XII: The Hypoglossal Nerves Fibers from the medulla exit the skull via the hypoglossal canal Innervate extrinsic and intrinsic muscles of the tongue that contribute to swallowing and speech

69. Spinal Nerves 31 pairs of mixed nerves named according to their point of issue from the spinal cord 8 cervical (C1�C8) 12 thoracic (T1�T12) 5 Lumbar (L1�L5) 5 Sacral (S1�S5) 1 Coccygeal (C0)

71. Spinal Nerves: Roots Each spinal nerve connects to the spinal cord via two roots Ventral roots Contain motor (efferent) fibers from the ventral horn motor neurons Fibers innervate skeletal muscles)

72. Spinal Nerves: Roots Dorsal roots Contain sensory (afferent) fibers from sensory neurons in the dorsal root ganglia Conduct impulses from peripheral receptors Dorsal and ventral roots unite to form spinal nerves, which then emerge from the vertebral column via the intervertebral foramina

74. Spinal Nerves: Rami Each spinal nerve branches into mixed rami Dorsal ramus Larger ventral ramus Meningeal branch Rami communicantes (autonomic pathways) join to the ventral rami in the thoracic region

75. Spinal Nerves: Rami All ventral rami except T2�T12 form interlacing nerve networks called plexuses (cervical, brachial, lumbar, and sacral) The back is innervated by dorsal rami via several branches Ventral rami of T2�T12 as intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall

77. Cervical Plexus Formed by ventral rami of C1�C4 Innervates skin and muscles of the neck, ear, back of head, and shoulders Phrenic nerve Major motor and sensory nerve of the diaphragm (receives fibers from C3�C5)

79. Brachial Plexus Formed by ventral rami of C5�C8 and T1 (and often C4 and T2) It gives rise to the nerves that innervate the upper limb Major branches of this plexus: Roots�five ventral rami (C5�T1) Trunks�upper, middle, and lower Divisions�anterior and posterior Cords�lateral, medial, and posterior

81. Brachial Plexus: Nerves Axillary�innervates the deltoid, teres minor, and skin and joint capsule of the shoulder Musculocutaneous�innervates the biceps brachii and brachialis and skin of lateral forearm Median�innervates the skin, most flexors and pronators in the forearm, and some intrinsic muscles of the hand Ulnar�supplies the flexor carpi ulnaris, part of the flexor digitorum profundus, most intrinsic muscles of the hand, and skin of medial aspect of hand Radial�innervates essentially all extensor muscles, supinators, and posterior skin of limb

84. Lumbar Plexus Arises from L1�L4 Innervates the thigh, abdominal wall, and psoas muscle Femoral nerve�innervates quadriceps and skin of anterior thigh and medial surface of leg Obturator nerve�passes through obturator foramen to innervate adductor muscles

87. Sacral Plexus Arises from L4�S4 Serves the buttock, lower limb, pelvic structures, and perineum Sciatic nerve Longest and thickest nerve of the body Innervates the hamstring muscles, adductor magnus, and most muscles in the leg and foot Composed of two nerves: tibial and common fibular

91. Innervation of Skin Dermatome: the area of skin innervated by the cutaneous branches of a single spinal nerve All spinal nerves except C1 participate in dermatomes Most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness

93. Innervation of Joints Hilton�s law: Any nerve serving a muscle that produces movement at a joint also innervates the joint and the skin over the joint

94. Motor Endings PNS elements that activate effectors by releasing neurotransmitters

95. Review of Innervation of Skeletal Muscle Takes place at a neuromusclular junction Acetylcholine (ACh) is the neurotransmitter ACh binds to receptors, resulting in: Movement of Na+ and K+ across the membrane Depolarization of the muscle cell An end plate potential, which triggers an action potential

97. Review of Innervation of Visceral Muscle and Glands Autonomic motor endings and visceral effectors are simpler than somatic junctions Branches form synapses en passant via varicosities Acetylcholine and norepinephrine act indirectly via second messengers Visceral motor responses are slower than somatic responses

99. Levels of Motor Control Segmental level Projection level Precommand level

101. Segmental Level The lowest level of the motor hierarchy Central pattern generators (CPGs): segmental circuits that activate networks of ventral horn neurons to stimulate specific groups of muscles Controls locomotion and specific, oft-repeated motor activity

102. Projection Level Consists of: Upper motor neurons that direct the direct (pyramidal) system to produce voluntary skeletal muscle movements Brain stem motor areas that oversee the indirect (extrapyramidal) system to control reflex and CPG-controlled motor actions Projection motor pathways keep higher command levels informed of what is happening

103. Precommand Level Cerebellum Acts on motor pathways through projection areas of the brain stem Acts on the motor cortex via the thalamus Basal nuclei Inhibit various motor centers under resting conditions

104. Precommand Level Neurons in the cerebellum and basal nuclei Regulate motor activity Precisely start or stop movements Coordinate movements with posture Block unwanted movements Monitor muscle tone Perform unconscious planning and discharge in advance of willed movements

106. Reflexes Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus Learned (acquired) reflexes result from practice or repetition, Example: driving skills

107. Reflex Arc Components of a reflex arc (neural path) Receptor�site of stimulus action Sensory neuron�transmits afferent impulses to the CNS Integration center�either monosynaptic or polysynaptic region within the CNS Motor neuron�conducts efferent impulses from the integration center to an effector organ Effector�muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

109. Spinal Reflexes Spinal somatic reflexes Integration center is in the spinal cord Effectors are skeletal muscle Testing of somatic reflexes is important clinically to assess the condition of the nervous system

110. Stretch and Golgi Tendon Reflexes For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary Muscle spindles inform the nervous system of the length of the muscle Golgi tendon organs inform the brain as to the amount of tension in the muscle and tendons

111. Muscle Spindles Composed of 3�10 short intrafusal muscle fibers in a connective tissue capsule Intrafusal fibers Noncontractile in their central regions (lack myofilaments) Wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers

112. Muscle Spindles Contractile end regions are innervated by gamma (?) efferent fibers that maintain spindle sensitivity Note: extrafusal fibers (contractile muscle fibers) are innervated by alpha (?) efferent fibers

114. Muscle Spindles Excited in two ways: External stretch of muscle and muscle spindle Internal stretch of muscle spindle: Activating the ? motor neurons stimulates the ends to contract, thereby stretching the spindle Stretch causes an increased rate of impulses in Ia fibers

116. Muscle Spindles Contracting the muscle reduces tension on the muscle spindle Sensitivity would be lost unless the muscle spindle is shortened by impulses in the ? motor neurons ?�? coactivation maintains the tension and sensitivity of the spindle during muscle contraction

118. Stretch Reflexes Maintain muscle tone in large postural muscles Cause muscle contraction in response to increased muscle length (stretch) How a stretch reflex works: Stretch activates the muscle spindle IIa sensory neurons synapse directly with ? motor neurons in the spinal cord ? motor neurons cause the stretched muscle to contract All stretch reflexes are monosynaptic and ipsilateral

119. Stretch Reflexes Reciprocal inhibition also occurs�IIa fibers synapse with interneurons that inhibit the ? motor neurons of antagonistic muscles Example: In the patellar reflex, the stretched muscle (quadriceps) contracts and the antagonists (hamstrings) relax

125. Golgi Tendon Reflexes Polysynaptic reflexes Help to prevent damage due to excessive stretch Important for smooth onset and termination of muscle contraction Produce muscle relaxation (lengthening) in response to tension Contraction or passive stretch activates Golgi tendon organs Afferent impulses are transmitted to spinal cord Contracting muscle relaxes and the antagonist contracts (reciprocal activation) Information transmitted simultaneously to the cerebellum is used to adjust muscle tension

130. Flexor and Crossed-Extensor Reflexes Flexor (withdrawal) reflex Initiated by a painful stimulus Causes automatic withdrawal of the threatened body part Ipsilateral and polysynaptic

131. Flexor and Crossed-Extensor Reflexes Crossed extensor reflex Occurs with flexor reflexes in weight-bearing limbs to maintain balance Consists of an ipsilateral flexor reflex and a contralateral extensor reflex The stimulated side is withdrawn (flexed) The contralateral side is extended

133. Superficial Reflexes Elicited by gentle cutaneous stimulation Depend on upper motor pathways and cord-level reflex arcs Plantar reflex Stimulus: stroking lateral aspect of the sole of the foot Response: downward flexion of the toes Tests for function of corticospinal tracts

134. Superficial Reflexes Babinski�s sign Stimulus: as above Response: dorsiflexion of hallux and fanning of toes Present in infants due to incomplete myelination In adults, indicates corticospinal or motor cortex damage

135. Superficial Reflexes Abdominal reflexes Cause contraction of abdominal muscles and movement of the umbilicus in response to stroking of the skin Vary in intensity from one person to another Absent when corticospinal tract lesions are present

136. Developmental Aspects of the PNS Distribution and growth of spinal nerves correlate with the segmented body plan Sensory receptors atrophy with age and muscle tone lessens due to loss of neurons, decreased numbers of synapses per neuron, and slower central processing Peripheral nerves remain viable throughout life unless subjected to trauma