Chapter 13 – The Peripheral Nervous System and Reflex Activity

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Chapter 13 - The Peripheral Nervous
System and Reflex Activity

 1. Define peripheral nervous system and identify its components.
Sensory Receptors
 2. Classify general sensory receptors by structure, stimulus detected, and body location.

Overview: From Sensation to Perception
 3. Outline the events that lead to sensation and perception.
 4. Explore the levels of neural integration in the somatosensory system.
 5. Identify the main aspects of sensory perception.
Nerves and Associated Ganglia
 6. Define nerve and ganglion and indicate the general body location of ganglia.
 7. Describe the general structure of a nerve and follow the process of nerve regeneration.

Cranial Nerves
 8. Name the 12 pairs of cranial nerves; indicate the body regions and structures innervated by each.

Spinal Nerves
 9. Describe the general features of spinal nerves and the distribution of their rami.
10. Define plexus. Name the major plexuses and describe the distribution and function of the peripheral
    nerves arising from each plexus.
Peripheral Motor Endings
11. Compare and contrast the motor endings of somatic and autonomic nerve fibers.

Overview of Motor Integration: From Intention to Effect
12. Outline the three levels of the hierarchy of motor control.
13. Compare the roles of the cerebellum and basal nuclei in controlling motor activity.
The Reflex Arc
14. Name the components of a reflex arc and distinguish between autonomic and somatic reflexes.

Spinal Reflexes
15. Compare and contrast stretch, flexor, crossed extensor, and superficial reflexes.

Developmental Aspects of the Peripheral Nervous System
16. List changes that occur in the sensory system with aging.
17. Describe the developmental relationship between the segmented arrangement of the peripheral nerves,
    skeletal muscles, and skin dermatomes.

Lecture Outline
  I. Sensory Receptors (pp. 491494; Fig. 13.1; Table 13.1)
    A. Sensory receptors are specialized to respond to changes in their environment called stimuli (pp.
         1.   Receptors may be classified according to the activating stimulus.
         2.   Receptors may be classified based on their location or the location of the activating stimulus.
         3.   Receptors may be classified based on their overall structural complexity.
    B. Free, or naked, nerve endings are present everywhere in the body and respond primarily to pain
       and temperature. (p. 492)
    C. Encapsulated Dendritic Endings (pp. 492494; Table 13.1)
         1.   Meissner's corpuscles are receptors for discriminatory and light touch in hairless areas of the
         2.   Pacinian, or lamellated, corpuscles, are stimulated when deep pressure is first applied.
         3.   Ruffini endings respond to deep and continuous pressure.
         4.   Muscle spindles detect when a muscle is being stretched and initiate a reflex that resists the
         5.   Golgi tendon organs are stimulated when the associated muscle stretches the tendon.
         6.   Joint kinesthetic receptors monitor the stretch in the articular capsules of synovial joints.

 II. Overview: From Sensation to Perception (pp. 494498; Fig. 13.2)
    A. The somatosensory system, the part of the sensory system serving the body wall and limbs,
       involves the receptor level, the circuit level, and the perceptual level.
         1.   Processing at the receptor level involves a stimulus that must excite a receptor in order for
              sensation to occur.
         2.   Processing at the circuit level is involved with delivery of impulses to the appropriate region
              of the cerebral cortex for stimulus localization and perception.
         3.   Processing at the perceptual level involves interpretation of sensory input in the cerebral

  I. Nerves and Associated Ganglia (pp. 498500; Figs. 13.313.4)
    A. A nerve is a cordlike organ consisting of parallel bundles of peripheral axons enclosed by
       connective tissue wrappings.
    B. Ganglia are collections of neuron cell bodies associated with nerves in the PNS.
   C. If damage to a neuron occurs to the axon and the cell body remains intact, cut or compressed axons
      can regenerate.

II. Cranial Nerves (pp. 500508; Fig. 13.5; Table 13.2)
   A. Olfactory nerves are responsible for smell.
   B. Optic nerves are responsible for vision.
   C. Oculomotor nerves play a role in eye movement.
   D. Trochlear nerves play a role in eye movement.
   E. Trigeminal nerves are general sensory nerves of the face.
   F.   Abducens nerves play a role in eye movement.
   G. Facial nerves function as the chief motor nerves of the face.
   H. Vestibulocochlear nerves are responsible for hearing and equilibrium.
   I.   Glossopharyngeal nerves innervate part of the tongue and pharynx.
   J.   Vagus nerves innervate the heart, lungs, and the abdominal organs.
   K. Accessory nerves move structures associated with the head and neck.
   L. Hypoglossal nerves are mixed nerves that arise from the medulla and serve the tongue.

III. Spinal Nerves (pp. 508518; Figs. 13.613.12; Tables 13.313.6)
   A. Thirty-one pairs of mixed spinal nerves arise from the spinal cord and serve the entire body except
      the head and neck.
   B. Innervation of Specific Body Regions
        1.   Each spinal nerve connects to the spinal cord by a dorsal root and a ventral root.
        2.   Rami lie distal to and are lateral branches of the spinal nerves that carry both motor and
             sensory fibers.
        3.   The back is innervated by the dorsal rami with each rami innervating the muscle in line with
             the point of origin from the spinal column.
        4.   Only in the thorax are the ventral rami arranged in a simple segmental pattern corresponding
             to that of the dorsal rami.
        5.   The cervical plexus is formed by the ventral rami of the first four cervical nerves.
        6.   The brachial plexus is situated partly in the neck and partly in the axilla and gives rise to
             virtually all the nerves that innervate the upper limb.
        7.   The sacral and lumbar plexuses overlap and because many fibers of the lumber plexus
             contribute to the sacral plexus via the lumbosacral trunk, the two plexuses are often referred to
             as the lumbosacral plexus.
        8.   The area of skin innervated by the cutaneous branches of a single spinal nerve is called a
        9.   Hinton's law states that any nerve serving a muscle that produces movement at a joint also
             innervates the joint and the skin over the joint.

 I. Peripheral Motor Endings (p. 519)
   A. Peripheral motor endings are the PNS element that activates effectors by releasing
   B. The terminals of the somatic motor fibers that innervate voluntary muscles form elaborate
      neuromuscular junctions with their effector cells and they release the neurotransmitter
   C. The junctions between autonomic motor endings and the visceral effectors involve varicosities and
      release either acetylcholine or epinephrine as their neurotransmitter.
II. Overview of Motor Integration: From Intention to Effect (pp. 519521; Fig.
   A. Levels of Motor Control
       1.   The segmental level is the lowest level on the motor control hierarchy and consists of the
            spinal cord circuits.
       2.   The projection level has direct control of the spinal cord.
       3.   The precommand level is made up of the cerebellum and the basal nuclei and is the highest
            level of the motor system hierarchy.

 I. The Reflex Arc (pp. 521522; Fig. 13.14)
   A. Reflexes are unlearned, rapid, predictable motor responses to a stimulus, and occur over highly
      specific neural pathways called reflex arcs (pp. 521522; Fig. 13.14).
II. Spinal Reflexes (pp. 522527; Figs. 13.1513.19)
   A. Spinal reflexes are somatic reflexes mediated by the spinal cord (pp. 522 527; Figs. 13.1513.19).
       1.   In the stretch reflex the muscle spindle is stretched and excited by either an external stretch or
            an internal stretch.
       2.   The Golgi tendon reflex produces muscle relaxation and lengthening in response to
       3.   The flexor, or withdrawal, reflex is initiated by a painful stimulus and causes automatic
            withdrawal of the threatened body part from the stimulus.
       4.   The crossed extensor reflex is a complex spinal reflex consisting of an ipsilateral withdrawal
            reflex and a contralateral extensor reflex.
       5.   Superficial reflexes are elicited by gentle cutaneous stimulation.
III. Developmental Aspects of the Peripheral Nervous System (p. 527)
   A. The spinal nerves branch from the developing spinal cord and
      adjacent neural crest and exit between the forming vertebrae. Each
      nerve becomes associated with the adjacent muscle mass.
   B. Cranial nerves innervate muscles of the head in a similar way.
   C. Sensory receptors atrophy to some degree with age, and there is a
      decrease in muscle tone in the face and neck; reflexes occur a bit
      more slowly.

Cross References
Additional information on topics covered in Chapter 13 can be found in the chapters
listed below.
1. Chapter 3: Membrane functions
2. Chapter 4: Nervous tissue
 3. Chapter 5: Cutaneous sensation and sensory receptors
 4. Chapter 9: Neuromuscular junction
 5. Chapter 11: Membrane potentials; neural integration; serial and parallel processing; synapses;
 6. Chapter 12: Ascending and descending tracts of the spinal cord; spinal roots; gray and white matter of
    the spinal cord
 7. Chapter 15: Sensory receptors for the special senses and generator potentials; cranial nerves associated
    with their special senses; reflex activity of the special senses
 8. Chapter 23: Reflex activity and control of digestive secretions; nerve plexuses involved in digestion;
    function of the vagus nerve in parasympathetic control
 9. Chapter 25: Spinal reflex control of micturition
10. Chapter 27: Spinal reflexes and the physiology of the sexual response

Laboratory Correlations
 1. Marieb, E. N. Human Anatomy & Physiology Laboratory Manual: Cat and Fetal Pig Versions. Eighth
    Edition Updates. Benjamin Cummings, 2006.
    Exercise 22: Human Reflex Physiology
 2. Marieb, E. N. Human Anatomy & Physiology Laboratory Manual: Main Version. Seventh Edition
    Update. Benjamin Cummings, 2006.
   Exercise 22: Human Reflex Physiology
Zuker, Charles S. "A Cool Ion Channel." Nature 416 (6876) (March 2002): 27 28.
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