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CHAPTER
13
The Peripheral Nervous System
and Reflex Activity
Objectives
1. Define peripheral nervous system and list its components.
Part 1: Sensory Receptors and Sensation
Sensory Receptors
2. Classify general sensory receptors by structure, stimulus detected, and body location.
Sensory Integration: From Sensation to Perception
3. Outline the events that lead to sensation and perception.
Part 2: Transmission Lines: Nerves and Their Structure and Repair
Nerves and Associated Ganglia
6. Define ganglion and indicate the general body location of ganglia.
Cranial Nerves
Spinal Nerves
10. Describe the general structure of a spinal nerve and the general distribution of its rami.
Part 3: Motor Endings and Motor Activity
Peripheral Motor Endings
12. Compare and contrast the motor endings of somatic and autonomic nerve fibers.
Motor Integration: From Intention to Effect
13. Outline the three levels of the motor hierarchy.
Part 4: Reflex Activity
The Reflex Arc
15. Name the components of a reflex arc and distinguish between autonomic and somatic
reflexes.
Spinal Reflexes
16. Compare and contrast stretch, flexor, crossed-extensor, and tendon reflexes.
Developmental Aspects of the Peripheral Nervous System
Suggested Lecture Outline
Part 1: Sensory Receptors and Sensation
I. Sensory Receptors (pp. 484–487; Table 13.1)
A. Sensory receptors are specialized to respond to changes in their environment called
stimuli (p. 484).
B. Receptors may be classified according to the type of stimulus (p. 484):
1. Mechanoreceptors are stimulated by mechanical force, such as touch, pressure,
vibration, and stretch.
2. Thermoreceptors respond to changes in temperature.
C. Receptors may be classified according to their location or location of stimulus
(pp. 484–485):
1. Exteroceptors are located at or near the body surface and detect stimuli arising from
3. Proprioreceptors are found within skeletal muscles, tendons, joints, ligaments, and
connective tissue coverings of bones and muscles and relay information concerning
body movements.
D. Receptors may be classified according to structural complexity (pp. 485–487; Table 13.1)
1. Simple receptors are general senses and may be nonencapsulated or encapsulated
dendritic endings.
II. Sensory Integration: From Sensation to Perception (pp. 487–490;
Figs. 13.2–13.3)
A. The somatosensory system, the part of the sensory system serving the body wall and
limbs, receives input from exteroreceptors, proprioreceptors, and interoreceptors
(p. 487; Fig. 13.2).
B. There are three main levels of neural integration in the somatosensory system: the
receptor level, circuit level, and perceptual level (pp. 487–490; Fig. 13.3).
1. Processing at the receptor level requires a stimulus to excite a receptor within its
receptive field, causing generation of graded potentials in order for sensation to occur.
2. Processing at the circuit level involves delivery of impulses along first-, second-, and
third-order neurons to the appropriate region of the cerebral cortex for stimulus local-
ization and perception.
3. Processing at the perceptual level involves several aspects:
a. Perceptual detection sums input from several receptors and is the simplest level of
perception.
particular sensation.
f. Pattern recognition is the ability to recognize a pattern in a complete scene.
4. The perception of pain protects the body from damage and is stimulated by extremes
of pressure and temperature, as well as chemicals released from damaged tissues.
5. The pain threshold is the stimulus intensity at which we begin to perceive pain and is
the same for most people, although pain tolerance is a genetically determined trait that
varies from person to person.
Part 2: Transmission Lines: Nerves and Their Structure and Repair
III. Nerves and Associated Ganglia (pp. 490–492; Figs. 13.4–13.5)
A. A nerve is a cordlike organ consisting of parallel bundles of peripheral axons enclosed by
connective tissue wrappings (p. 490; Fig. 13.4).
1. Each axon within a nerve is surrounded by a thin layer of loose connective tissue, the
endoneurium.
3. An epineurium bundles all fascicles into a nerve.
B. Peripheral nerves, either cranial or spinal, are classified according to the direction in
which they transmit impulses (p. 490).
C. Ganglia are collections of neuron cell bodies associated with nerves in the PNS
(pp. 490–491).
D. If damage to a PNS nerve fiber occurs and the cell body remains intact, cut or
compressed axons can regenerate; damaged CNS nerve fibers almost never regenerate
(pp. 491–492; Fig. 13.5).
IV. Cranial Nerves (pp. 492–501; Fig. 13.6; Table 13.2)
A. There are twelve pairs of cranial nerves that originate from the brain and, with the
exception of the vagus nerve, serve areas of the head and neck (pp. 492–501; Fig. 13.6;
Table 13.2).
1. Olfactory nerves (cranial nerve I) detect odors.
2. Optic nerves (cranial nerve II) are responsible for vision.
6. Vestibulocochlear nerves (cranial nerve VIII) are responsible for hearing and balance.
V. Spinal Nerves (pp. 501–511; Figs. 13.7–13.13; Tables 13.3–13.6)
A. Thirty-one pairs of mixed spinal nerves arise from the spinal cord and serve the entire
body except the head and neck (pp. 501–503; Figs. 13.7–13.8).
1. Each spinal nerve connects to the spinal cord by a ventral root, containing motor
fibers, and a dorsal root, containing sensory fibers.
B. Innervation of Specific Body Regions (pp. 503–511; Figs. 13.9–13.13; Tables 13.3–13.6)
1. Rami lie distal to and are lateral branches of the spinal nerves that carry both motor
and sensory fibers.
2. The back is innervated by the dorsal rami with each ramus innervating the muscle in
line with the point of origin from the spinal column.
to virtually all the nerves that innervate the upper limb.
6. The sacral and lumbar plexuses overlap and because many fibers of the lumbar plexus
contribute to the sacral plexus via the lumbosacral trunk, the two plexuses are often
referred to as the lumbosacral plexus.
7. The area of skin innervated by the cutaneous branches of a single spinal nerve is called
a dermatome.
Part 3: Motor Endings and Motor Activity
VI. Peripheral Motor Endings (p. 511)
A. Peripheral motor endings are the PNS element that activates effectors by releasing
neurotransmitters (p. 511).
VII. Motor Integration: From Intention to Effect (pp. 511–513; Fig. 13.14)
A. Levels of Motor Control (pp. 511–513; Fig. 13.14)
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 and acts on direct and
3. The precommand level is made up of the cerebellum and the basal nuclei and is the
highest level of the motor system hierarchy.
a. The cerebellum acts on motor pathways through projection areas of the brain stem,
and on the motor cortex via the thalamus.
Part 4: Reflex Activity
VIII. The Reflex Arc (p. 513; Fig. 13.15)
A. Reflexes are unlearned, rapid, predictable motor responses to a stimulus and occur over
highly specific neural pathways called reflex arcs (p. 513; Fig. 13.15).
1. Inborn, or intrinsic, reflexes are unlearned, unpremeditated, and involuntary.
2. Learned, or acquired, reflexes result from practice, or repetition.
autonomic, which activate visceral effectors (p. 513).
IX. Spinal Reflexes (pp. 513–519; Figs. 13.16–13.20)
A. Spinal reflexes are somatic reflexes mediated by the spinal cord (pp. 513–519;
Figs. 13.16–13.20).
2. The tendon reflex produces muscle relaxation and lengthening in response to
contraction.
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.
X. Developmental Aspects of the Peripheral Nervous System (p. 519)
A. The spinal nerves branch from the developing spinal cord and adjacent neural crest and
exit between the forming vertebrae (p. 519).
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
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
Lecture Hints
1. Emphasize the distinction between the central and peripheral nervous system, but stress
2. Many students will have difficulty with the difference between receptor potentials,
3. As the anatomy of the nerve is discussed, point out the similarity between the basic
4. Students often have problems with neuron regeneration and myelination (i.e., understand-
5. Using a model of the spinal cord, describe the pathway of a spinal reflex. Emphasize the
origin of the reflex with a stimulus at sensory receptors, the relay into the spinal cord
6. Emphasize to students that the CNS is receiving sensory input from many sources
simultaneously, all of which may be distilled to a single, specific motor output.
Activities/Demonstrations
2. Select a student to help in the illustration of reflexes such as patellar, plantar, and
abdominal.
4. Obtain a sheep brain with the cranial nerves intact to illustrate their locations.
6. Obtain a 3-D model of a spinal cord cross section to illustrate the five components of a
reflex arc and to illustrate terms such as ipsilateral, contralateral, and monosynaptic.
Critical Thinking/Discussion Topics
1. How can the injection of novocaine into one area of the lower jaw anesthetize one entire
side of the jaw and tongue?
2. How can seat belts for both the front and back seat passengers of a car prevent serious
neurological damage? How can using only lap belts cause severe damage?
Library Research Topics
1. How does acupuncture relate to the distribution of spinal nerves?
2. Will all victims of polio suffer paralysis? What different forms are there?
List of Figures and Tables
All of the figures in the main text are available in JPEG format, PPT, and labeled & unlabeled
format on the Instructor Resource DVD. All of the figures and tables will also be available in
Transparency Acetate format. For more information, go to www.pearsonhighered.com/educator.
Figure 13.1 Place of the PNS in the structural organization of the nervous system.
Figure 13.2 Three basic levels of neural integration in sensory systems.
Figure 13.8 Formation of spinal nerves and rami distribution.
Figure 13.9 The cervical plexus.
Figure 13.10 The brachial plexus.
Figure 13.11 The lumbar plexus.
Figure 13.12 The sacral plexus.
Figure 13.13 Map of dermatomes.
Figure 13.19 The tendon reflex.
Figure 13.20 The crossed-extensor reflex.
Table 13.1 General Sensory Receptors Classified by Structure and Function
Answers to End-of-Chapter Questions
Multiple-Choice and Matching Question answers appear in Appendix H of the main text.
Short Answer Essay Questions
13. The PNS includes all nervous tissue outside the CNS, and consists of the sensory recep-
14. Sensation is simply the awareness of a stimulus, whereas perception also understands the
meaning of the stimulus. (p. 484)
15. a. Central pattern generators (CPGs) control often repeated locomotion and motor
16. For a diagram of the hierarchy of motor control, see Figure 13.14. (p. 512)
17. The cerebellum is called a precommand area because it integrates inputs from all ascend-
18. In the PNS, damaged fibers can be replaced or repaired by physical and chemical proc-
19. See pp. 501–502.
a. Spinal nerves form from dorsal and ventral roots that unite distal to the dorsal root
20. a. A plexus is a branching nerve network formed by roots from several spinal nerves that
ensures that any damage to one nerve root will not result in total loss of innervation to
21. Ipsilateral reflexes involve a reflex initiated on and affecting the same side of the body;
contralateral reflexes involve a reflex that is initiated on one side of the body and affects
the other side. (p. 518)
22. The flexor, or withdrawal, reflex is a protective mechanism to withdraw from a painful
stimulus, leading to a loss of pain. (p. 518)
23. Flexor reflexes are protective ipsilateral and polysynaptic reflexes that are designed to
24. The sensory input of a crossed-extensor reflex illustrates parallel processing, an ipsilateral
25. Reflex tests assess the condition of the nervous system. Exaggerated, distorted, or absent
26. Dermatomes are related to the sensory innervation regions of the spinal nerves. The
spinal nerves correlate with the segmented body plan, as do the muscles (at least embry-
ologically). (pp. 509–510)
Critical Thinking and Clinical Application Questions
1. Precise realignment of cut, regenerated axons with their former effector targets is highly
2. Damage to his common fibular nerve would result in problems dorsiflexing his right foot,
3. Damage to the brachial plexus occurred when he suddenly stopped his fall by grabbing
the branch. (p. 505)
4. The left trochlear nerve (IV), which innervates the superior oblique muscle responsible
for this action. (p. 495)
6. The specific ascending pathways of the fasciculus cuneatus carry discriminative touch
7. Referred pain occurs because visceral and somatic pain fibers travel along the same neu-
ral pathways. Mr. Jake felt pain in his left arm because the heart, located on the left side
of the thoracic cavity, would share pathways with the left arm. (p. 490)
Suggested Readings
Cox, James J., et al. “An SCN9A Channelopathy Causes Congential Inability to Experience
Pain.” Nature 444 (7121) (Dec. 2006): 894–898.
Foell, J., et al. “Phantom Limb Pain After Lower Limb Trauma: Origins and Treatments.”
The International Journal of Lower Extremity Wounds 10 (4) (Dec. 2011): 224–235.
Julius, D., and A. I. Basbaum. “Molecular Mechanisms of Nociception.” Nature 413 (6852)
(Sept. 2001): 203–210.
Konstantinos, Meletis, et al. “Spinal Cord Injury Reveals Multilineage Differentiation of
Ependymal Cells.” PLoS Biology 6 (7) (July 2008).
Kwon, B. K., C. G. Fisher, M. F. Dvorak, and W. Tetzlaff. “Strategies to Promote Neural
Repair and Regeneration After Spinal Cord Injury.” Spine 30 (17) (Sept. 2005): S3–S13.
