The Peripheral
Nervous System
and Reflex Activity
13
Suggested Lecture Outline
PART 1: SENSORY RECEPTORS AND SENSATION
I. Sensory Receptors (pp. 490–493; Figs. 13.1–13.4; Table 13.1)
A. Sensory receptors are specialized to respond to changes in their environment called stimuli (p. 490–493).
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.
C. Encapsulated Dendritic Endings (pp. 491–493; Table 13.1)
II.
1.
Meissner’s corpuscles are receptors for discriminatory and light touch in hairless areas of the body.
2.
Pacinian, or lamellated, corpuscles, are stimulated when deep pressure is first applied.
3.
Ruffini’s corpuscles respond to deep and continuous pressure.
4.
Muscle spindles detect when a muscle is being stretched and initiate a reflex that resists the stretch.
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.
Overview: From Sensation to Perception (pp. 493–497; 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 cortex.
PART 2: TRANSMISSION LINES: NERVES AND THEIR STRUCTURE AND REPAIR
I. Nerves and Associated Ganglia (pp. 497–499; Figs. 13.3–13.4)
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. 499–507; 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. 508–518; Figs. 13.6–13.12; 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.
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 dermatome.
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.
PART 3: MOTOR ENDINGS AND MOTOR ACTIVITY
I. Peripheral Motor Endings (pp. 518–520; Fig. 13.13)
A. Peripheral motor endings are the PNS element that activates effectors by releasing neurotransmitters.
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 acetylcholine.
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. 519–520;
Fig. 13.13)
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.
PART 4: REFLEX ACTIVITY
I. The Reflex Arc (pp. 520–526; Figs. 13.14–13.20)
A. Reflexes are unlearned, rapid, predictable motor responses to a stimulus, and occur over highly specific neural
pathways called reflex arcs (pp. 520–521; Fig. 13.14).
II.
Spinal Reflexes (pp. 521–526; Figs 13.15–13.20)
A. Spinal reflexes are somatic reflexes mediated by the spinal cord (pp. 521–526; Figs. 13.15–13.20).
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 contraction.
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. 526)
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.