Chapter 13
The Peripheral Nervous System and Reflex Activity
521
13
4.
Te projection level consists of descending fibers that project
to and control the segmental level. Tese fibers issue from the
brain stem motor areas (indirect system) and cortical motor areas
[direct (pyramidal) system]. Neurons in the brain stem appear to
turn CPGs on and off, or to modulate them.
5.
Te cerebellum and basal nuclei constitute the precommand
areas that subconsciously integrate mechanisms mediated by the
projection level.
PART 4
Reflex Activity
The Reflex Arc
(p. 513)
1.
A reflex is a rapid, involuntary motor response to a stimulus. Te
reflex arc has five elements: receptor, sensory neuron, integration
center, motor neuron, and effector.
Spinal Reflexes
(pp. 513–519)
1.
±esting of somatic spinal reflexes provides information on the
integrity of the reflex pathway and the degree of excitability of the
spinal cord.
2.
Somatic spinal reflexes include stretch, tendon, flexor, crossed-
extensor, and superficial reflexes.
3.
A stretch reflex, initiated by stretching of muscle spindles, causes
contraction of the stimulated muscle and inhibits its antagonist. It
is monosynaptic and ipsilateral. Stretch reflexes maintain muscle
tone and body posture.
4.
±endon reflexes, initiated by stimulation of tendon organs by
excessive muscle tension, are polysynaptic reflexes. Tey cause
relaxation of the stimulated muscle and contraction of its
antagonist to prevent muscle and tendon damage.
5.
Flexor reflexes are initiated by painful stimuli. Tey are
polysynaptic, ipsilateral reflexes that are protective in nature.
6.
Crossed-extensor reflexes consist of an ipsilateral flexor reflex and
a contralateral extensor reflex.
7.
Superficial reflexes (plantar and abdominal reflexes) are elicited
by cutaneous stimulation. Tey require functional cord reflex arcs
and corticospinal pathways.
Developmental Aspects of the Peripheral Nervous
System
(p. 519)
1.
Each spinal nerve provides the sensory and motor supply of an
adjacent muscle mass (destined to become skeletal muscles) and
the cutaneous supply of a dermatome (skin segment).
2.
Reflexes slow down with age; this probably reflects neuronal loss
or sluggish CNS integration circuits.
5.
Te cervical plexus (C
1
–C
4
) innervates the muscles and skin of
the neck and shoulder. Its phrenic nerve serves the diaphragm.
6.
Te brachial plexus serves the shoulder, some thorax muscles,
and the upper limb. It arises primarily from C
5
–±
1
. Proximal
to distal, the brachial plexus has roots, trunks, divisions, and
cords. Te main nerves arising from the cords are the axillary,
musculocutaneous, median, radial, and ulnar nerves.
7.
Te lumbar plexus (L
1
–L
4
) provides the motor supply to the
anterior and medial thigh muscles and the cutaneous supply
to the anterior thigh and part of the leg. Its chief nerves are the
femoral and obturator.
8.
Te sacral plexus (L
4
–S
4
) supplies the posterior muscles and skin
of the lower limb. Its principal nerve is the large sciatic nerve
composed of the tibial and common fibular nerves.
9.
Dorsal rami serve the muscles and skin of the posterior body
trunk. ±
1
–±
12
ventral rami give rise to intercostal nerves that
serve the thorax wall and abdominal surface.
10.
Joints are innervated by the same nerves that serve the muscles
acting at the joint. All spinal nerves except C
1
innervate specific
segments of the skin called dermatomes.
PART 3
Motor Endings and Motor Activity
Peripheral Motor Endings
(p. 511)
1.
Motor endings of somatic nerve fibers (axon terminals) form
neuromuscular junctions with skeletal muscle cells. Axon
terminals contain synaptic vesicles filled with acetylcholine,
which (when released) signals the muscle cell to contract. An
elaborate basal lamina fills the synaptic cle².
2.
Autonomic motor endings, called varicosities, are functionally
similar, but structurally simpler, beaded terminals that innervate
smooth muscle and glands. Tey do not form specialized
neuromuscular junctions and the motor responses elicited are
generally slower.
Motor Integration: From Intention to Effect
(pp. 511–513)
1.
Motor mechanisms operate at the level of the effectors (muscle
fibers), descending circuits, and control levels of motor behavior.
2.
Te motor control hierarchy consists of the segmental level, the
projection level, and the precommand level.
3.
Te segmental level is the spinal cord circuitry that activates
ventral horn motor neurons to stimulate the muscles. It consists
of reflexes and central pattern generators (CPGs), segmental
circuits controlling locomotion.
Multiple Choice/Matching
(Some questions have more than one correct answer. Select the best
answer or answers from the choices given.)
1.
Te large onion-shaped receptors that are found deep in the
dermis and in subcutaneous tissue and that respond to deep
pressure are
(a)
tactile discs,
(b)
lamellar corpuscles,
(c)
free
nerve endings,
(d)
muscle spindles.
2.
Proprioceptors include all of the following except
(a)
muscle spindles,
(b)
tendon organs,
(c)
tactile discs,
(d)
joint kinesthetic receptors.
3.
Te aspect of sensory perception by which the cerebral cortex
identifies the site or pattern of stimulation is
(a)
perceptual
detection,
(b)
feature abstraction,
(c)
pattern recognition,
(d)
spatial discrimination.
4.
Te neural machinery of the spinal cord is at the
(a)
precommand
level,
(b)
projection level,
(c)
segmental level.
5.
Dorsal root ganglia contain
(a)
cell bodies of somatic motor
neurons,
(b)
axon terminals of somatic motor neurons,
(c)
cell bodies of autonomic motor neurons,
(d)
axon terminals
of sensory neurons,
(e)
cell bodies of sensory neurons.
6.
Te connective tissue sheath that surrounds a fascicle of nerve
fibers is the
(a)
epineurium,
(b)
endoneurium,
(c)
perineurium,
(d)
epimysium.
Review Questions
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