The Peripheral Nervous System and Reﬂex Activity
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.
Te cerebellum and basal nuclei constitute the precommand
areas that subconsciously integrate mechanisms mediated by the
The Reﬂex Arc
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.
±esting of somatic spinal reﬂexes provides information on the
integrity of the reﬂex pathway and the degree of excitability of the
Somatic spinal reﬂexes include stretch, tendon, ﬂexor, crossed-
extensor, and superﬁcial reﬂexes.
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.
±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.
Flexor reﬂexes are initiated by painful stimuli. Tey are
polysynaptic, ipsilateral reﬂexes that are protective in nature.
Crossed-extensor reﬂexes consist of an ipsilateral ﬂexor reﬂex and
a contralateral extensor reﬂex.
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
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).
Reﬂexes slow down with age; this probably reﬂects neuronal loss
or sluggish CNS integration circuits.
Te cervical plexus (C
) innervates the muscles and skin of
the neck and shoulder. Its phrenic nerve serves the diaphragm.
Te brachial plexus serves the shoulder, some thorax muscles,
and the upper limb. It arises primarily from C
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.
Te lumbar plexus (L
) 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.
Te sacral plexus (L
) 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.
Dorsal rami serve the muscles and skin of the posterior body
ventral rami give rise to intercostal nerves that
serve the thorax wall and abdominal surface.
Joints are innervated by the same nerves that serve the muscles
acting at the joint. All spinal nerves except C
segments of the skin called dermatomes.
Motor Endings and Motor Activity
Peripheral Motor Endings
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.
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
Motor Integration: From Intention to Effect
Motor mechanisms operate at the level of the eﬀectors (muscle
ﬁbers), descending circuits, and control levels of motor behavior.
Te motor control hierarchy consists of the segmental level, the
projection level, and the precommand level.
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.
(Some questions have more than one correct answer. Select the best
answer or answers from the choices given.)
Te large onion-shaped receptors that are found deep in the
dermis and in subcutaneous tissue and that respond to deep
Proprioceptors include all of the following except
joint kinesthetic receptors.
Te aspect of sensory perception by which the cerebral cortex
identiﬁes the site or pattern of stimulation is
Te neural machinery of the spinal cord is at the
Dorsal root ganglia contain
cell bodies of somatic motor
axon terminals of somatic motor neurons,
cell bodies of autonomic motor neurons,
of sensory neurons,
cell bodies of sensory neurons.
Te connective tissue sheath that surrounds a fascicle of nerve
ﬁbers is the