Anatomy & Physiology 34A
Chapter 13 – PNS: Spinal Nerves & Reflexes
I. Overview
A. Introduction to the PNS
B. Nerve roots & rami
C. Nerve Plexuses
1. Cervical Plexus
2. Brachial Plexus
3. Lumbar Plexus
4. Sacral Plexus
D. Reflexes
II. Introduction to the PNS
A. The PNS conveys impulses to and from the CNS and includes the:
1. Sensory receptors within sensory organs
2. Nerves & their associated ganglia
3. Nerve plexuses
B. Nerves of the PNS are classified as either
1. Cranial nerves - arise from the brain
2. Spinal nerves - arise from the spinal cord
C. The terms sensory, motor, or mixed nerve refer to the direction
that nerve impulses are conducted
1. Sensory (afferent) nerves conduct impulses toward the CNS
2. Motor (efferent) nerves conduct impulses away from the CNS
3. Mixed nerves are composed of both sensory & motor nerves,
thus conduct impulses to & from the CNS
III. Spinal Nerves
A. Each of 31 pr. of spinal nerves is formed by the union of a dorsal
and ventral spinal root that emerges from the SC through an
intervertebral foramen to innervate a part of the body
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B. Spinal nerves are named according to the vertebrae they emerge
through
1. 8 pr. cervical - C1 is between occipital bone & atlas; C2-C7
leave above their cervical bone, C8 and the rest leave below
namesake vertebrae
2. 12 pr. thoracic (T1-T12)
3. 5 pr. lumbar (L1-L5)
4. 5 pr. sacral (S1-S5)
5. 1 pr. coccygeal (Co)
C. A spinal nerve is a mixed nerve attached to spinal cord by a
1. Dorsal (posterior) root, which contains the dorsal root
ganglion - a collection of sensory cell bodies. Sensory neurons
carry impulses to the CNS
2. Ventral (anterior) root is composed of motor fibers that
convey impulses away from the CNS
D. After spinal nerves exit a root, they divide into peripheral branches
1. Dorsal ramus contains mixed nerves that innervate muscles,
joints, & skin of the posterior neck & trunk.
2. Ventral ramus contains mixed nerves that innervate the
muscles & skin of the anterior & lateral neck & trunk, as well as
the limbs.
3. Rami communicantes are two spinal nerve branches from the
ventral ramus to a sympathetic trunk ganglion, part of the
autonomic nervous system. Communicantes are mixed nerves.
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IV. Nerve Plexuses
A. Except for T2-T12 (intercostal nerves), the ventral rami of spinal
nerves split again & again as nerve networks called plexuses.
B. There are four plexuses; nerves emerging from the plexuses are
named according to the structures they innervate or the course they
take.
1. Cervical plexus - formed by the ventral rami of C1-C5
a. Branches innervate the skin & muscles of the neck, head, &
shoulders
b. Fibers from the 3rd-5th cervical nerves become the phrenic
nerve, which innervates the diaphragm
2. Brachial plexus - formed by C5-T1; passes over the 1st rib
behind the clavicle and enters the axilla; innervates the upper
extremities, and some shoulder & neck muscles. Major
sudivisions include:
a. Roots – five ventral rami (C5-T1), which unite to form three
b. Trunks – upper, middle, and lower; each trunk divides into
c. Divisions – anterior and posterior, then each of the 6
divisions merge to form three
d. Cords – posterior, medial, and lateral, which form the
following nerve branches
1) Axillary – from the posterior cord, posterior to the
humerus neck; innervates the deltoid and teres minor
muscles, skin and joint capsule of the shoulder
2) Radial – from the posterior cord, wraps around the
humerus in the radial groove; innervates posterior arm
skin, and the extensors of the arm, forearm and wrist
3) Median – from the medial cord, lies between the radial
and ulnar nerves; innervates anterior forearm skin and
flexor muscles
4) Ulnar (funny bone nerve) – from the medial cord,
descends along the medial humerus, over the medial
epicondyle, then along the medial ulna; innervates forearm
muscles not innervated by the median nerve
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5) Musculocutaneous – from the lateral cord, descends
along the anterior arm to just below the elbow; innervates
elbow flexors and lateral forearm skin
3. Lumbar plexus - formed by T12-L4; innervates lower
abdomen, anterior & medial lower extremity. Major nerves
include:
a. Femoral – courses near the femoral artery and vein;
innervates anterior & lateral thigh, medial leg & foot
b. Obturator – passes through the obturator foramen to the
medial thigh; innervates medial thigh, adductors of thigh
c. Saphenous – courses near the great saphenous vein;
innervates skin of the medial leg & foot and knee joint
4. Sacral plexus - formed by L4-S4; innervates lower back, pelvis,
perineum, posterior thigh & leg, & foot. Major nerves include:
a. Sciatic – largest nerve, passes through the greater sciatic
notch, then deep to the biceps femoris; innervates the
hamstrings, and branches into two nerves
1) Tibial – posterior to tibia; innervates knee flexors and foot
plantar flexors (branches into medial & lateral plantar
nerves)
2) Common peroneal (fibular) lateral to fibula; innervates
fibularis muscles and branches into
a) Superficial peroneal, which innervates fibularis muscles
b) Deep peroneal, which innervates tibialis anterior
(Note: Sciatica involves injury to the sciatic nerve, causing
pain to radiate from the buttock down the posterior leg; may
be caused by herniated disc, dislocated hip, lumbosacral
osteoarthritis, pregnancy, and gluteal muscle injection)
b. Pudental – innervates the perineal area (external genitals)
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V. Reflexes – fast, involuntary reactions of glands or muscles to
stimlation.
A. Reflexes may be somatic or autonomic
1. Somatic (spinal) reflexes (somatic NS) involve an involuntary
contraction of skeletal muscle in response to a stimulus
2. Visceral reflexes (autonomic NS) involve involuntary
contractions of smooth muscle, cardiac muscle, or glandular
secretion
B. Components of a reflex arc
1. Stimulus
2. Receptor - dendrite of a sensory neuron and the place the
impulse is initiated
3. Sensory neuron - relays the impulse through the dorsal root to
the CNS
4. Integration center - located in the CNS and usually involves an
association neuron (interneuron); here impulses are sent through
synapses to other parts of the body
5. Motor neuron - conducts the impulse from the CNS to an
6. Effector – a muscle or gland
7. Response
C. Reflexes may involve ipsilateral or contralateral pathways
1. In ipsilateral reflex arcs, the incoming sensory neuron and
outgoing motor neuron are on the same side of the spinal cord
2. In contralateral reflex arcs, the sensory and motor neurons
involved are on opposite sides of the spinal cord
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D. Some somatic reflexes include
1. Monosynaptic reflex arcs have one synapse between the sensory
neuron entering the SC and the motor neuron exiting the SC, and
are generally ipsilateral. Two monosynaptic myotactic (stretch)
reflexes are
a. Knee Jerk (patellar) reflex – tap on patellar ligament stimulates
proprioceptors, which stimulate sensory neuron, which synapses
with motor neuron, which stimulates quadriceps femoris muscles
to contract, extending the knee
b. Ankle jerk reflex – tap on Achilles tendon stimulates
proprioceptors, which stimulate a sensory neuron, which
synapses with a motor neuron, which stimulates gastrocnemius
and soleus to contract, plantar flexing the foot
c. In more complex myotactic tendon reflexes, sensory neurons
synapse with interneurons in ascending tracts that lead to the
cerebellum, then interneurons in descending tracts synapse with
motor neurons which stimulate the skeletal muscle
2. Flexor (withdrawal) reflex example: step on a tack stimulates
nociceptors, which stimulate a sensory neuron, which synapses
with several interneurons, which synapse with several motor
neurons (ipsilateral), which signal thigh and knee flexors to
contract
3. Crossed extensor reflex – in above example, the extensor muscles
of the opposite leg must be stimulated too, thus the incoming
sensory neuron also synapses with interneurons that cross to the
other side of the SC (contralateral) and synapse with motor
neurons to the opposite leg extensors
4. Reciprocal inhibition – when a muscle in a reflex is stimulated, its
antagonists are usually inhibited because the incoming sensory
neurons transmit IPSPs to interneurons, which inhibits the motor
neuron to the antagonistic muscle
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E. Several ipsilateral visceral reflexes are involved in eye focusing for
close vision
1. Accomodation reflex of the lenses – proprioceptors signal sensory
neuron, which synapses with parasympathetic motor neurons in
occulomotor nerve (CN III), which stimulate ciliary smooth
muscle to contract, causing lens to become rounder
2. Photopupil reflex – photoreceptors signal sensory neuron, which
synapses with parasympathetic motor neurons in CN III, which
stimulates smooth circular muscle of iris to contract, constricting
pupil
3. Convergence reflex – proprioceptors signal sensory neuron, which
synapses with motor neuron in trigeminal nerve (CN V), which
stimulate medial rectus muscles to move eyes medially