Peripheral Nerves and Reflexes

Spinal Nerves

review the functional anatomy of the spinal cord

• 31 pairs of mixed nerves arise from spinal cord to supply all parts of the body except head

• named according to their point of issue

8 cervical (C1-C8)

12 thoracic (T1-T12)

5 Lumbar (L1-L5)

5 Sacral (S1-S5)

1 Coccygeal (Co)

Spinal Nerves — Roots

• each spinal nerve connects to the spinal cord via two medial roots

• each root forms a series of rootlets that attach to spinal cord

ventral roots arise from the anterior horn and contain motor (efferent) fibers

dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers

Spinal Nerves — Rami

• short spinal nerves branch into three or four mixed, distal rami:

small dorsal ramus

larger ventral ramus

tiny meningeal branch

rami communicantes at base of ventral rami in thoracic region

Nerve Plexuses

• all ventral rami, except T2–T12, form interlacing nerve networks

• plexuses are found in cervical, brachial, lumbar, and sacral regions

each resulting branch of a plexus contains fibers from several spinal nerves

fibers travel to the periphery via several different routes

• each muscle receives a nerve supply from more than one spinal nerve

• therefore, damage to one spinal segment cannot completely paralyze a muscle

Spinal Nerve Innervation — Back, Anterolateral Thorax, & Abdominal Wall

• back is innervated by dorsal rami via several branches

• thorax is innervated by ventral rami T1–T12 as intercostal nerves

intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall

Cervical Plexus

• formed by ventral rami of C1–C4

most branches are cutaneous nerves of the neck, ear, back of head, and shoulders

most important nerve of this plexus is the phrenic nerve

major motor and sensory nerve of the diaphragm

Brachial Plexus

• formed by C5–C8 and T1 (C4 and T2 may also contribute to this plexus)

• gives rise to nerves that innervate upper limb

• major branchings:

roots — five ventral rami (C5–T1)

trunks — three, upper, middle, and lower, which form divisions

divisions — six, an anterior and posterior from each trunk; serve the front and back of the limb

cords — three, lateral, medial, and posterior; from the divisions

Brachial plexus — nerves

axillary n — innervates deltoid and teres minor mm

musculocutaneous n — sends fibers to the biceps brachii and brachialis mm

median n — branches to most of the flexor muscles of forearm

ulnar n — supplies the flexor carpi ulnaris and part of the flexor digitorum profundus mm

radial n — innervates essentially all extensor muscles of arm and forearm

Diagram of the brachial plexus
diagram of the brachial plexus

© Mikael Häggström, used under the terms of the GNU Free Documentation License, version 1.2 or any later version

Lumbar Plexus

• arises from L1–L4 and innervates the thigh, abdominal wall, and psoas muscle

• major nerves are femoral n and obturator n

Sacral Plexus

• arises from L4–S4 and serves the buttock, lower limb, pelvic structures, and the perineum

• major nerve is the sciatic n, the longest and thickest nerve of the body

composed of two nerves:

tibial n

common fibular (peroneal) n

Innervation of Joints

• Hilton’s law — a nerve providing motor signals to a muscle that produces movement at a joint also innervates the joint itself and skin over the joint


• dermatome — area of skin innervated by cutaneous branches of a single spinal nerve

all spinal nerves except C1 participate in dermatomes


• reflex — rapid, predictable motor response to a stimulus

may be inborn or learned (acquired)

may involve only peripheral nerves and the spinal cord

may involve higher brain centers as well

Reflex Arc

• components of a reflex arc:

receptor — site of stimulus

reflex arc

sensory neuron — transmits the afferent impulse to the CNS

integration center — either monosynaptic or polysynaptic region within the CNS

motor neuron — conducts efferent impulses from the integration center to an effector

effector — muscle fiber or gland that responds to the efferent impulse

Stretch and Deep Tendon Reflexes

• for skeletal muscles to perform normally:

Golgi tendon organs (proprioceptors) must constantly inform the brain as to the state of the muscle

stretch reflexes initiated by muscle spindles must maintain healthy muscle tone

Muscle spindles

composed of 3–10 intrafusal muscle fibers that lack myofilaments in their central regions, are noncontractile, and serve as receptive surfaces

histology of the spindle apparatus

muscle spindle apparatus

wrapped with two types of afferent endings:

primary sensory endings of type Ia fibers

secondary sensory endings of type II fibers

are innervated by gamma (γ) efferent fibers

Note:  contractile muscle fibers are extrafusal fibers and are innervated by alpha (α) efferent fibers

Operation of the muscle spindles

stretching muscles activates muscle spindle

there is an increased rate of action potential in Ia fibers

contracting the muscle reduces tension on muscle spindle

there is a decreased rate of action potential on Ia fibers

Stretch Reflex

• stretching muscle activates muscle spindle

excited γ-motor neurons of the spindle cause stretched muscle to contract

afferent impulses from spindle result in inhibition of antagonist

• Example:  patellar reflex

tapping patellar tendon stretches the quadriceps and starts the reflex action

quadriceps m contracts and antagonistic hamstrings relax

Deep Tendon Reflex

• opposite of stretch reflex

contracting the muscle activates Golgi tendon organs

afferent Golgi tendon neurons are stimulated, neurons inhibit contracting muscle, and antagonistic muscle is activated

as a result, contracting muscle relaxes and antagonist contracts

Flexor and Crossed Extensor Reflexes

• flexor reflex is initiated by a painful stimulus (actual or perceived) that causes automatic withdrawal of the threatened body part

• crossed extensor reflex has two parts:

stimulated side is withdrawn

contralateral side is extended

Superficial Reflexes

• initiated by gentle cutaneous stimulation

• example:

plantar reflex — initiated by stimulating lateral aspect of sole

response is downward flexion of toes

indirectly tests for proper corticospinal tract functioning

spontaneous Babinski in a newborn infant

abnormal Babinski sign

Babinski’s sign:  abnormal plantar reflex indicating corticospinal damage where great toe dorsiflexes and smaller toes fan laterally

Developmental Aspects of the PNS

• spinal nerves branch from developing spinal cord and neural crest cells

supply motor and sensory function to developing muscles

• cranial nerves innervate muscles of the head

• distribution and growth of spinal nerves correlate with segmented body plan (4th week)

• sensory receptors atrophy with age, and muscle tone lessens

• peripheral nerves remain viable throughout life unless subjected to trauma

Table of the Cranial Nerves

Number Name
0 nervus terminalis
I olfactory n
II optic n
III oculomotor n
IV trochlear n
V trigeminal n
VI abducens n
VII facial n
VIII vestibulocochlear n
IX glossopharyngeal n
X vagus n
XI spinal accessory n
XII hypoglossal n

Questions for thought
1.   What is the functional relationship of the peripheral nervous system to the central nervous system?
2.   Describe the formation and composition of a spinal nerve.
3.   Diagram a spinal nerve and its branches, labelling each component.
4.   What is meant by a plexus?
5.   List the four major plexuses arising from spinal nerves, and indicate the general body regions supplied by each.
6.   Compare and contrast flexor and crossed extensor reflexes.
7.   While watching the Olympic weight-lifting trials, you notice that on several occasions a competitor lifts a weight to the chest and then suddenly drops it. Using your knowledge of reflexes, explain what is happening.
Other questions to test your knowledge