PNS

31 pairs

How many pairs of spinal nerves are there (C1 to Co1)

Anterior (ventral) root and posterior (dorsal) root

Each nerve is formed from merger of what

Anterior root

Many axons of motor neurons who’s somas are in anterior and lateral horns

Posterior root

Many axons of sensory neurons whose somas are in posterior root ganglion

Part of spinal cord it comes from and number

What is each nerve named for

The vertebra of the same number (C2 nerve exits between C2 and C1 vertebra)

Cervical nerves exit intervertebral foramina superior to what

Inferior to the vertebra of the same number (T2 nerve exits between T2 and T3 vertebra)

Below C8, nerves exit where

C8 nerve, exits below C7 vertebra

Exception is what nerve and where does it exit

Lumbar, sacral, coccygeal

3 spinal nerves that have long roots that extend inferiorly before exiting vertebrae; for the cauda equina

After intervertebral foramen

Where does the spinal nerve split

Posterior ramus

Small branch that innervate muscles and skin of back

Anterior ramus

Large branch that splits into multiple other branches; at different levels it innervates anterior and lateral trunk, upper limb, lower limb; participates in plexuses

Rami communicates

Small branches of autonomic fibers; extend between spinal nerves and sympathetic trunk ganglion; ganglia interconnected in sympathetic trunk parallel to vertebral column

Dermatomes

Segments of skin supplied by single spinal nerves; can help localize damage to one or more spinal nerves; involved in referred visceral pain

Nerve plexus

Network of interweaving anterior rami of spinal nerves; four main plexuses; individual rami branch repeatedly

Cervical, brachial, lumbar, and sacral

Four main plexuses that occur bilaterally

Shingles

Reactivation of chickenpox infection; virus remaining latent in posterior root ganglia; reactivated, travels through sensory axons to dermatome; rash and blisters along the dermatome; burning and tingling pain; antiviral medication to reduce celerity; vaccine to prevent or reduce disease severity

Reflexes

Involuntary responses; a stimulus is required to initiate a reflex; response is rapid, involves a chain of only a few neurons; the response is preprogrammed, always the same; the response is involuntary, no intent or awareness of the reflex before it happens; a survival mechanism

Reflex arc

Neural pathway responsible for generating the response

Stretch reflex

Reflexive contraction of a muscle after it is stretched; stretch (or tendon tap) is detected by a muscle spindle receptor

Intrafusal muscle fibers; extrafusal muscle fibers

Spindle Contains what innervated by gamma motor neurons and wrapped by sensory neurons; fibers not within spindle are what innervated by large alpha motors

Spindles sensory axons fire impulses that are conducted to the spinal cord; in spinal cord, the sensory axon excited alpha motor neurons of the same muscle, causing contraction (monosynaptic)

What happens when muscle is stretched

Spinal, somatic, ipsilateral, innate

Stretch reflex classified as what

Golgi tendon reflex

Prevents muscles from contracting excessively; some excited interneurons inhibit motor neurons of same muscle; some excited interneurons excite motor neurons of antagonist muscle (reciprocal activation)

Golgi tendon organs

Detect excessive tension; proprioceptors with sensory ending at muscle tendon junction; their sensory axons excite interneurons in the spinal cord

Withdrawal reflex

Pulls a body part away from painful stimulus; stimulus excited nociceptor sensory neuron that transmits signal to spinal cord and excites interneurons; interneurons excite motor neurons neurons of flexors so flexor muscles contract and limb is withdrawn; simultaneously, other interneurons reciprocally inhibit motor neurons do extensors so that extensors muscles relax and withdrawal happens quickly

Cross-extensor reflex

Occurs in conjunction with withdrawal reflex; some interneurons excited by the nociceptor sensory neuron cross midline and excite extensor motor neurons on other side; allows the opposite side limb to support body weight while the hurt limb withdraws

Specific muscles, nerves, spinal segments

Reflexes can test function of what

Hypoactive reflex

Diminished or absent; may indicate damage to spinal cord, or muscle disease, or damage to neuromuscular junction

Hyperactive reflex

Abnormally strong response; may indicate damage to brain or spinal cord; especially if accompanies by clonus (rhythmic oscillating movements with reflex testing)