KIN 336 - Peripheral Nerves

spinal nerves

• 31 pairs

• all are mixed nerves (motor and sensory) named for where they come out of the spinal cord

• supply all body parts except head

How many pairs of nerves in each section?

• cervical: 8

• thoracic: 12

• lumbar: 5

• sacral: 5

• coccyx: 1

Why do 7 cervical vertebrae give rise to 8 pairs of cervical spinal nerves?

• each of the first 7 pairs (C1 to C7) exits the vertebral canal superior to vertebra for which it is named

• last spinal nerve (C8) exits canal inferior to C7

roots

• formed by merging rootlets from spinal cord

• types: dorsal and ventral

dorsal root

sensory fibers (cell bodies in dorsal root ganglion)

ventral root

motor fibers

spinal nerve

when roots unite (mixed sensory and motor)

rami

when spinal nerve splits (branches, mixed sensory and motor)

dorsal rami

supply posterior trunk (skin & intrinsic/deep back muscles)

ventral rami

form intercostal nerves (thorax) and major plexuses (cervical, brachial, lumbar, sacral)

meningeal branch

re-enters vertebral canal → innervates meninges, blood vessels

rami communicantes

autonomic fibers (sympathetic)

dermatomes

• area of skin innervated by cutaneous branches of single spinal nerve

• all spinal nerves except C1 participate

• affected units ascertain extent of spinal cord injuries

• most overlap, so destruction of a single nerve will not cause complete numbness

plexus

• network of nerves that contains motor and sensory neurons from the spinal cord

• multiple nerves from different spinal cord levels join together (intermingling)

• provides backup or redundancy in case one nerve is damaged

• not all nerves come from these, but some do

cervical plexus

• formed by ventral rami of C1-C5

• innervates skin/muscles of neck, ear, back of head, shoulders

hiccups aka spasms

caused by irritation of phrenic nerve

respiratory arrest

• caused by injury to phrenic nerves or C3-C5 spinal cord that paralyzes diaphragm

  • survival requires mechanical respirator that forces air into lungs

stingers

a temporary symptom of nerve damage that involves shooting pains down arm or hand caused by damage to cervical plexus

brachial plexus

• formed by ventral rami of C5-C8 and T1 (and often C3 and/or T2)

• supplies shoulder, arm, forearm, and hand

organization (proximal → distal) of brachial plexus

• roots: C5-T1 ventral rami

• trunks: upper (C6), middle (C7), lower (C8-T1)

divisions of brachial plexus

• anterior (flexors)

• posterior (extensors)

cords of brachial plexus

• named by relation to axillary artery 

  • lateral

  • medial

  • posterior

terminal branches (major nerves) of brachial plexus

• axillary

• musculocutaneous

• radial

• median

• ulnar

axillary terminal branch controls

• deltoid

• teres minor

• shoulder sensation

musculocutaneous terminal branch controls

• flexor arm muscles

• lateral forearm sensation

radial terminal branch controls

• extensors of arm/forearm

• posterior limb sensation

median terminal branch controls

• most forearm flexors

• hand intrinsics

• lateral palm

ulnar terminal branch controls

• intrinsic hand muscles

• medial hand

median nerve anatomy

• runs through carpal tunnel

• supplies forearm flexors, thenar muscles, lateral palm (digits 1-3, half of 4)

motor loss in median nerve injury

weak forearm flexion, impaired pincer grasp (thumb-index opposition)

sensory loss in median nerve injury

numbness in thumb, index, middle, and lateral half of ring finger

classic signs of median nerve injury

• hand of benediction (when making fist)

• ape hand (thenar atrophy, thumb pulled into plane of palm)

carpal tunnel syndrome

• compression of median nerve → pain, numbness, weakness

• seen in wrist lacerations

lumbar plexus

• L1-L4

• formed by anterior/ventral rami (divisions) of the lumbar spinal nerves

• innervates thigh, abdominal wall, psoas muscle

lumbar plexus major nerves

• femoral

• obturator

femoral nerve

runs anteriorly down front leg to innervate quadriceps, skin of anterior thigh, medial leg

obturator nerve

goes medially to innervate adductors of thigh (passes through obturator foramen)

sacral plexus

• L4-S4

• innervates buttock, lower limb, pelvis, perineum

• lies anterior to piriformis muscle, on the surface of the posterior pelvic wall

• gives rise to sciatic nerve

sciatic nerve

• L4-S3

• longest, thickest nerve in body

• innervates hamstrings, adductor magnus, most of leg and foot

• 2 components: tibial and common fibular

sciatica

• stabbing pain radiating along nerve

• caused by: falls, disc herniation, trauma, poor injection placement

• severe injury → paralysis of hamstrings & foot drop

• rehab and weight-bearing activity help prevent disc injury and promote nerve recovery

intercostal nerves

• T1-T11

• somatic nerves arising from anterior division (ventral rami) of the thoracic spinal nerves from T1-T11

• supply intercostal muscles, thoracic wall, abdominal wall, plus cutaneous branches to skin along course

• also innervate pleura and peritoneum 

central pattern generators (CPGs)

• spinal cord circuits producing rhythmic motor patterns

• networks of excitatory and inhibitory interneurons

• drive automatic behaviors: locomotion, breathing chewing

• modulated by descending input, but can function w/o it

• integrate sensory feedback (spindles, GTOs) to adjust rhythm