Anatomy Module 9

spinal cord

part of the CNS

functions: sensory and motor innervation of body, two-way conduction pathway for signals between body and brain, major center for reflexes

extends from foramen magnum to level of L1 or L2 (lumbar vertebrae)

31 pairs of spinal nerves (PNS): 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

cervical, lumbar enlargements for nerves supplying upper and lower limbs

spinal nerve and spinal cord organization

Cervical: most cervical spinal nerves emerge superior to their respective vertebrae. (C8 emerges inferior to C7)

Thoracic to coccygeal spinal nerves emerge inferior to respective vertebrae

inferior end of spinal cord = conus medullariss (ends at L1 in adult, L4 in kids)

bundle of nerve roots at inferior end of vertebral canal = cauda equina

long filament that extends from conus medullaris to coccyx and acts as a tether = filum terminale

spinal cord protection

spinal cord is protected by body, meninges and CSF.
Spinal cord has epidural space that is filled with fat and veins (not in CNS)

subdural space, arachnoid mater, subarachnoid space, and pia mater same as brain.

denticulate ligaments anchor spinal cord to dura mater

epidural nerve block

needle between l4 and l5 vertebrae permits injection of anesthesia into epidural space w/o risk of damage to spinal cord. the cauda equina gets “pushed” out of the way if necessary

spinal cord gray matter

gray matter forms “H” shape surrounded by white matter.

Grey commisure: unmyelinated axons crossing from on side to the other

dorsal/posterior arms of H are dorsal horns

ventral/anterior arms of H are ventral horns

spinal cord roots

dorsal horns receive information from sensory neurons. signals from sensory neurons reach spinal cord via dorsal roots.

ventral horns send out info to skeletal muscle. signals going out to motor neurons travel via ventral roots.

how roots form spinal nerves

dorsal and ventral roots merge to form a spinal nerve.

spinal nerve has both sensory and motor axons

exits vertebral column at intervertebral foramina

immediately splits into dorsal and ventral ramido

dorsal and ventral rami

ventral and dorsal rami carry both sensory and motor fibers

ventral rami innervate most of the body, including trunk and limbs. dorsal rami innervate small portion of the back

reflexes and the reflex arc

very simple neruon chain

basic structural plan of nervous system

reflexes usa=ually exclude brain; rapid, automatic, unlearned motor response to stimulus.

can be somatic (hot stove) or visceral (vomiting)

spinal comes in and synapses at spinal cord

motor neurons stimulate and inhibit as appropriate

reflex testing

patellar reflex, etc

used in clinical setting to check muscle groups and spinal nerves, spinal cord

dermatomes

area of skin supplied by a single spinal nerve

all spinal nerves have a dermatome except C1

why are they important?

numbness in the area can pinpoint spinal cord injuries

referred pain: when pain from an organ is mistakenly referred to dermatome (Ie, appendicitis starts as pain in T10 dermatome)

nerve plexuses

networks of nerved formed by ventral rami or spinal nerves only

all spinal nerves except T2-T12 branch and rejoin

each muscle in a limb receives its nerve supply from more than one spinal nerve, damage to one spinal nerve cannot completely paralyze any limb muscle

includes: cervical plexus, brachial plexus, lumbar plexus, sacral plexus

cervical plexus

ventral rami of first 4 cervical nerves

most branches are cutaneous sensory

some motor to anterior neck

phrenic nerve receives fibers from C3, C4, and C5; innervates diaphragm for respiration

brachial plexus

partly in neck, partly in axilla

gives rise to most nerves of the upper limb

mixing of ventral rami of C5-C8 and most of the ventral ramus of T1

main components: ventral rami, trunks, divisions, cords

brachial plexus

5 roots (C5, C6, C7, C8, T1) merge into 3 trunks (Upper - C5,C6; Middle - C7; Lower - C8, T1)

each trunk branches into 2 divisions (anterior and posterior)

anterior division composed of nerves that innervate anterior compartment muscles of upper limb and skin

posterior division composed of nerves that serve posterior compartment and skin

terminal branches: axillary nerve, radial nerve, ulnar nerve, median nerve, musculocutaneous nerve

musculocutaneous nerve

sensory innervation: skin sensation for lateral forearm

motor innervation: anterior compartment of arm

ulnar nerve

sensory innervation: skin of medial hand, medial side of digit 4, all of digit 5

motor innervation: flexors and intrinsic hand muscles (interossei, hypothenar mm.)

median nerve

sensory innervation: skin of lateral palm and digits 1-3 and lateral side of digit 4

motor innervation: anterior compartment of forearm (with ulnar) thenar muscles of palm (muscles that oppose the thumb)

carpal tunnel

median nerve lesion that affects motor nerve to thenar muscles.

pain/numbness in lateral palm

from: compression of nerve near hand

axillary nerve

sensory innervation: shoulder joint and skin overlying part of deltoid m.

motor innervation: teres minor and deltoid

radial nerve

sensory innervation: skin over dorso-lateral arm, forearm and hand

motor innervation: posterior compartment of arm and forearm (extensors)

lumbar plexus

L1-L4

iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous, femoral, obturator

femoral nerve

sensory innervation: skin of antero-medial thigh, skin of medial leg and foot

motor innervation: muscles of anterior thigh (thigh flexors, leg extensors)

obturator nerve

sensory innervation: skin of medial thigh

motor innervation: muscles of medial thigh (adductors)

sacral plexus

L4-S4

anterior: Tibial, pudendal, sciatic (sciatic feels like neither lowkey)

posterior: superior gluteal, lumbosacral trunk, inferior gluteal, common fibular, posterior femoral cutaneous

sciatic nerve

tibial nerve and common fibular nerve together in common sheath

longest and thickest nerve of the body. serves all of lower limb except anterior/medial thigh

tibial nerve

sensory innervation: skin of postero-laterla leg, skin of sole of foot (subdivides into plantar nerves)

motor innervation: muscles of posterior thigh, muscles of posterior leg

adductor magnus (with obturator n.) and biceps femoris (with common fibular n.)

common fibular nerve

sensory innervation: skin of antero-lateral leg, skin on dorsum of foot

motor innervation: muscles of anterior leg (deep fibular), muscles of lateral leg (superficial fibular)

common fibular nerve also innervates the biceps femoris (along with the tibial nerve)

Foot drop

caused by lesion of the common fibular/peroneal nerve

phrenic nerve lesion

innervates diaphragm

results in inability/difficult breathinga

axillary nerve lesion

innervates deltoid

results in diffuculty abducting arm to 90 degrees

musculocutaneous nerve lesion

innervates anterior arm

results in weakness flexing elbow

median nerve lesion

innervates anterior arm and thumb

results in weakness flexing wrist, difficulty opposing thumb, carpal tunnel syndrome

radial nerve lesion

innervates posterior arm and forearm

results in difficulty extending elbow, wrist drop

ulnar nerve lesion

innervates deep hand muscles

results in innability to adduct/abduct fingers, “funny bone” sensation

femoral nerve lesion

innervates anterior thigh muscles

walking problems, knee buckles

tibial nerve lesions

innervates posterior thigh and leg

results in shuffling gait, difficulty with plantar flexing

autonomic nervous system

the general visceral motor systme of the PNS

involuntary control of visceral functions - regulates smooth and cardiac msucle and glands

regulates heart rate, BP, breathing, body temp, other functions neccessary for maintaining homeostasis

bottom line: controls all the “routine” functions of physiological systems

sympathetic - fight or flight

parasympathetic - resting and digesting

somatic vs autonomic nervous systems

somatic:

one neuron, voluntary, sensory from skin, special senses, motor to skeletal muscle

autonomic:

two neurons (synapses in ganglia) , involuntary, visceral sensory from organs, viscerla motor to organs (viscera)

sympathetic nervous system

causes widespread, long-lasting mobilization of the fight or flight response

thoracolumbar: preganglionic neuron cell bodies in thoracice and lumbar spinal cord

effects: increase heart rate, respirartion, adrenaline, causes pupils to dilate, liver releases sugars (energy) into the blood, digestion slows, blood redirected to skeletal muscles and brain (results in cold skin)

sympathetic trunk

paravertabral ganglia

chain of ANS ganglia

lateral to vertebral column from neck to pelvis

connect to ventral rami of spinal nerves by rami communicans. White rami are laterla to gray rami.

~1 ganglia per spinal nerve (some fused, in total 22-24)

parasympathetic nervous system

rest and digest

effects are highly localized and shorter lived

“craniosacral” bc the preganglionic neurons are in brain and sacral region of spinal cord

effects: heart rate and respiration slows, blood flow to digestive system increases, pupils constricted

parasympathetic vs sympathetic

sympathetic (thoracolumnar): emergencies, exercise, excitement

parasympathetic (craniosacral): normal everyday functioning (house keeping)