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Spinal Cord

Anatomy & Physiology

spinal cord provides a vital link between brain and rest of body, exhibits some functional independence from the brain
spinal cord and spinal nerves pathway for sensory and motor nerve signals and responsible for reflexes, quickest reactions to stimulus
spinal cord description extends inferiorly from brain through vertebral canal, ends at the level of L1 vertebra, has rootlets coming off that will form spinal nerves (interweaving nerves or nerve plexuses)
cauda equina nerve roots projecting inferiorly from spinal cord
nerve plexus group of nerves
number of pairs of spinal nerves 31
spinal nerves contain both sensory and motor axon, identified by first letter of spinal cord part and number
breakdown of spinal nerves 8 cervical (C1-C8), 12 thoracic nerves (T1-T12), 5 lumbar nerves (L1-L5), 5 sacral nerves (S1-S5), and 1 coccygeal nerve (Co)
protection of spinal cord encapsulated by spinal cord meninges
spinal cord meninges continuous with cranial meninges
structures encircle the spinal cord from outer to inner vertebra, epidural space, dura mater, subdural space, arachnoid mater, subarachnoid space, and pia mater
epidural space lies between dura mater and inner walls of the vertebra, has areolar connective tissue, blood vessels, and adipose tissue, and epidural anesthetics introduced here
dura mater deep to epidural space, provides stability to spinal cord, fuses with connective tissue surrounding nerves
subdural space potential space between dura mater and arachnoid mater, seen only in histologic preparations
subarachnoid space deep to arachnoid mater, space filled with cerebrospinal fluid
pia mater deep to subarachnoid space, innermost meningeal layer of elastic and collagen fibers, adheres to spinal cord and supports blood vessels
denticulate ligaments triangular extensions of spinal pia mater that attach to dura matter and help suspend and anchor spinal cord laterally
gray matter dendrites and cell bodies of neurons, glial cells, and unmyelinated axons in a shape that resembles a letter H
subdivisions of gray matter anterior horns, lateral horns, posterior horns and gray commissure
anterior horns left and right anterior masses of gray matter, house cell bodies of somatic motor neurons innervating skeletal muscles
lateral horns found in T1-L2 parts of spinal cord and contain cell bodies of autonomic motor neurons (innervate cardiac muscle, smooth muscle, glands
posterior horns left and right posterior masses of gray matter, axons of sensory neurons and cell bodies of interneurons
gray commissure horizontal band of gray matter surrounding central canal, contains primarily unmyelinated axons, communication route between left and right gray matter
nuclei within gray matter sensory nuclei, somatic sensory nuclei, visceral sensory nuclei, motor nuclei, somatic motor nuclei, and autonomic motor nuclei
sensory nuclei contain interneuron bodies in posterior horn
somatic sensory nuclei receive nerve signals from sensory receptors
visceral sensory nuclei receive nerve signals from blood vessels and viscera
motor nuclei contain motor neuron cell bodies in anterior and lateral horns
somatic motor nuclei innervate skeletal muscle in anterior horn
autonomic motor nuclei innervate smooth muscle, cardiac muscle, glands
white matter external to gray matter, composed of myelinated axons going to and from the brain
subdivision of white matter posterior funiculus, lateral funiculus, and anterior funiculus, each function with both motor and sensory axons (both ascending and descending tracts)
posterior funiculus between posterior gray horns and posterior medium sulcus
lateral funiculus on lateral side of spinal cord
anterior funiculus between anterior gray horns and anterior median fissure, interconnected by white commissure
conduction pathways characteristics travel through white matter of the spinal code, most decussate (cross over from one side to the other), composed of paired tracts
tract axon bundle
nucleus collection of neuron cell bodies
sensory pathways aka ascending pathways, signals from sensory receptors ascending to brain (carry info from proprioreception, touch, temperature, pressure, and pain and include somatosensory pathways and viscerosensory pathways)
motor pathways aka descending pathways, signals from brain to muscles or glands
contralateral relationship to opposite
ipsilateral relationship to same side
conduction pathways composed of series of two or three neurons sensory pathways with primary, secondary, and tertiary neurons; motor pathways with upper motor neuron and lower motor neuron, cell bodies located in nuclei
somatosensory pathways process stimuli from skin, muscles, and joints
viscerosensory pathways process stimuli from viscera
primary neuron first neuron in the chain of sensory pathways, dendrites part of receptor that detects stimulus, and axon projecting to secondary neuron within CNS
secondary neuron interneuron, within posterior horn of spinal cord or brainstem nucleus, projects to thalamus or cerebellum, and projects to tertiary neuron
tertiary neuron interneuron residing in thalamus, axon projects to primary somatosensory cortex
motor pathways descending pathways in the brain and spinal cord, may be formed from cerebral cortex, cerebral nuclei, cerebellum, descending projection tracts, or motor neurons
at least two motor neuron present in motor pathway upper motor neuron and lower motor neuron
motor pathway components innervation of limb and trunk muscles (neurons in anterior horn and nearby gray matter), innervation of head and neck muscles (neurons in motor nuclei of cranial nerves and reticular formation)
direct pathways responsible for conscious control of skeletal activity of motor pathways
indirect pathways responsible for subconscious or unconscious control of motor pathways
spinal cord characteristics formed from motor and sensory axons, enveloped in successive connective tissue wrappings
anterior cord (ventral root) of spinal cord formed from multiple anterior rootlets arising from spinal cord, contains motor axons only
posterior root (dorsal root) of spinal cord formed from posterior rootlets entering spinal cord, contains sensory axons only
anterior root and posterior root unite within intervertebral foramen and become spinal nerve
dermatomes specific segment of skin supplies by single spinal nerve (all spinal nerves, except C1), skin divided into segments into a dermatome map (i.e. horizonal skin around umbilicus [supplied by anterior ramus of T10]
dermatomes can indicate potential damage to one or more spinal nerves (i.e. loss of sensation on medial arm and forearm [C8 may be damaged]), involved in visceral pain (pain from one organ referred to a dermatome)
example of visceral pain appendix innervated by axons from T10, and referred visceral pain in T10... but appendix in T12
nerve plexus network of interweaving anterior rami of spinal nerves (form plexuses on both right and left sides)
main plexuses cervical plexuses, brachial plexuses, lumbar plexuses, and sacral plexuses
cervical plexuses located deep on each side of the neck, formed primarily by anterior rami of C1-C4 (C5 contributes some axons), innervates anterior neck muscles, skin of neck, and portions of head and shoulders)
brachial plexuses networks of nerves supplying upper limb and pectoral girdle, formed by anterior rami of C5-T1, extend laterally from neck, pass superiorly to first rib, continue into axilla
5 terminal branches of the brachial plexus axillary nerve (from posterior cord), median nerve (from medial and lateral cords), musculocutaneous nerve (from lateral cord), radial nerve (from posterior cord), ulnar nerve (from medial cord)
lumbar plexus formed from anterior rami of spinal nerves L1-L4, located along psoas major muscle, subdivided into anterior division and posterior division
nerves in the lumbar plexus femoral nerve and obturator nerve
femoral nerve supplies anterior thigh muscles and sartorius (knee extensor and hip flexors), sensory signal from anterior and inferomedial thigh and medial leg
obturator nerve supplies medial thigh muscles (adduct the thigh), sensory signals from superomedial skin of thigh
sacral plexus formed from anterior rami of spinal nerves L4-S4, immediately inferior to lumbar plexuses, innervate gluteal region, pelvis, perineum, posterior thigh, almost all leg and foot
anterior division of the sacral plexus tend to innervate muscles that flex part of lower limb
posterior division of the sacral plexus tend to innervate muscles that extend part of lower limb
sacral plexus nerves sciatic nerve, tibial nerve, common fibular nerve, deep fibular nerve, and superficial fibular nerve
sciatic nerve largest and longest nerve in body, projects through greater sciatic notch, extends into posterior thigh
tibial nerve innervates hamstrings, adductor magnus, plantar muscles of foot (extends thigh and flex legs), sensory signals from skin on sole of foot
common fibular nerve supplies short head of biceps femoris muscle (knee flexor), splits into deep fibular nerve and superficial fibular nerve
deep fibular nerve supplies anterior leg muscles and muscles on dorsum of foot (dorsiflex and extend the toes), sensation from skin between dorsum of first and second toes
superficial fibular nerve innervates lateral compartment muscles of leg (evertors and weakly plantar flexes), sensation from dorsal foot and anteroinferior leg
reflex properties rapid, pre-programmed, involuntary reactions of muscles or glands (i.e. touch burner of stove and automatically remove hand from stimulus), survival mechanism, enables quick response without waiting for brain to process, and requires stimulus
rapid response of reflex few neurons involved and minimal synaptic delay
pre-programmed response of reflex occurs same way every time
involuntary response of reflex requires no conscious intent, awareness occurring after completion of reflex act
reflex arc neural "wiring" of a single reflex, begins at a receptor in PNS, communicates with CNS, ends at a peripheral effector (muscle or gland), varying number of intermediate steps
1st 3 steps of reflex arc 1) stimulus activates a receptor (i.e. temperature, pressure) 2) nerve signal travels through sensory neuron to CNS (in spinal cord) 3) information from the nerve signal is processed by interneurons
last 2 steps of reflex arc 4) motor neuron transmits nerve signal to effector (transmits signal through ant root to gland or muscle) 5) effector responds to the nerve signal from the motor neuron (peripheral target organ responding to nerve signal, counteract original stimulus)
information from the nerve signal is processed by interneurons more complex reflexes have a number of interneurons, integrate sensory signals and transmit information to motor neurons, simplest reflexes without interneurons
reflex types monosynaptic and polysynaptic reflexes
monosynaptic reflexes simplest of all reflexes, sensory axons directly synapsing on motor neurons, very minor synaptic delay (i.e. patellar reflex)
polysynaptic reflexes more complex neural pathways, more prolonged synaptic delay before response (i.e. withdrawal reflex initiated by painful stimulus)
stretch reflex (aka deep tendon reflex or the muscle spindle reflex) monosynaptic reflex, regulates skeletal muscle length, reflexive muscle contraction after stretching of a muscle
muscle spindles located mainly in the muscle belly, found laying between and parallel with muscle fibers, more numerous in muscles controlling precise movements
responses of muscle spindles respond to both length (tonic response) and rate of change in length (phasic response)
sensory receptor (afferent) response to spindle stretching send impulse to CNS, stops sending feedback to CNS when the muscle fiber shortens (if lengthens too much or too quickly the feedback they send to the CNS results in involuntary muscle contraction and work reflexively, signals only travel to spinal cord
positive and negative effects of stretch reflex beneficial in preventing muscle injury, but can be detrimental during therapeutic stretching (i.e. initial 6 seconds of static stretch)
biceps reflex monosynaptic stretch reflex, muscles stretched with a tap, sensory neurons synapsing with alpha motor neuron, transmit signals to extrafusal muscle fibers, and initiate muscle contraction and flex elbow joint
reciprocal inhibition synapsing of sensory axons with interneurons, these synapsing with alpha motor neurons, inhibit antagonistic muscle contraction (i.e. inhibit triceps brachii muscle in biceps reflex), polysnaptic
Created by: Nicolekr



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