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Spinal Cord

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general nervous system function
sensory information to information processing center to motor information transmitted to muscles then to muscles
embryonic spinal cord
develops from caudal neural tube; plates expand both dorsally and ventrally
alar plate neurons
become interneurons and sprout axons that become white matter
basal plate neurons
become motor neurons and sprout axons to effector organs
h shaped
center; gray matter; central canal; mostly cell bodies
gray matter
cell bodies
white matter
myelinated tracts
dorsal root ganglia
develop from neural crest cells; contain cell bodies from sensory neurons; receive sensory input; axons go to dorsal spinal cord; afferent fibers from sensory neurons form dorsal root and carry information here
spinal cord
extends from brain stem through foreman magnum to first or second lumbar vertebra; ends just inferior to the ribs; 42 inches long, 3/4 inch thick; elastic and flexible but delicate
protect spinal cord
bone, spinal fluid, meninges
meninges
pia mater, arachnoid, dura mater
dura mater
single, fibrous layer, unattached to bone
arachnoid
delicate, weblike structure
pia mater
contiguous with outer layer of spinal cord
between bone and dura
epidural space contains fat padding and veins
between arachnoid and pia
subarachnoid space contains cerebral spinal fluid
inferior cord
conus medullaris and filum terminale
conus medullaris
tapering cone shaped structure
filum terminale
fibrous extension of conus, covered with ; anchors spinal cord to coccyx-prevents jostling of cord
denticulate ligaments
dura and tooth like pia that secure the cord in place
epidural space
cord ends between L1 and L2; subarachnoid space within allows penetration by needles without damage to to cord
cauda equina
collection of nerve roots at the inferior end of spinal cord angle sharply downward and inferiorly
fetal development
vertebral column grows faster than spinal cord; lower nerve roots catch up to exit points
spinal nerves
31 pairs which all exit from the intravertebral foramen; travel to corresponding body region; define a spinal cord segment
cervical/lumbar enlargements
increased innervation to arms (cervical), increased innervation to legs (lumbar), function determines form
internal structure
slightly flattened from front to back; anterior- ventral median fissure; posterior- dorsal median sulcus(more shallow); partially divide spinal cord in half; connected by central gray commissure
dorsal horn
cell bodies of interneurons and projection neurons, receives efferent sensory information,sends sensory info to brain
dorsal root ganglia axons
may project to the dorsal horn interneurons or projection neurons; axons may go directly to tracts in white matter and then to the brain
ventral roots
motor neurons in this horn send their axons out through these roots that fuse to become the ventral roots of spinal nerves
gray matter organization
divided into 10 layers (laminae); each layer receives different information; I-VI and X are sensory; VII-IX generally are motor
4 zones of relative involvement of lamina
input from somatic sensory neurons; input from visceral sensory; visceral motor neurons; somatic motor neurons
white matter organization
bundles of myelinated and unmyelinated axons; ascending, descending and transversely; bundles divided on each side into white columns or funiculi (dorsal, lateral and ventral)
ascending
up cord or to brain (sensory)
descending
from brain or higher cord (motor)
transversely
from one side to the other (commissural fibers)
white matter tracts
multineuron pathways (peripheral neurons, spinal cord neurons, brain neurons); decussation, relay, somatotopy and symmetry in common
decussation
most paths cross from one side to the contralateral side; usually some axons also travel on the ipsilateral side; sensory tracts takes place at sensory input or at the medulla; motor tracts take place at the medulla
pyramidal tract
motor tract of the medulla
relays
most pathways have a chain of two or three neurons that relay information along the pathway or tract
somatotopy
a precise spatial relationship among tract fibers that demonstrates an orderly mapping of the body
receptor neurons
for extensors are ventral to receptor neurons for the flexors
motor information
to the appendages is lateral to motor information to the trunk
sensory information
from the arm enters lateral to that of the leg; carried through different fibers in the tract
cerebellum somatotopy
more complex
brain somatotopy
extremely specific
symmetry
all pathways and tracts are paired; one of the pair is on each side of the spinal cord or brain
first order neurons
ascending pathway; from periphery
second order neurons
ascending pathway; with cell bodies in dorsal horn or medulla that project to brain
third order neurons
ascending pathway; with cells in thalamus or elsewhere that project to other parts of the brain
dorsal column-medial lemniscus tract
ascending tract; discriminative touch, vibration and conscious proprioception; pathway formed by paired tracts of fascicles cuneatus and fascicles gracilis and the medial lemniscus; pathway decussates in medulla
spinothalamic tract
ascending tract; pain, temperature and coarse touch; primary afferent neurons synapse with second order projection neurons in dorsal horn; terminates in thalamus; decussates at the level of the first order neuron
spinocerebellar tract
ascending tract; muscle or tendon stretch from golgi tendon organs, muscle fibers and joint receptors to cerebellum, subconscious proprioception
medial lemniscus
begins in the medulla and ends in the thalamus; info from thalamus transmitted to the primary somatosensory cortex
ventral
spinocerebellar tracts; crossed fibers that cross back again, transmit information from trunk and legs to same side of cerebellum
dorsal
spinocerebellar tracts; fibers do not cross; transmit information from trunk and less to same side of body to cerebellum
direct system (pyramidal)
descending pathway; pyramidal and corticospinal tracts only; no synapses until the ventral horn; contacts and influences the extrapyramidal system; decussates in medulla; only synapse in neutral horn on lower motor neurons
indirect system (extrapyramidal)
descending pathway; all brain and brain stem motor nuclei pathways except pyramidal tract; multineuronal
motor pathways
consist of two neurons; upper motor and lower motor
upper motor neurons
pyramidal cells of the motor cortex; subcortical neurons that give rise to the descending motor tracts to the ventral horn; do not project to muscles; damage=spastic paralysis (multiple sclerosis)
lower motor neurons
ventral horn neurons in the spinal cord ventral horn that innervate skeletal muscle; damage=flaccid paralysis (polio)
direct system organization
originates from cells in the primary motor cortex; fibers become in the pyramidal tract in the brain stem; regulates fast, precise and skilled movements
indirect system organization
influence ventral horn neurons; receive input from the premotor areas and basal ganglia; not part of the corticospinal tract
rubrospinal tract
indirect pathway; begins in red nucleus of midbrain and ends in ventral horn; does not synapse with direct pathway; regulates muscle tone and control flexors
tectospinal tract
indirect pathway; begins in the superior colliculus of the midbrain and ends in the ventral horn of the cervical spine; helps head move during visual tracking; does not synapse with corticospinal tract
reflexes
inborn vs learned
inborn
intrinsic reflex; rapid predictable response to a stimulus, unconscious (maintained by brain stem and spinal cord); postural changes, avoiding pain, controlling visceral activities
learned
acquired reflex; rapid predictable response to a stimulus from practice or experience; driving a car and getting dressed in the morning
spinal reflexes processing
serial processing and parallel processing
serial processing
reflex arcs; the whole system works together to bring about a predictable response
parallel processing
allows conscious responses; input from reflex ascends and allows decision making
reflex arcs
highly specific neural pathways; have five components; somatic or autonomic reflexes
somatic reflexes
activate muscle
autonomic reflexes
activate visceral effectors (smooth muscle or glands)
receptor
site of stimulus
sensory neuron
transmits impulse
integration center
may be monosynaptic or polysynaptic; decision center
motor neuron
conducts efferent impulses
effector
muscle fiber or gland
myotatic reflex (stretch)
maintains muscles length set by brain; maintains muscle tone; all monosynaptic but have polysynaptic aspects; all ipsilateral
patellar reflex
keeps your knees from buckling; if knees buckle, quad lengthens with conscious thought
stretch reflex homeostasis
positive reflex response= sensory and motor components are intact; vigor of response indicates the level of excitability of spinal cord
hypoactive response
peripheral nerve damage or ventral horn injury
hyperactive response
lesions of upper motor neurons can reduce inhibition
spinal cord function homeostasis
receives sensory input and transmits to brain; processes some sensory information; receives motor info from brain and transmits to muscles; reflex center; reflex learning
spinal cord function
damage to spinal cord produces functional loss
cervical
7; head, neck, diaphragm, wrist extensors, triceps, hand
thoracic
12; chest, abdominal muscles
lumbar
5; legs
sacral
5; bowel, bladder, sexual function
spinal cord transection
complete loss of sensory and motor function below level of damage
spinal injury
spinal shock; transient loss of all function below injury (motor and visceral, bowel and bladder, reflexes)
cervical vertebra damage
quadriplegia
damage between T1 and L1
paraplegia
stroke
hemiplegia
polio
viral attack of ventral horn cells; irreversible flaccid paralysis and atrophy; deadly; preventable with oral vaccination
anencephaly
without brain; cerebrum and brain stem did not develop in utero; child is vegetative; muscles flaccid and no voluntary movement
spina bifida
forked spine; incomplete formation of vertebral arches (lumbosacral); maternal folic acid inadequacy diet

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