Spinal Cord 2 Flashcards
where are the motornuerons found?
in the ventral horn
2 kinds of motorneurons
- alpha
- gamma
alpha motorneurons
- go to skeletal muscle
- somatic efferent fibers
- go to extrafusal fibers
- end as motor end plate
gamma motorneurons
- go to intrafusal fibers (muscle spindle)
- send sensory info to tell how contracted the muscle is
distribution of motorneurons
- shoulder muscles: upper cervical region
- hand muscles: low cervical/upper thoracic
*what’s the point? explains our segmental arrangement
what is a similarity of the motorneurons in the ventral horn to the dosral horn?
both are arranged somatitopically
intermediolateral column contains what?
visceral motor nuerons
lateral motor column supplies:
muscles of the limb
medial motor column supplies:
axial muscles
describe the convergence of afferents to a ventral horn motorneuron
- 4 descending tracts come down to converge on the neuron (corticospinal, reticulospinal, vestibulospinal, rubrospinal and tectospinal)
- an inhibitor and excitatory spinal reflex converges on it
- the motorneuron axon extends as the final common pathway to a muscle
lower motor neurons =
alpha motor neurons - arise from anterior horn cells and project to skeletal muscle
upper motor neurons =
arise in the CNS, stay in CNS, and descend to act on lower motor neurons
what would happen if there were a lesion on a lower motor neuron? and why?
- hyporeflexive
- reflex arc is no longer in tact
what would happen if there were a lesion on a upper motor neuron? and why?
- hyperreflexive
- reflex arc still intact
- think of McNeils example of the women w/ massive stroke that kicked him in the head
what are the afferent fibers that enter the dorsal root?
- group A fibers
- C fibers
group A fibers
- large and myelinated
- come into dorsal column w/o synapsing and ascend
- FAST
what do group A fibers carry?
- proprioception
- discriminative touch
- vibration
- 2 point sensory info
C fibers
- tiny unmyelinated fibers
- from spinothalamic tract
- slower
what do C fibers carry?
signals related to . . .
- pain
- temp
- poorly localized touch
how is the group A fibers / C fibers interaction significant in real life?
- when you hurt yourself, there is a second where you know what happened but it doesn’t hurt, you just know the pain is coming
- the “something touched me” sense is faster than the pain sense
where does sensory start?
at the sensory endings for primary afferents in the DRG
- there are specialized receptors for everything
- examples: pacinian corpuscle, ruffini endings, meissner corpuscle, etc.
fasciculus proprius
- interconnects segments of SC - helps w/ reflexes
- has axons that don’t leave SC
- sends info up and down more segments
Why is a spinal cord injury not a precise cut off?
- the fasciculus proprius
- the fact that dermatomes also overlap
What are the clinically important long tract systems (note: this is repetitive)
SENSORY -spinothalamic -dorsal column aka medial lemniscal -spinocerebellar MOTOR -corticospinal
spinothalamic tract
-transmits pain, temp and poor localization into the CNS and up to thalamus
dorsal column aka medial lemniscal system
-transmits discriminative touch, proprioception, vibration, 2 pt discrimination to consciousness into the CNS and up to thalamus
spinocerebellar projections
-relay info about muscle tension, limb position and spinal cord level of activity to cerebellum for planning of movements
corticospinal projections
-most important of the upper motorneurons in the control of fine movement
where are primary afferent (first order) sensory neurons?
in the DRG
where do primary afferent axons synapse?
on the same side as the DRG cell body (ipsilateral)
what do secondary afferent neurons (second order) receive?
primary afferent synapses form ipsilateral DRG cells
secondary afferent neurons project where?
to the contralateral ventral thalamus to synapse on 3rd order neurons
tertiary afferent (3rd order) neurons project where?
from thalamus to cerebral cortex
spinothalamic tract
- enters where?
- synapses where?
enters and synapses in lamina 2 then crosses (decussates in cord)
dorsal column aka medial lemniscal pathway (group A fibers)
- enter?
- synapse?
- go where?
- enter and ascend on same side
- synpase w/ 2nd order neuron in MEDULLA
- cross there and got to VPL then cortex
what must happen for us to be aware of these pathways?
they MUST reach the cortex
if you see a pathway crossing in the white commissure, what does it tell you?
it’s spinothalamic tract
once the pathways reach the VPL of thalamus, where specifically do they go?
post central gyrus
post central gyrus is aka
primary somatosensory area
distribution of the sensory homunculus
- more nervous system in the post central gyrus is dedicated to face and index finger than anything else
- indicates importance
what is the origin of the corticospinal tract?
motor cortex aka pre central gyrus
motor homunculus
-closely resembles the sensory homunculus
blood supply of motor homunculus that supplies the face and upper extremity?
middle cerebral a.
blood supply of motor homunculus that supplies the leg?
anterior cerebral a.
corticospinal projections
- what it provides
- what happens when lost
- where decussation occurs
- cortex provides volitional control over intended movements
- w/o it, one loses fine fractionated control
- pyramidal decussation occurs at the spinomedullary junction
describe the pathway of the corticospinal projections
precentral gyrus –> bottom of medulla to decussate –> descend down lateral corticospinal tract –> synapse on motor nuerons –> muscle
spinal injury above the medulla would effect what? give example
- upper motor neurons
- ex: stroke
spinal injury below the medulla would effect what? give example
- lower motor neurons
- ex: SC injury
UMN lesion
- disuse atrophy
- fasciculations (quivering)
- spastic paralysis
- hyperreflexia
LMN lesion
- atrophy
- flaccid
- flaccid paralysis
- hyporeflexia
syringomyelia
- damage around the central canal
- pressure on crossing fibers causes a loss of pain and temp to those segments on both sides
tabes dorsalis
- damage to dorsal column
- almost always secondary to tertiary syphilis
- lose touch and proprioception
- when they walk they can’t feel their feet touch the ground
in a drug deal gone bad when the bullet hits the lateral faniculus, what does it affect?
corticospinal and spinothalamic tracts but NOT dorsal column
brown-sequard syndrome
- hemisection of SC
- causes 4 neural deficits
what are the 4 neural deficits caused by brown-sequard syndrome?
- loss of pain and temp at opposite side and below
- loss of light tough and 2pt ipsilateral and below
- upper motor lesion ipsilateral below level of lesion
- lower motor lesion at level of lesion d/t injury of cells directly, not axons
