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Motor Systems (Spinal Cord) (Functions (Flexor crossed-extensor reflex…
Motor Systems (Spinal Cord)
Functions
1) carries motor information from brain to periphery
2) carries sensory input from periphery to brain
3) mediates reflexes for body
Reciprocal Inhibition
Inhibitory motor neuron synapses on alpha motor neuron of antagonistic muscle to perform contraction of targeted muscle & relaxation of antagonistic muscle
Flexor crossed-extensor reflex
ipsilateral: excite flexors, inhibit extensors
contralateral: excite extensors, inhibit flexors
Organization & Anatomy
organized hierarchically
forebrain –> brainstem –> spinal cord
all meet @ final common pathway
motor neurons in spinal cord (body)
brainstem cranial nerves (head)
Basal ganglia & cerebellum act on cerebral cortex though relay nuclei in thalamus (VL/VA)
Anatomy
appears segmented due to dorsal & ventral roots on surface
Dorsal
dorsal root – sensory (afferents)
DRG – contains somas of sensory afferents entering cord
Ventral
ventral root = motor efferents
spinal nerves = mix of sensory & motor
gray matter = nerve cell bodies, dendrite, axons
dorsal horn
mainly neurons responding to sensory input
intermediate grey
some sensory & motor neurons + interneurons
ventral horn
mainly motor neurons w/ axons exiting spinal cord
contains 2 types of lower motor neurons
alpha motor neurons
innervate extrafusal muscles
trigger generation of force in muscle
3 sources of input
spinal interneurons
muscle spindles
upper motor neurons in brain
gamma motor neurons
innervate intrafusal muscle fiber of muscle spindle
loading spindle – stretching muscle stretches Ia nerve endings & activates alpha motor neuron
unloading spindle – extrafusal muscle contracts & activity in Ia axon stops; spindle no longer sensitive to stretch
GMN activity reset length of spindle by contracting intrafusal muscle fiber; Ia continue to inform CNS of changes to length
muscle spindle = proprioceptor - stretch receptor
4 more items...
motor neuronal pools = particularly in cord segments that innervate upper & lower limbs
white matter = bundles of myelinated axons organized into tracts
each muscle fiber = innervated by single axon
motor unit
one alpha motor neuron & all muscle fibers it innervates
motor neuron pool
collection of alpha motor neurons that innervates single muscle
Categories of Movement
Reflexive
involuntary coordinated patterns of muscle contraction & relaxation elicited by peripheral stimuli (i.e knee jerk)
Rhythmic
repetitive motor patterns (i.e breathing, chewing, swallowing etc)
circuits lie in brainstem & spinal cord
Voluntary
goal-directed
improve w/ practice
Voluntary Contraction
requires interaction of 2 neurons
an upper motor neuron (UMN) –> lower motor neuron (LMN)
UMN: contralateral to LMN (i.e. corticospinal tract)
voluntary contraction = UMN –> LMN
neurons synapse striated muscle clinically referred to "lower motor neuron"
Reflex Contraction = muscle sensory neurons –> LMN
LMN: ipsilateral to innervated muscle
Descending pathway from brain
modulate spinal reflex or directly drive motor output = upper motor neurons
lateral pathways
voluntary movement of distal musculate
under direct cortical control
innervate distal musculature
ventromedial pathways
control of posture, locomotion, orienting, and balance
under brainstem control
innervate axial & proximal musculature
force of muscle contraction
1) by varying firing rate of motor units
1 AP in alpha motor neuron –> endplate potential (EPP) = large enough to cause postsynaptic AP in muscle
AP in muscle causes muscle twitch & rapid relaxation/contraction
increasing AP frequencies –> twitch summation & eventually sustained contraction
2) recruitment of synergistic motor units
size principle = small motor units are recruited first (slow) then larger motor unit (fast)
fast twitch fibers = generate more fore but tire easily
slow twitch fibers = generate less force but endure
Golgi Tendon Organs
situated in series w/ muscle fiber (top of extrafusal muscle fiber)
Ib afferents encode muscle tension info
"reverse myotatic reflex"
extreme cases – reflex protects muscle from being overloaded
normal function = regulate muscle tension w/in normal range