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Chapter 13: Motor Systems - Spinal Cord (Spinal cord (spinal nerves is a…
Chapter 13: Motor Systems - Spinal Cord
Categories of movement
reflexive
involuntary coordinated patterns of muscle contraction and relaxation elicited by stimuli
reflex contraction
muscle sensory neurons --> LMN
rhythmic
repetitive motor patterns
circuits lie in brainstem and spinal cord
voluntary
goal directed
improve with practice
Voluntary contraction of skeletal muscle requires interaction of 2 neurons
what's an Upper motor neuron
descending spinal pathways from brain, modulate spinal reflexes or directly drive motor output
lateral pathways
voluntary movement of distal musculature
under direct cortical control
innervate distal musculature
ventromedial pathways
control of posture, locomotion, orienting and balance
under brainstem control
innervate axial and proximal musculature
lower motor neuron
neurons that synapse on striated muscle
graded control of the force of muscle contraction 2 mechanisms
By varying firing rate of motor units
Action potential in alpha motor neuron causes endplate potential large enough to generate postsynaptic AP in muscle
AP in muscle causes TWITCH
increasing AP frequency --> twitch summation and eventually SUSTAINED CONTRACTION
recruitment of synergistic motor units
small motor units are recruited first (SLOW) and then larger motor units (FAST)
voluntary contraction UMN --> LMN
UMN is contralateral to LMN, UMN descends
at medulla it crosses midline
goes through lateral corticospinal tract
LMN (LMN ipsilateral to innervated muscle)
Motor systems organized hierarchically
forebrain, brainstem, spinal cord
Final common pathway
motor neurons in spinal cord (body)
brainstem cranial nerves (head)
Spinal cord
carries motor info from brain to periphery
carries sensory input from periphery to brain
mediates reflexes for body
spinal nerves is a mix of sensory and motor
Dorsal root (sensory afferents)
Ventral root motor efferents
anatomy
gray matter
nerve cell bodies, dendrites, axons
divisions of spinal gray matter
dorsal horn
sensory input
intermediate gray
some sensory, some motor (interneurons)
ventral horn
motor input
white matter
myelinated axons organized into tracts
Each muscle fiber is innervated by single axon
motor unit
one alpha motor neuron and all the muscle fibers it innervates
motor neuron pool
collection of alpha motor neurons that innervates a single muscle
2 types of lower motor neurons in spinal cord
alpha motor neurons
innervate extrafusal muscle
trigger generation of force in muscle
gamma motor neurons
innervate intrafusal muscle fibers of muscle spindle
Alpha motor neuron has 3 sources of input
spinal interneurons
muscle spindles
proprioceptor - stretch receptor
detect changes in MUSCLE LENGTH
INTRAFUSAL muscle in a fibrous capsule
Ia sensory afferents wrap around intrafusal muscle fibers and inform CNS about MUSCLE LENGTH
situated in parallel with EXTRAFUSAL MUSCLE FIBERS
UMN
Monosynaptic myotonic reflex arc (stretch reflex): when a muscle is pulled on, it contracts
as muscle is STRETCHED, discharge rate INCREASES
as muscle is SHORTENED, discharge rate DECREASES
when weight is placed on muscle, muscle LENGTHENS, muscle spindles STRETCH
increased AP discharge, depolarizes alpha motor neuron
increase AP frequency
muscle contracts
Gamma motor neurons innervate intrafusal muscle fibers in the muscle spindle and ADJUST THE SENSITIVITY of the muscle spindle
loading spindle: STRETCHING muscle, STRETCHES Ia nerve endings, activated alpha motor neuron
unloading spindle: EXTRAFUSAL muscle CONTRACTS, Ia axon activity STOPS
Gamma motor neuron activity RESETS length of spindle by CONTRACTING INTRAFUSAL muscle fiber
Golgi tendon organs: monitor muscle TENSION (force of contraction)
Golgi tendon organs situated in series with muscle fiber
Ib afferents encode MUSCLE TENSION
Reverse myotatic reflex: in extreme circumstances, reflex PROTECTS muscle from being overloaded
Reciprocal inhibition
CONTRACTION of 1 set of muscles is accompanied by RELAXATION of antagonist muscles
Flexor crossed-extensor reflex
ipsilateral: EXCITE flexors, INHIBIT extensors
Contralateral: EXCITE extensors, INHIBIT flexors