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Chapter 13: Spinal Control of Movement (Spinal Cord Anatomy (Dorsal Root…
Chapter 13: Spinal Control of Movement
"Final Common Pathway"- motor neurons in spinal cord
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Spinal Cord Anatomy
Dorsal Root Ganglia- contains somas of sensory afferents entering the cord
Ventral Root- motor (efferents)
Dorsal root- sensory (afferents)
Spinal nerves- mixed sensory & motor
Two muscle types
Striated: Cardiac, skeletal,
Smooth: digestive tract, arteries, viscera
Axial Muscles- trunk movement (posture)
Distal Muscles- manipulation of objects
Proximal Muscles- locomotion
Lower Motor Neurons- somatic motor neurons, in ventral horn.
Flexors lie dorsal to extensors, axial MNs medial to distal MNs
Upper Motor Neurons- soma in cerebral cortex or brainstem; provide input to LMNs
Limb Enlargements- regions of cord where innervation of muscles of arms & legs occurs- horns are swollen
Alpha Motor Neurons- trigger generation of force by muscles
Three sources of AMN input
Upper motor neurons
Interneurons in spinal cord (largest)
DRG neurons
To control muscle contraction, 2 mechanisms
Varying firing rate of AMNs (ACh at NMJ)
Recruiting additional motor units
Size principle- small motor units recruited first
Only extrafusal fibers innervated by AMNs
Gamma Motor Neurons- innervate intrafusal fibers
Activation of AMNs and GMNs has opposite effect on 1a output- alpha alone decreases, while gamma alone increases
Motor Unit- one Alpha motor neuron & all the muscle fibers it innervates
Motor neuron pool- all alpha motor neurons that innervate a single muscle
Two Muscles Types (the sequel)
Red muscle fibers- many mitochondria, rich in myoglobin, fatigue resistant "slow twitch"
White muscle fibers- sparse mitochondria, rapid fatigue "fast twitch"
Types of motor units
Fast fatigable- (FF)- large motor units, brief exertions
Fast fatigue-resistant motor units (FR)- intermediate
Slow motor units- small, sustained contractions
Muscle Spindles- 8-10 intrafusal fibers
1a sensory axons wrap around muscle fibers- mechanosensitive channels
Myotatic reflex- when a muscle is pulled it tends to contract
Golgi Tendon Organs- Strain gauges in series with muscles
Innervated by 1b sensory axons- branches entwined in collagen fibrils
1b axons enter spinal cord, synapse on 1b inhibitory interneurons that synapse on AMNs innervating the same muscle
Spinal Interneurons- involved in reciprocal inhibition & antagonist muscles
Reflexes
Flexor withdrawal reflex- 1d nociceptive axons; slower than stretch reflex
Crossed-Extensor Reflex (reciprocal Inhibition)
Central Pattern Generators are circuits that give rise to rhythmic motor activity
NMDA receptors are important in spinal interneurons
Glutamate causes NMDA to open, Ca2+ to enter; causes K+ channels to open; Mg2+ enters, membrane resets