7. Motor Neuron Circuits

long distance circuit neuron

NS cell located in medial ventral horn, which controls axial muscles bilaterally, body position, and location

local circuit neuron

NS cell in lateral ventral horn, which controls limbs ipsilaterally, mediates fine control of distal muscles

γ motor neuron

small diameter NS cell that innervates muscle spindles in intrafusal capsules, sends sensory info about muscle length and its speed of change to 1a and 2a; adjusts tension level in intrafusal muscle fibers

α motor neuron

large diameter NS cell that innervates extrafusal muscle fibers and sends motor info, such as generating force for movement in striated muscles

innervation ratio

how many fibers innervated by a single α motor neuron, low is good for fine muscle control, high good for lots of force, forms motor unit w/ muscle fibers

motor unit

α motor neuron & its fibers

slow

red, rich in myoglobin and mitochondria, fatigue resistant, muscle unit responsible for sustained muscle activation (tonically active); low activation threshold

intermediate

muscles in between S and FF, used for tasks such as walking

fast fatiguable

easily tired, pale, low mt, motor units needed for high force in rapid movements

motor unit plasticity

ability to acquire new motor skills depending on upper MNs in motor units

extrafusal

motor muscle fiber outside spindle; large diameter for produce force via α motor neurons

intrafusal

sensory muscle fiber inside spindle; regulates muscle length via γ motor neurons (dynamic or static), w/ 2 stretch receptors: group Ia and II sensory afferents

group Ia sensory afferent

spiraled bag and chain fibers responding phasically to small stretches, forming excitatory connections on α MNs, which innervate synergistic muscles

reciprocal innovation

contraction of the streched synergistic muscle and relaxation of the antagonist muscle

group II sensory afferent

neuron on chain fibers which fire tonically proportionally to stretch, mediates muscle tone

muscle tone

steady tension level in a muscle

stretch reflex

muscles provide resistance and maintain muscle length as objects get heavier

Golgi tendon organ

reflex regulator located at junction of muscle and tendon, made of collagen 😡, which is innervated by 1b sensory axons, signal slowly during passive muscle stretch, rapidly during contraction; monitor and maintain force

stance

extending muscle, limb stays in contract w/ ground

swing

muscle is flexed, off ground

vestibular nucleus

motor control center responsible for vestibulospinal tract

reticular formation

motor control center responsible for colliculospinal tract

red nucleus

motor control center responsible for rubrospinal tract

medial IZ

region of spinal cord w/ local circuit neurons that synapse w/ medial lower motor neurons

lateral IZ

region of spinal cord where neurons synapse w/ the lateral ventral horn

lateral white matter

spinal cord region responsible for voluntary movement from the cortex

medial white matter

spinal cord region whose axons come from brainstem and control postural muscles

primary motor cortex

where upper MNs mediate planning and initiation of movements in precentral gyrus receiving input from basal ganglia and cerebellum via thalamus

Betz cell

pyramidal cell, w/ largest soma in CNS responsible for distal muscle control found in primary motor cortex

non Betz pyramidal cell

pyramidal cell found in both motor and premotor cortex (all divisions)

Montreal procedure

Dr. Penfield stimulated different brain regions while patients were awake and described reactions, mapped site of precentral gyrus

corticobular tract

pathway from brainstem to local circuit neurons that controls head, neck, and face muscles

lateral corticospinal tract

direct pathway terminating in lateral ventral horn and IZs w/ most axons synapsing on local circuit neurons

ventral corticospinal tract

pathway from medial and dorsal regions of the motor cortex going to the spinal cord

spike triggered avaeraging

method to correlate the activity of an individual neuron w/ muscle activity

premotor cortex

brain region that exerts both indirect (reciprocal projections to primary motor cortex) and direct (axons to corticobular and spinal tracts) control on behavior

closed loop movement

movement encoded by lateral premotor cortex, where external cue is needed to move

open loop movement

movement encoded by medial premotor cortex triggered by internal cues

feedforward adjustment

movement to stabilize posture during movement, where compensatory movements precede primary one, w/ both intiated by cortical upper MNs