Motor System

neurons from central motor centers in the motor cortex, brainstem centers (midbrain, pons, medulla), cerebellum, basal ganglia, thalamus

located in brainstem from cranial nerve nuclei and the spinal cord (form motor nerves)

1. supraspinal or descending motor pathways
2. spinal neurons (segmental interneurons and intersegmental neurons)
3. primary afferent (sensory fibers)

planning, initiating, and directing voluntary movements

gating proper initiation of movement

basic movements and postural control

sensory motor coordination (coordination/timing of movement)

reflex coordination

all of the motor neurons innervating a single muscle

sensory (laminae I-VI)

descending/sensory neurons (upper lamina VII)

interneurons/motor neurons (lower VII, VIII, IX)

M1/area 4 caudal precentral gyrus into anterior bank of central sulcus; directs movement has a hommunculus, contralateral prjections, functional aspect (interjoint coordination), distributed so different muscles/joints at different areas

M2/area 6b on the medial surface of the hemisphere above the cingulate gyrus; initiates movements specified by internal cues (spontaneous)

PM/area 6a just anterior to primary motor cortical strip of precentral gyrus; movements based on external cues

M3/area 24&25 on medial surface of hemisphere in limbic cortex; movements based on emotional state or reward information

relay info from somatosensory system, vestibular system, cerebellum, and basal ganglia to motor areas

to M1/PMC after synapsing at VLp in posterior thalamus

to SMA (some M1) after synapsing at VLa in anterior thalamus

ballistic bidirectional movements, all-or-none, no sensory feedback, 10% of cortical layer V

slow, accurate, unidirectional movement, tight sensory control, graded firing, 90% of cortical later V

M1 neurons can influence multiple muscles

M1 cortical spinal neurons synapse directly onto spinal motoneurons (for distal skilled movements)

cause permanent loss of fine motor control such as finger individuation

a motor neuron and all of the muscle fibers to which it connects (basic unit of movement)

impaired self-initiated tasks, bimanual coordination affected like manipulating fingers in both hands

M1 neurons posses cutaneous or deep receptive fields via projections from S1

smallest motor units recruited first and then bigger as force increases

proportional to the number of motor units active at any given time

muscle fibers rich in mitochondria and myoglobin; energy efficient and fatigue resistant to produce small forces

sparse mitochondria; fatigue easily and produce large forces for brief periods of time

intermediate motor units that produce twice as much force as slow motor units

impairment in planning self-initiated motor tasks; bimanual coordination impaired

impairment in planning externally-cued motor tasks (object-related)

defects in emotional state or reward information effects on motor control

generate force; innervated by alpha motor neurons

part of muscle spindle; innervated by gamma motor neurons

stretch receptors; activated by change in muscle length and in rate of change of length; arranged in parallel with the extrafusal muscle fibers surrounding them

contractile, receive gamma motor neuron innervation (controls the sensitivity of the spindle by contracting polar intrafusal fibers)

noncontractile, receive sensory information from Groups Ia and II afferents

regulate sensitivity of the spindles to muscle stretch by setting the intrafusal muscle fibers to an appropriate length

large diameter, fast conducting nerve fibers that connect monosynpatically with the alpha motor neuron

terminate on interneurons in the spinal cord

gamma motor neurons activate spindle contraction as alpha neurons contract the muscle; maintains sensory feedback to CNS

records the tension generated at the myotendinous junction during contraction (in series with extrafusal muscle fibers)

afferent fiber from GTO that contact inhibitory interneurons that decrease alpha motor activity innervating that muscle

flaccid paralysis, fasiculations, fibrillations (polio, ALS, spinal shock)

weakness, hypotonia, hyporeflexia or areflexia, muscle atrophy

spontaneous brief contractions of motor units of injured alpha motor neurons (possibly normal, can be ALS)

spontaneous twitches of single denervated muscle fibers; not visible (electromyography), usually pathogenic

midbrain superior colliculus

midbrain red nucleus

pons lateral vestibular nucleus

pons medial pontine reticular nuclei

medulla medial medullary reticular nuclei

lower limb extensors, upper limb flexors, axial extensors (Medial RST activates, Lateral RST inhibits)

resting level of tension in a muscle (depends on alpha neurons--depends on Ia spindles--depends on gamma motor neurons--gamma inhibited by UMN LReST)

cut below midbrain to block LReST (MReST and LVST still able to work) leading to uninhibited gamma motor neurons, increased Ia/II sensory and alpha (hypertonia/spasticity)

characterized by hyperactive stretch reflexes (increase in sensitivity to rate of change of muscle length = velocity-dependent reflex activity)

initial high level of resistance which suddenly yields as GTOs start firing

rhythmic, oscillatory contraction and relaxations of muscles in response to muscle stretching

characterized by hypertonicity that is equally evident in both extensors and flexors (stretch reflexes are hypoactive)

spastic paralysis (weakness, hypertonia, hyperreflexia, positive Babinski's sign, spasticity, muscle atrophy)

LVST excitatory to alpha anti-gravity muscles but normally inhibited at vestibular nuclei by cerebrum/cerebellum (effect on alpha only, not reduced by gamma reflex)

Parkinson's rigidity in moving limb like a ratchet with tremors (defect in basal ganglia)