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| cell bodies lie in the brainstem or cerebral cortex; axons descend to synapse with the local circuit neurons or with the lower motor neurons directly; second motor subsystem; initiation of voluntary movements & complex spatiotemporal sequences of skilled movements |
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| primary motor cortex & premotor cortex |
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| essential for planning, initiating, & directing sequences of voluntary movements involving the head, trunk, & limbs; part of the upper motor neuron pathways |
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| overlies the pons & fourth ventricle in the posterior cranium; larger of the subsystems that regulate the activity of the upper motor neurons |
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| embedded in the depths of the forebrain; suppress unwanted movement & prepare upper motor neuron circuits for the initiation of movements |
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| innervate specialized muscle fibers that are actually sensory receptors called muscle spindles; small |
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| Why are muscle spindles also called intrafusal muscle fibers? |
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| they are embedded within connective tissue capsules in the muscle; fusal means capsular |
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| innervates the extrafusal muscle fibers (striated muscle fibers that actually generate the forces needed for posture & movement); the second type of lower motor neuron |
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| the relationship between an a motor neuron & the muscle fiber it innervates wherein a single a motor neuron & its associated muscle fibers constitute the smallest unit of force that can be activated to produce movement; recognized by Sherrington |
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| slow motor units/S motor units |
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| in most skeletal muscles, the smaller motor units comprise small "red" muscle fibers that contract slowly & generate relatively small forces; especially important for activities that require sustained muscular contraction (i.e. maintenance of an upright posture) |
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| fast fatigable motor units/FF motor units |
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| larger a motor neurons innervate larger, pale muscle fibers that generate more force; especially important for brief exertions that require large forces (i.e. running or jumping) |
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| fast fatigue-resistant motor units/FR motor units |
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| intermediate size; not as fast as FF units; generate more force than S motor units |
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| as the synaptic activity driving a motor neuron pool increases, low threshold S motor units are recruited first, then FR motor units, and finally the FF motor units |
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| the sensory receptors embedded within most muscles; the sensory signal for the stretch reflex originates here |
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| Ia afferent activity is dominated by signals transduced by the _____ subtype of nuclear bag fiber whose biomechanical properties emphasize the velocity of fiber stretch |
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| Group II afferents innervate _____ nuclear bag fibers & the nuclear chain fibers |
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| Group II afferents signal the level of _____ fiber stretch by firing tonically at a frequency in proportion to the degree of the stretch, with little dynamic sensitivity |
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| the centrally projecting branch of the sensory neuron forms excitatory connections with the a motor neurons that innervate the same muscle & form inhibitory connections with those a motor neuron that innervate antagonistic muscles; results in rapid contraction of the stretched muscle & simultaneous relaxation of the antagonistic muscle |
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| the steady level of tension in muscles; is caused by the reflex circuit mediated by group II afferents which keeps muscles under some degree of stretch at all times |
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| The level of y motor neuron activity is often referred to as _____ & can be adjusted by upper motor neuron pathways as well as by local reflex circuitry |
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| Golgi tendon organs are exquisitely sensitive to increases in muscle _____ that arise from muscle contraction |
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| Unlike spindles, Golgi tendon organs are relatively insensitive to _____ |
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| The muscle spindle system is a feedback system that monitors & maintains muscle _____ |
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| The Golgi tendon system is a feedback system that monitors & maintains muscle _____ |
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