NPB+112+11++motor+lecture+notes

NPB+112+11++motor+lecture+notes - NPB 112, Fall 2011 Feb....

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1 NPB 112, Fall 2011 Feb. 8-15, 2011 Motor systems (Chs. 16, 17) I. Overview II. Motor neurons A. rate coding B. spatial recruitment by size principle III. Spinal reflexes A. stretch (monosynaptic, myotatic) reflex B. gamma loop and movement initiation C. Ib fibers D. withdrawal reflex I. OVERVIEW of motor system: A. Movement is ultimate output of the nervous system. B. Types of movement: 1. Voluntary, conscious, goal-directed (eating, etc.). 2. Involuntary, subconscious reflexes: withdrawal, posture (some voluntarily control) 3. Rhythmic (locomotion, chewing) initiated voluntarily, proceed automatically. C. 5 hierarchies of organization associated with movements. Extensive interactions; not strictly categorized. 1. Lowest level: SPINAL CIRCUITS. Spinal motoneurons and muscles they innervate a. Directly activated by motor cortex for voluntary contraction. b. Excited or inhibited by spinal reflexes. c. activated by spinal pattern generator for walking d. Under control of pathways from brainstem to control posture 2. middle level: brainstem: reticular formation, vestibular system: balance, posture 3. CEREBELLUM: coordinates movement. a. compares motor command (from cortex) with sensory feedback (from movement) b. If movement inaccurate, sends correction to motor cortex c. Involved in learning motor skills (e.g., play instrument)
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2 4. BASAL GANGLIA: initiate movement; damage leads to motor deficits such as Parkinson's disease (tremor, akenesia) or Huntingtons (uncontrolled writhing movements) 5. MOTOR and PREMOTOR cortex: involved in planning, initiating and executing voluntary movements a. corticospinal tract: direct pathway from motor cortex to spinal cord motoneurons and interneurons II. Motoneurons A. spinal cord organization B. 2 major classes of motoneuron: alpha (to extrafusal muscle fibers) and gamma (to muscle spindle). C. Definitions related to motoneurons: 1. Motor neuron pool: group (pool) of motoneurons innervating individual muscle. Fig. 16.2 a. Arranged in longitudinal columns along venteral horn of spinal cord 2. Motor unit: single motor neuron and ALL muscle fibers is synapses with. Fig. 16.5 a. Slow, small non-fatiguing: connects only to a few muscle fibers (red, aerobic, small, constantly contracting: antigravity); non-fatiguing to contract constantly b. large, fast fatiguing: motoneuron connects to many to pale (glycolytic, fast) muscle fibers to generate large force which fatigues quickly c. in-between type: fast, fatigue-resistant D. Force of muscle contraction controlled by: 1. Rate coding: firing rate of individual motor unit related to degree of force generated in muscle (temporal summation) 2. Spatial recruitment: more motor units fire to contract more muscle fibers (spatial summation)
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3 3. Size principle: smaller (aerobic, red) motor units recruited first, larger (anaeorobic, pale) later: Fig. 16.7 III. Spinal Reflexes: monosynaptic (myotatic) reflex A. REFLEX: involuntary, stereotyped movement evoked by a specific sensory
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NPB+112+11++motor+lecture+notes - NPB 112, Fall 2011 Feb....

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