13b Motor system

13b Motor system - MOTOR SYSTEM page 1 AC Brown A7b...

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MOTOR SYSTEM AC Brown page 1 A7b INTRODUCTION TO SOMATIC MOTOR SYSTEMS A. Functions 1. Posture: maintain appropriate body position against external forces 2. Movement: change of body position to accomplish desired act 3. Coordination: control pattern and sequence of muscle contraction for smooth, effective action B. Initiation of Function 1. Involuntary: motor act, initiated by specific internal or external stimulus, generally stereotyped, need not involve conscious volition 2. Voluntary: sometimes initiated without any obvious stimulus, often complex and varied, requires conscious volition Note: voluntary and involuntary are not separate systems, since they involve shared effector organs and pathways, and interact with each other C. Effector Structures 1. skeletal muscle: regular ( extrafusal , ordinary) skeletal muscle fibers 2. neurons innervating skeletal muscle fibers: alpha (large) motoneurons in the spinal cord and cranial motor nerve nuclei ( motor unit , final common pathway, lower motor neuron, always excitatory) a. spinal cord: alpha motoneurons in ventral horn, innervating muscles of movement and posture b. cranial nerves: motor nuclei in the bulb/brainstem (medulla, pons, midbrain) controlling 1) eye movement 2) mastication (chewing) 3) muscles of facial expression 4) muscles of the soft palate and larynx 5) muscles supporting the head 6) tongue muscles Note: the brainstem is sometimes called the "bulb"
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MOTOR SYSTEM AC Brown page 2 A7b INTRODUCTION (continued) D. Influences on Motoneurons (= motor neurons) 1. reflexes: simple, stereotyped, involuntary movements (e.g. flexion reflex, myotatic reflex) 2. brain stem nuclei (tone) 3. brain stem pattern generators: more complex movements involving several muscles, often voluntarily initiated (e.g. chewing, swallowing, walking, vomiting) 4. motor cortex (primary motor cortex, M1; essential for voluntary activity) 5. premotor cortex (input to motor cortex) 6. supplementary motor cortex (input to motor cortex) 7. cerebellum (feedback to motor cortex; coordination, balance, posture/tone) 8. basal ganglia (elaboration of movement & suppression of unwanted movements) E. Terms and Definitions tone muscle tension in the absence of voluntary activity hypertonia / hypotonia increased / reduced muscle tone compared to normal dystonia tonically rigid posture hyper- / hyporeflexia increased (brisk) / reduced reflex response clonus oscillatory contraction and relaxation of muscle (reflex) spasticity hypertonia, hyperreflexia, clonus tremor involuntary, small, rapid motions passive tremor tremor associated with rest intention tremor tremor accompanying voluntary motion
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13b Motor system - MOTOR SYSTEM page 1 AC Brown A7b...

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