MotorFunctionandtheMotorUnit.pptVIP

Motor Function and the Motor Unit Organization of the Nervous System Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Afferent (sensory) division [periphery ? CNS] Efferent (motor) division [CNS ? periphery] Somatic [motor neurons] Autonomic Sympathetic Parasympathetic Neurons Motor Unit The motor neuron and all the muscle fibers it innervates. Motor neuron determines fiber type Only ONE fiber type per motor unit FG FOG SO MU Classifications Motor Unit The number of muscle fibers in a motor unit (innervated by 1 motor neuron) varies Gastrocnemius 2,000 muscle fibers per motor neuron Extraocular muscles 10 muscle fibers per motor neuron Ratio of muscle fibers to motor neurons Affects the precision of movement Motor Units and Muscle Force Production The All-or-None Law (Bowditch’s Law) for motor units Applies to individual motor units, but not the entire muscle. The all-or-none law is based upon the difference between graded potentials and action potentials Stimulation threshold A motor unit is either activated completely or is not activated at all If there is enough graded potential to create an action potential that travels down the α-motor neuron of a motor unit, then all of the fibers in that motor unit will contract. The level of force production of a single motor unit is independent of the intensity of the stimulus, but it is dependent on the frequency of the stimulus This law implies a stimulation threshold ? important for the Size Principle Gradation of Muscle Force Two neural mechanisms responsible for force gradations: Recruitment Spacial summation Rate coding Temporal summation Recruitment Varying the number of motor units activated. Rate Coding Rate coding refers to the motor unit firing rate. Active motor units can discharge at higher frequencies to generate greater tensions. Recruitment vs. rate coding Smaller muscles (ex: first dorsal interosseous) rely more on rate coding Larger muscles of mixed fiber types (ex: deltiod