Notes (2) - Chapter 8 Skeletal Muscle Mechanisms Figure 8-18 Motor units in a skeletal muscle Lower part of spinal cord sends out motor fibers to

Notes (2) - Chapter 8 Skeletal Muscle Mechanisms Figure...

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Chapter 8 Skeletal Muscle Mechanisms ° ° Figure 8-18 Motor units in a skeletal muscle. Lower part of spinal cord sends out motor fibers to skeletal muscle cell Motor units are a single nerve fiber + all the individual muscle cells it innervates ° ° SKELETAL Motor Unit: One motor fiber and all the muscle cells it innervates. Number of muscle fibers/motor unit variable: o For delicate & precise movements – a single motor unit may be composed of as few as a dozen motor fibers (e.g. external eye musles; hand muscles) o For powerful, coarse movements – a single motor unit may contain 1500-2000 muscle fibers. Motor Unit Recruitment: o For weak contraction of whole muscle —only one or a few motor units are activated; o For stronger contractions – additional motor units are recruited! ° Comparison of motor unit recruitment in skeletal muscles with small motor units and muscles with large motor units (Figure 8.19) Small incremental increases in strength of contraction occur during motor unit recruitment in muscles with small motor units because only a few additional fibers are called into play as each motor unit is recruited Large incremental increases in strength of contraction occur during motor unit recruitment in muscles with large motor units, because so many additional fibers are stimulated with recruitment of each additional motor unit °
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° FATIGUE : TO PREVENT FATIGUE : asynchronous recruitment of motor fibers occurs. Asynchronous activation – helps prevent fatigue, motor fibers activated at different times Degree of muscle contraction decreases to protect a muscle from reaching a point it which it cannot produce sufficient ATP (resulting in rigor mortis!). Caused by: Accumulation of lactic acid (glycolysis) affecting excitation contraction coupling Accumulation of [K + ] 0 due to inability of the N+/K+ pump to keep up with the results of muscle activity—partially depolarizing muscle cells Depletion of glycogen energy reserves Effects if CENTRAL fatigue: decreased activation of motor neurons that is psychologically based; i.e. the discomfort associated with the activity, boredom, tiredness. Neuromuscular fatigue: depletion of Ach in presynaptic terminal (not under normal conditions). White muscle – glycolysis for energy, not very much oxygen accumulate lactic acid à build up leads to fatigue External potassium leads to depolarizing cell à fatigue Neuromuscular fatigue– more experimental, not enough Ach to activate the muscle ° ° Relationship of an action potential to the resultant muscle twitch The duration of the action potential is not drawn to scale but is exaggerated Note that the resting potential of a skeletal muscle fiber is -90mV, compared to a resting potential of -70mV in a neuron Transient contraction- twitch Tension is a bell shaped curve After the action potential is done the muscle starts its function °
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