The vertebrate motor unit The vertebrate motor unit muscles performing powerful

The vertebrate motor unit the vertebrate motor unit

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The vertebrate motor unit
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The vertebrate motor unit muscles performing powerful, coarsely controlled movements have larger number of fibers per motor unit example : a single calf muscle motor neuron may connect to ~2000 muscle fibers muscles that produce more precise, delicate movements contain fewer fibers per motor unit example : extraocular muscles have only ~12 muscle fibers per motor neuron A single motor unit consists of a single motor neuron and all the muscle fibers that it controls . The numbers of muscle fibers per motor unit and numbers of motor units per muscle vary widely.
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Motor units in extraocular muscles Extraocular muscles (which control the direction of gaze) produce very precise, delicate movements. Each motor neuron connects to only ~12 muscle fibers.
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The vertebrate motor units Increasing the number of active motor units is recruitment . Asynchronous recruitment of motor units delays fatigue: alternating motor activities gives motor units that have been active the opportunity to rest while others take over.
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Action potentials and muscle fiber contractions A single action potential in the muscle fiber causes an all-or- none muscle fiber contraction (a twitch ). The length and the strength of a sustained contraction ( tetanus ) depends on the frequency of action potentials.
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Treppe and summation The frequency of action potentials determines the tension . The graduated series of increasingly vigorous contractions that results when a series of identical stimuli are applied to a rested muscle is called treppe . It does not result in tetanus, and is important in cardiac muscle activity. When a muscle is stimulated by the motor neuron more than once in a short period (more frequently than in treppe), the successive twitches add to each other so the force generated is greater. This is called summation .
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Summation Treppe Treppe and summation
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High frequency stimulation can cause a smoothly fused contraction called tetanus . This extreme shortening is because all of the elastic muscle fibers of the muscle become completely shortened. incomplete tetanus : there are brief relaxation phases between stimulations but the muscle does not relax completely complete tetanus : relaxation phases are eliminated Tetanus
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Incomplete tetanus Complete tetanus Tetanus
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Fatigue In skeletal muscles, repetitive stimulation leads to fatigue when the fibers run out of enough energy, evident as a drop in tension. Rest overcomes fatigue, but it will reoccur sooner if inadequate recovery time is allowed.
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  • muscle tension, Extensor muscle, Calf Muscle

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