- The sliding of filamentous proteins within the muscle fiber results in contraction. - The force of contraction depends on the length of the muscle. Twitch contraction:Fig. 12-12. A motor unit is activated in an all-or-none fashion by a single action potential, which travels from the motoneuron along the axon to the muscle fibers. The neural action potential leads to an action potential in each muscle fiber innervated by that motoneuron. A muscle action potential in turn produces a brief contraction of the muscle fiber called a twitch. The duration of the twitch depends on the muscle fiber type. The duration of both the contraction and relaxation phases of the twitch are longer for slow-twitch (type I) than fast-twitch (type II) fibers. D.2. Frequency-Tension relation:Fig. 12-17•Muscle force is also a function of the rate coding that is the frequency of activation (firing rate) of motor units. An increase in the firing rate of a motor neuron translates into an increase in the frequency of action potential. The force
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