![]() ![]() ![]() Muscles stretched beyond this point will produce less tension. Muscles will generate more force when stretched beyond their resting length to a point. The length of a muscle is related to the tension generated by the muscle. One stimulus will affect all of the muscle fibers innervated by a given motor unit. Motor units act in a coordinated fashion. There are numerous motor units throughout skeletal muscles. In skeletal muscles a motor neuron can innervate many muscle fibers. The muscle responds to stronger stimuli by producing the same force. The muscle will not contract with greater force if the stimulus is greater. Finally if we continue to increase the stimulus so that it well exceeds the threshold the fiber will respond by contracting with the same force as when we just reached the stimulus. Now if we increase the stimulation so that enough is produced to reach the threshold the muscle fiber will respond by contracting. The muscle fiber will respond by remaining relaxed, it will not contract. We begin with a low amount of stimulation that does not reach the threshold to produce a contraction. Let’s say that we are electrically stimulating a muscle fiber. ![]() Muscle Stimulus and Contraction StrengthĪ skeletal muscle fiber will produce a given amount of force if the stimulus is strong enough to reach the threshold for muscle contraction. The troponin moves back into position blocking the myosin binding site on the actin and the muscle passively lengthens. During this phase calcium is actively transported back into the sarcoplasmic reticulum using ATP. This phase is called the relaxation phase. When the muscle relaxes the tension decreases. Myosin is now available for another cross-bridge formation. Myosin releases from actin when a second ATP attaches to myosin. ATP is used during this phase and energy is released as heat. Myosin moves actin, releases and reforms cross-bridges many times as the sarcomere shortens and the muscle contracts. During the contraction phase the cross-bridges between actin and myosin form. This next phase is called the contraction phase. This is followed by the actual muscle contraction that develops tension in the muscle. The depolarization results in the release of calcium by the sarcoplasmic reticulum and subsequent binding of calcium to troponin which causes the myosin binding site to be exposes. This results in release of acetylcholine and depolarization of the motor end plate. During the lag phase a signal called an action potential moves to the end of the motor neuron (axon terminal). The time between the activation of a motor neuron until the muscle contraction occurs is called the lag phase (sometimes called the latent phase). A twitch occurs when one muscle fiber contracts in response to a command (stimulus) by the nervous system. We can improve our understanding of muscle contraction by examining the contraction of one muscle fiber. ![]()
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