Question

Skeletal muscle has a defined contraction period, causing the full contraction of every stimulated fiber. (8 pts)

Part 1 - What determines the length of time during which the fiber contracts?

Part 2 - What ends the contraction, so relaxation can begin? (not including the termination of the NMJ stimulation)

Part 3 - Taking the previous two parts, and now adding in that the NMJ has stopped inducing depolarizations at the motor end plate, describe the events as the contraction cycle (each single twitch) ends.

Part 4 - Taking the previous three parts, and now adding in that the NMJ continues to induce depolarizations at the motor end plate, explain the generation of increased muscle tension as summation occurs.

Answer 1

1. Muscle activity is the repetition of the contraction and relaxation phase. Contraction or the shortening occurs when the calcium ions are released from the sarcoplasmic reticulum into the muscle fiber. And the relaxation occurs with the active uptake of the calcium back into the sarcoplasmic reticulum. Thus the length of the contraction of the muscle fiber is between the calcium release and calcium uptake events, it is based on the stimulation of the motor unit.

2. The relaxation event begins with the ATP binding to the actin-myosin cross-bridge to detach. Then the Ca is returned to the sarcoplasmic reticulum with the help of energy from the ATP molecule. As the Ca is removed from troponin blocking action of the tropomyosin occurs. Thus there is a relaxation phase.

3. The neuromuscular block usually occurs when there is a low concentration of Ca is available in the extracellular fluid. This happens because fewer Ca ions are available to trigger the binding of synaptic vesicles to the axon endplate and cause the release of acetylcholine. As the acetylcholine release is required for the depolarization of the muscle fiber there will be depression in the continuation of the depolarization. So that the release of Ca ions from the sarcoplasmic reticulum will not occur. As a result the contraction or shortening of the muscle will not happen.

4. The release of the acetylcholine causes the depolarization of the muscle fiber with the increasing the permeability of Na ions into the sarcolemma, whereby the action potential is propagated by the opening and closing of the Ca and Na channels and it proceeds in all directions from the neuromuscular junction, which is seen in the center of the muscle fiber. The action potential is transported to the interior of the muscle fiber with the help of T tubules. Thus the release of the Ca from the sarcoplasmic reticulum to the cytosol occurs, so the contraction of the muscle fiber occurs. Thus the signal transfer from the sarcolemma to T tubule to the sarcoplasmic reticulum to the myofibrils within the muscle fiber so that all the myofibrils contract in a synchronized manner. If the axon is supplying to a large number of the muscle fibers through its axon, the stimulation of the axon that is depolarisation cause the contraction of the large number of the muscle fibers which increase the strength of the muscle contraction that is increased muscle tencsion. This is known as the motor units summation.