Final answer:
Muscle fatigue can occur during the refractory period or due to a Ca2+ imbalance, leading to the inability of a muscle to contract. ATP depletion, lactic acid buildup, and electrolyte imbalances can all contribute to this condition, with prolonged exercise potentially damaging the muscle's ability to regulate Ca2+.
Step-by-step explanation:
Muscle fatigue occurs when a muscle can no longer contract in response to signals from the nervous system. This can happen during the refractory period or from Ca2+ imbalance. The refractory period is a time immediately following an impulse in a nerve or muscle during which a neuron or muscle cell cannot generate another action potential, allowing voltage-sensitive ion channels to return to their resting configurations.
During periods of intense muscle activity, ATP reserves may be depleted, leading to muscle fatigue. Lactic acid buildup can also lower intracellular pH, negatively affecting enzyme function and muscle contraction. Lastly, imbalances in Na+ and K+ levels may disrupt Ca2+ flow out of the sarcoplasmic reticulum (SR), impairing muscle function.
Long-term or sustained exercise can damage the SR and sarcolemma, resulting in impaired Ca2+ regulation. The relaxation of a muscle fiber, as shown in Figure 10.9, involves Ca2+ ions being pumped back into the SR, leading to the reduction of muscle contraction. Muscle contraction can also stop and lead to fatigue when ATP is exhausted.