Final answer:
A twitch refers to the contraction of a muscle in response to a single action potential from a motor neuron, characterized by a latent period, contraction phase, and relaxation phase. Increased frequency and number of motor neuron signals can lead to stronger muscle contractions and even sustained contraction, or tetanus.
Step-by-step explanation:
The synchronous contraction of a single motor unit is known as a twitch. This biological concept relates to the way muscles respond to stimuli from motor neurons. A single action potential from a motor neuron will produce a single twitch in the muscle fibers it innervates.
The twitch can be described as having three distinct phases: a latent period, the contraction phase, and the relaxation phase. During the latent period, the action potential travels along the sarcolemma, and calcium ions are released from the sarcoplasmic reticulum, prepping the muscle for contraction.
The contraction phase involves the binding of calcium to troponin, the repositioning of tropomyosin, the formation of cross-bridges, and the shortening of sarcomeres. The relaxation phase occurs as calcium ions are reabsorbed by the sarcoplasmic reticulum, causing the cross-bridges to detach and muscle tension to decrease.
Graded muscle responses occur when the frequency of action potentials and the number of motor neurons signaling increase the tension produced in skeletal muscles.
Successive twitches can build upon each other, a concept known as summation. If the stimulus frequency is high enough, it can result in a smooth and sustained contraction called tetanus.