Question

The longest phase in a single muscle twitch in a vertebra is about 50 millisecond and is ________ phase
A.Latent
B. Contraction
C. Relaxation
D. All are equal

Answer 1

Muscle twitch refers to the response of a muscle to a single electrical stimulation. It consists of three phases: a latent period, a contraction phase, and a relaxation phase.

Step by step answer: A muscle twitch occurs when one muscle fiber contracts in response to a command or stimulus by the nervous system.
-Option A- Latent phase
The lag phase or latent phase is the time between the activation of a motor neuron until the muscle contraction occurs. During this phase, a signal called an action potential moves to the end of the motor neuron (axon terminal) resulting in the release of acetylcholine and depolarization of the motor endplate. The depolarization results in the release of calcium by the sarcoplasmic reticulum and subsequent binding of calcium to troponin. This causes the myosin binding site to be exposed.
-Option B- Contraction
The latent phase is followed by the actual muscle contraction that develops tension in the muscle. This phase is known as the contraction phase. During this phase, the cross-bridges between actin and myosin are arranged. Myosin shifts action, releases, and reforms cross-bridges several times as the sarcomere shortens and the muscle contracts. ATP is utilized during this phase and energy is released as heat. Myosin is released from actin when a second ATP attaches to myosin. Myosin is then available for another cross-bridge formation.
-Option C- Relaxation
The last phase is Relaxation. This happens when the muscle relaxes and tension reduces. During the relaxation phase, calcium is actively transported back into the sarcoplasmic reticulum using ATP. The troponin changes positions back into position hindering the myosin-binding site on the actin and the muscle passively lengthens.
-Option D- All are equal
The latent period is a short delay of 1-2 msec from the time when the action potential reaches the muscle until tension can be identified in the muscle. Whereas relaxation time is when the muscle is returning to resting length and it is the longest phase. They are not equal.
So, the correct Option is C- Relaxation

Note: Muscle twitches can occur because of many reasons, like stress, too much caffeine, a poor diet, exercise, or as a side effect of some medicines. A maximum number of people get twitches in the eyelid, thumb, or calf muscles. These types of twitches mostly go away after a few days.