Question

Lack of relaxation between successive stimuli in sustained muscle contraction is known as.
(a) Tonus
(b) Spasm
(c) Fatigue
(d) Tetanus

Answer 1

Hint: In this state, $Ca^{++}$ ion concentration present in the sarcoplasm will permit all the sarcomeres to develop cross-bridges and truncate, thereby facilitate continuous muscle contraction till the muscles become fatigued and cannot create further muscle tension.

Complete answer:
Tetanus is a phenomenon where there is an absence of relaxation between successive stimuli. Muscles will undergo periodic contraction and relaxation to perform the desired function such as locomotion, pumping of blood by heart, etc. Continuous muscle contraction generates an elevated amount of action potential which in turn will motivate the muscle tension to reach the peak. Therefore, the muscle fibers will not get time to undergo relaxation.
The diagram shown below represents the graph of a single muscle twitch, wave summation, and tetanus muscle contraction.

The contraction and relaxation of a muscle by the stimulation of a single action potential are called a single muscle twitch.
When the muscle is triggered by a new stimulus before it relaxes from the previous muscle twitch, then at this point the strength of the muscle twitch increases which will further enhance the muscle tension. This phenomenon is called wave summation or temporal summation.
Tetanus contraction can either be fused or unfused. In unfused tetanus, there will be an incomplete relaxation of the muscle fibers. It is also known as incomplete tetanus.
Fused tetanus: Upon a high stimulation rate, the muscle fibers will not undergo relaxation between subsequent stimuli. It is also known as incomplete tetanus.

So, the correct answer is option (d) Tetanus.

Note:
Tetanus is caused by the sudden contractions of the voluntary muscles whereas a spasm is a rapid involuntary muscle contraction. An individual contraction is known as a muscle twitch whereas tetanus is a combination of multiple contractions to create persistent contractions.