The smallest functional unit of muscle fibre is

Question

Accepted Answer

The correct option is (2):
The **Sarcolemma** is the specialized cell membrane that surrounds every muscle fiber. This option is considered correct when defining the fundamental unit necessary for the entire muscle *fiber's* operation. The sarcolemma’s primary function is to receive and conduct the electrochemical signal, known as the action potential, from the motor neuron deep into the muscle cell via the transverse tubules. This is the critical first step—the trigger—that activates the entire cell’s machinery. Without the sarcolemma, the muscle fiber would be unable to sense or respond to nerve stimuli, making it the essential, functional outer boundary required for the muscle fiber to perform its role.

The incorrect option is (1):
The **Sarcomere** is widely recognized as the smallest *contractile* unit, containing the actin and myosin filaments that slide past each other to physically shorten the muscle. However, a single sarcomere is merely an **internal component** of a much larger muscle fiber, a repeating unit within the myofibril. It cannot function unless the entire muscle fiber is stimulated by an action potential, which is initiated by the sarcolemma. Therefore, while the sarcomere is the anatomical *engine* of contraction, the sarcolemma is the necessary *ignition system* that allows the entire muscle fiber to function as a responsive, excitable cell.

The incorrect option is (3):
The **Sarcoplasm** is simply the cytoplasm of the muscle cell. It is the fluid matrix enclosed by the sarcolemma, which contains various organelles, proteins, and stored nutrients like glycogen and oxygen (bound to myoglobin). While the sarcoplasm is the medium in which all the functional components, such as myofibrils and the sarcoplasmic reticulum, reside, it is a **substance** or **medium**, not a defined, structural *unit* that performs or initiates a specific physiological function like signal conduction or contraction.

The incorrect option is (4):
The option "All options are wrong" is incorrect because the question asks for the smallest functional unit of the *muscle fibre*. The muscle fiber is a cell, and its components include the sarcolemma (cell membrane), sarcoplasm (cytoplasm), and sarcomeres (contractile units). Since the sarcolemma represents the functional boundary and the initial point of excitability for the entire fiber, a correct and highly relevant answer is present among the choices.