If an individual nerve fibre plus all the muscle fibres are innervates nerve then it is called:

Question

Accepted Answer

The correct option is (2):
A Motor Unit is the fundamental structural and functional unit of neuromuscular control. It is precisely defined as a single motor neuron (an individual nerve fiber) and *all* of the individual muscle fibers that this neuron directly innervates. This collective unit acts as one functional group; when the motor neuron fires an action potential, all of the muscle fibers within its unit contract simultaneously, following the all-or-none principle. The size of the motor unit dictates the level of control: fine, delicate movements are performed by muscles with small motor units (fewer muscle fibers per nerve), while powerful, gross movements rely on large motor units (many muscle fibers per nerve). Therefore, the provided definition—an individual nerve fiber plus all the muscle fibers it innervates—is the precise anatomical and physiological description of a motor unit.

The incorrect option is (1):
Synaptic transmission is the *process* by which signaling molecules, called neurotransmitters, are released by the axon terminal and bind to receptors on another cell, in this case, the muscle fiber. This is the chemical event that allows the nerve impulse to successfully jump the synaptic cleft (the tiny gap) between the nerve ending and the muscle fiber. While synaptic transmission is the vital action that initiates muscle contraction within the motor unit, it describes *how* the signal is sent, not the structural grouping (*what* it is) of the nerve fiber and its muscle fibers.

The incorrect option is (3):
The Motor endplate, more accurately referred to as the neuromuscular junction, is the specialized area of the muscle fiber membrane where the axon terminal of the motor neuron connects. It is the postsynaptic side, a receiver site that contains the specific receptors for the neurotransmitter released by the nerve. Although the motor endplate is an absolutely essential part of the motor unit, it is only a single, small *location* on the surface of *one* muscle fiber. It does not encompass the entire nerve fiber, nor all the muscle fibers the nerve controls, making it an incomplete definition.

The incorrect option is (4):
Reciprocal innervation is a coordination *mechanism* controlled by the central nervous system (CNS), not a structural unit. It is the principle by which the contraction of the agonist (prime mover) muscle is accompanied by the simultaneous relaxation or inhibition of the antagonist (opposing) muscle. This neural reflex is critical for producing smooth, efficient, and coordinated movement patterns by preventing the opposing muscle groups from working against each other. Therefore, it is a neurophysiological *principle of coordination* between multiple motor units, rather than the definition of a single motor unit itself.