Question

Stimulation of a muscle fiber by a motor neuron occurs at
A. The neuromuscular junction
B. The transverse tubules
C. The myofibril
D. The sarcoplasmic reticulum

Answer 1

Hint: The neurons that innervate the muscle transmit a signal to the muscle fibers for the initiating of muscle contraction is called motor neuron. Stimulation of a muscle fiber by a motor neuron at a structure which is divided into three parts: Presynaptic, Synaptic cleft and Postsynaptic portions.

Complete answer:
The neuromuscular junction -The neuromuscular junction (NMC) is the connection between motor neurons and muscle fibers through which action potentials from neurons are transmitted to muscle fibers.
A motor unit consists of a motor neuron and muscle fibers it innervates. A muscle fiber is under the control of a single neuron.
Transverse tubule-The T-tubule (transverse tubule) is an extension of the cell membrane that penetrates into the centre of the skeletal and cardiac muscle cells. T-tubules are tubules formed by the surface membrane of skeletal or cardiac muscle cells or by a phospholipid bilayer identical to the sarcolemma.
Myofibrils-These are long, thread-like proteinaceous fibrils present in the sarcoplasm of muscular tissue.
Sarcoplasmic reticulum-The sarcoplasmic reticulum (SR) is a membrane-bound structure of muscle cells that resembles the endoplasmic reticulum of other cells. The main function of SR is to store calcium ions

Hence, from the above points it’s clear that the correct option is neuromuscular junction (A).

Note:
When the motor nerve approaches neuromuscular junction, it loses its myelin sheath and the exposed nerve expands. This extension is known as the area of ​​the foot, or telodendria axon, or terminal button. This part of the muscle membrane is an end plate bent into several folds. These folds contain the acetylcholine receptor site which contains receptors for acetylcholine. Wrinkles increase the area of ​​action of acetylcholine.