To stimulate muscle contraction, acetylcholine is released from the:

• A. synaptic cleft
• B. myofibrils within the muscle cell
• C. axon terminal of a motor neuron
• D. sarcolemma of the muscle fiber

Answer: (C)

Verified step by step guidance

1. Understand the context of the problem: Muscle contraction is initiated by the release of a neurotransmitter called acetylcholine, which plays a key role in transmitting signals from the nervous system to the muscle fibers.
2. Identify the structure responsible for releasing acetylcholine: Acetylcholine is released from the axon terminal of a motor neuron, which is part of the neuromuscular junction. This junction is the site where the motor neuron communicates with the muscle fiber.
3. Clarify the role of the synaptic cleft: The synaptic cleft is the small gap between the axon terminal of the motor neuron and the sarcolemma (membrane) of the muscle fiber. Acetylcholine diffuses across this cleft to bind to receptors on the sarcolemma.
4. Eliminate incorrect options: Myofibrils are structures within the muscle cell responsible for contraction, but they do not release acetylcholine. The sarcolemma is the muscle fiber's membrane that receives acetylcholine, not the structure that releases it.
5. Conclude the correct answer: The axon terminal of a motor neuron is the structure that releases acetylcholine to stimulate muscle contraction, ensuring the signal is transmitted effectively to the muscle fiber.