Question

Muscle contraction is initiated by a signal sent by the central nervous system by the release of __________.

• A. cyclic adenine monophosphate
• B. acetyl choline
• C. acetyl coenzyme A
• D. cyclic guanine monophosphate

Hint:

• Somatic Neurotransmitter: Acetylcholine is the universal neurotransmitter utilized by somatic motor neurons to activate skeletal muscle contraction.

• Signal Termination: To ensure that the muscle does not remain in a state of continuous contraction, the enzyme acetylcholinesterase (AChE) rapidly hydrolyzes acetylcholine in the synaptic cleft into acetic acid and choline, allowing the muscle fiber to repolarize and relax.

Answer 1

The Correct Option is B

Step 1: Frame the question.
The central nervous system sends a signal to start muscle contraction by releasing a chemical, and we need to name that chemical.
Step 2: Recall the junction.
A motor neuron meets a muscle fibre at the neuromuscular junction. The electrical signal must cross this gap as a chemical messenger.
Step 3: Trace the release.
When the action potential reaches the axon terminal, $\text{Ca}^{2+}$ rushes in, vesicles fuse with the membrane, and a neurotransmitter is released into the synaptic cleft.
Step 4: Name that neurotransmitter.
At skeletal muscle junctions the released neurotransmitter is acetylcholine, which binds receptors on the sarcolemma and triggers contraction.
Step 5: Rule out the others.
Cyclic AMP and cyclic GMP are intracellular second messengers, and acetyl coenzyme A is a respiration intermediate. None of these is the synaptic signal.
Step 6: Conclude.
The signalling molecule is acetyl choline, which is option (2).
\[ \boxed{\text{acetyl choline}} \]