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

Concept:

• Somatic Motor Signaling: Skeletal muscles are under voluntary control, with contraction initiated by neural signals generated in the Central Nervous System (CNS) and transmitted along somatic motor neurons.

• The Motor Unit: A single motor neuron and the group of skeletal muscle fibers it innervates function together as a unified motor unit.

• Neuromuscular Junction (NMJ): The specialized chemical synapse between the terminal end of a motor neuron and the motor end-plate region of the muscle fiber's sarcolemma (plasma membrane).

• Synaptic Transmission: At the NMJ, electrical action potentials are converted into chemical signals through the release of a specific neurotransmitter into the synaptic cleft.

Step 1: Trace the chemical sequence at the neuromuscular junction
When an action potential reaches the presynaptic axon terminal of the motor neuron:

• Voltage-gated calcium channels open, allowing calcium ions (\(\text{Ca}^{2+}\)) to enter the axon terminal.

• This influx triggers the fusion of synaptic vesicles with the presynaptic membrane, releasing

acetylcholine (ACh) into the synaptic cleft via exocytosis.

• ACh molecules diffuse across the cleft and bind to ligand-gated nicotinic acetylcholine receptors on the sarcolemma.

Step 2: Evaluate the options against the mechanism of action

• (A) cyclic adenine monophosphate (cAMP): An intracellular second messenger involved in signal transduction cascades, not a primary synaptic neurotransmitter.

• (B) acetyl choline: The primary neurotransmitter released at somatic motor synapses that depolarizes the sarcolemma to trigger skeletal muscle contraction.

• (C) acetyl coenzyme A (acetyl-CoA): A key biochemical intermediate in cellular respiration (Krebs cycle) and lipid metabolism, rather than a neural signaling molecule.

• (D) cyclic guanine monophosphate (cGMP): An intracellular second messenger that regulates cellular pathways, such as smooth muscle relaxation, but is not responsible for initiating skeletal muscle contraction at the NMJ.
Thus, acetylcholine is the neurotransmitter responsible for initiating the process, corresponding to option (B).