What is released by axon terminals into the synaptic cleft to stimulate a muscle to contract?

• A. Serotonin
• B. Dopamine
• C. GABA
• D. Acetylcholine

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the context of the question. Axon terminals are part of neurons, and they release specific chemical messengers called neurotransmitters into the synaptic cleft to communicate with other cells, such as muscle cells.
2. Step 2: Recall the role of acetylcholine in muscle contraction. Acetylcholine is a neurotransmitter that binds to receptors on the muscle cell membrane, initiating a cascade of events that lead to muscle contraction.
3. Step 3: Differentiate acetylcholine from other neurotransmitters mentioned in the question. Serotonin, dopamine, and GABA are neurotransmitters, but they are primarily involved in other functions, such as mood regulation, reward signaling, and inhibitory signaling in the nervous system, respectively.
4. Step 4: Focus on the mechanism of acetylcholine release. When an action potential reaches the axon terminal, it triggers the release of acetylcholine into the synaptic cleft via exocytosis.
5. Step 5: Understand the importance of acetylcholine binding to receptors. Acetylcholine binds to nicotinic receptors on the muscle cell membrane, leading to depolarization and subsequent muscle contraction.