Question

A substance called a ______ crosses the synaptic cleft and stimulates the muscle fiber to contract.

A) Acetylcholine
B) Dopamine
C) Serotonin
D) Endorphin

Answer 1

Acetylcholine (ACh) is the substance that stimulates the muscle fiber to contract.

Step-by-step explanation:

Acetylcholine (ACh) is the substance that crosses the synaptic cleft and stimulates the muscle fiber to contract. When a nerve impulse travels down the motor neuron's axon terminal, it causes the release of ACh from synaptic vesicles. The ACh then travels across the synaptic cleft and binds to ACh receptors on the muscle fiber, signaling the muscle fiber to contract.