Which substance acts at the NMJ to excite the muscle fibers of a motor unit?

Which substance acts at the NMJ to excite the muscle fibers of a motor unit?

• A. Acetylcholine
• B. ATP
• C. Creatine phosphate
• D. Serotonin

Answer: (A)

The primary signal from a motor neuron to skeletal muscle fibers at the neuromuscular junction is acetylcholine. When an action potential arrives at the motor neuron terminal, acetylcholine is released into the synaptic cleft and binds to nicotinic receptors on the motor end plate. This opens cation channels, depolarizing the muscle membrane to create an end-plate potential. If the depolarization is strong enough, it triggers a muscle action potential that travels along the membrane and into the T-tubules, leading to calcium release from the sarcoplasmic reticulum and ultimately muscle contraction. The other substances listed don’t serve this rapid excitatory role at the NMJ: ATP is mainly an energy molecule (and sometimes a co-transmitter in other contexts), creatine phosphate acts as an energy reserve, and serotonin is a neurotransmitter primarily involved in other systems, not the skeletal NMJ excitation.

The primary signal from a motor neuron to skeletal muscle fibers at the neuromuscular junction is acetylcholine. When an action potential arrives at the motor neuron terminal, acetylcholine is released into the synaptic cleft and binds to nicotinic receptors on the motor end plate. This opens cation channels, depolarizing the muscle membrane to create an end-plate potential. If the depolarization is strong enough, it triggers a muscle action potential that travels along the membrane and into the T-tubules, leading to calcium release from the sarcoplasmic reticulum and ultimately muscle contraction. The other substances listed don’t serve this rapid excitatory role at the NMJ: ATP is mainly an energy molecule (and sometimes a co-transmitter in other contexts), creatine phosphate acts as an energy reserve, and serotonin is a neurotransmitter primarily involved in other systems, not the skeletal NMJ excitation.