Triggering of the muscle action potential occurs after which of the following?
Select one:
a calcium ion binds to channels on the motor end plate
b. acetylcholine binds to chemically gated channels on the motor end plate
c. the action potential jumps across the NMJ
d. acetylcholinesterase is released from synaptic vesicles into the synaptic cleft
e. Any of these can produce an action potential in the muscle cell.
The Correct Answer and Explanation is :
Correct answer:
b. Acetylcholine binds to chemically gated channels on the motor end plate.
Explanation
The initiation of a muscle action potential begins at the neuromuscular junction (NMJ), where a motor neuron communicates with a skeletal muscle fiber. This process involves a series of steps that lead to muscle contraction.
- Release of Acetylcholine (ACh): When an action potential travels down the motor neuron, it reaches the axon terminal. This triggers voltage-gated calcium (Ca²⁺) channels to open, allowing Ca²⁺ to enter the neuron. The influx of Ca²⁺ causes synaptic vesicles to release acetylcholine into the synaptic cleft via exocytosis.
- Binding of Acetylcholine to Receptors: Acetylcholine then diffuses across the synaptic cleft and binds to chemically gated (ligand-gated) ion channels on the motor end plate of the muscle fiber. These receptors are nicotinic acetylcholine receptors, which allow sodium (Na⁺) ions to enter the muscle cell when activated.
- Depolarization and Muscle Action Potential: As Na⁺ enters the muscle fiber, the cell membrane becomes depolarized, generating a local end-plate potential. If this depolarization reaches the threshold level, it will trigger a muscle action potential that spreads along the sarcolemma and into the T-tubules, leading to muscle contraction.
Why the Other Answers Are Incorrect
- (a) Calcium ion binds to channels on the motor end plate: Calcium plays a role in neurotransmitter release at the presynaptic terminal but does not directly bind to receptors on the motor end plate to trigger the action potential.
- (c) The action potential jumps across the NMJ: The action potential does not “jump” across the synaptic cleft; instead, neurotransmitters carry the signal chemically across the gap.
- (d) Acetylcholinesterase is released from synaptic vesicles: Acetylcholinesterase breaks down acetylcholine to terminate the signal but does not initiate the action potential.
- (e) Any of these can produce an action potential: Only acetylcholine binding to its receptor can initiate the muscle action potential.
Thus, option (b) is correct because the binding of acetylcholine to chemically gated channels is the key step in triggering a muscle action potential.