Final answer:
When an action potential reaches the axon terminal in the sarcolemma, it depolarizes the membrane and opens voltage-gated sodium channels, allowing sodium ions to enter the cell.
Step-by-step explanation:
When an action potential reaches the axon terminal in the sarcolemma, it depolarizes the membrane and opens voltage-gated sodium channels, allowing sodium ions to enter the cell. This further depolarizes the presynaptic membrane, causing voltage-gated calcium channels to open. The entry of calcium ions initiates a signaling cascade that leads to the fusion of synaptic vesicles containing neurotransmitter molecules with the presynaptic membrane. The neurotransmitters are then released into the synaptic cleft and bind to receptors on the sarcolemma, generating an action potential and ultimately leading to muscle contraction.