Final answer:
An action potential at motor nerve endings causes an increase in sodium (Na+) conductance by activating voltage-dependent sodium channels, facilitating communication between nerves and muscles.
Step-by-step explanation:
An action potential at motor nerve endings causes a transient increase in sodium (Na+) conductance by activating voltage-dependent sodium channels. When the membrane depolarizes, voltage-gated sodium channels open, allowing Na+ ions to enter the muscle fiber. This triggers an action potential that rapidly spreads along the entire membrane, initiating the process known as excitation-contraction coupling. Additionally, acetylcholine (ACh) is released from the motor neuron, binds to receptors on the muscle cell, and further opens sodium ion channels, strengthening the development of an action potential in the muscle's sarcolemma.