Final answer:
An action potential in a muscle fiber triggers a Ca2+ signal through the depolarization of the muscle fiber membrane, which opens voltage-gated sodium channels, prompting the release of Ca2+ from the sarcoplasmic reticulum. The Ca2+ binds to troponin and initiates muscle contraction.
Step-by-step explanation:
When a motor neuron releases the neurotransmitter acetylcholine (ACh), it binds to receptors on the muscle fiber causing depolarization. This depolarization opens voltage-gated sodium channels, allowing Na+ ions to enter and trigger an action potential that rapidly spreads along the sarcolemma and into the T-tubules.
The action potential traveling down the T-tubules then prompts the adjacent sarcoplasmic reticulum (SR) to open its calcium channels. This results in Ca2+ ions diffusing out of the SR and into the sarcoplasm. When these Ca2+ ions bind to troponin, it leads to a conformational change that moves tropomyosin away from the binding sites on actin, thus allowing the myosin heads to attach to actin and initiate muscle contraction.
In smooth muscle cells, the process is slightly different due to their smaller diameter; T-tubules are not necessary to transmit the action potential deep into the fiber. Instead, calcium channels located in the sarcolemma open during the action potential phase, resulting in an influx of Ca2+ from outside the cell, which then activates contraction.