Final answer:
The T-tubular membrane contains voltage-sensitive receptors that are linked to calcium receptors in the sarcoplasmic reticulum, which respond to action potentials and release calcium, initiating muscle contraction.
Step-by-step explanation:
The T-tubular membrane contains voltage-sensitive receptors that are responsive to action potentials. This receptor is mechanically linked to calcium receptors in the adjacent sarcoplasmic reticulum (SR). An action potential travels down the T-tubules and triggers the opening of these calcium channels in the membrane of the adjacent SR. This results in the release of Ca2+ from the SR into the sarcoplasm, which is critical for initiating muscle fiber contraction by the contractile units, or sarcomeres. These processes are part of what is known as excitation-contraction coupling, a physiological mechanism where an electrical stimulus leads to muscle contraction. Acetylcholine released from the axon terminal binds to receptors on the sarcolemma, generating an action potential that propagates down the T-tubules, ultimately causing the release of calcium ions from the SR and leading to muscle contraction.