Question

In skeletal muscle, which of the following events occurs before depolarization of the T tubules in the mechanism of excitation-contraction coupling?

1) Depolarization of the sarcolemmal membrane
2) Opening of Ca₂⁺ release channels on the sarcoplasmic reticulum (SR)
3) Uptake of Ca₂⁺ into the SR by Ca₂⁺-adenosine triphosphatase (ATPase)
4) Binding of Ca₂⁺ to troponin C
5) Binding of actin and myosin

Answer 1

The event that occurs before depolarization of the T-tubules in skeletal muscle during excitation-contraction coupling is the depolarization of the sarcolemmal membrane.

Step-by-step explanation:

The sequence of events leading to muscle contraction begins with the excitation of the muscle fiber membrane, known as the sarcolemma. This is achieved by the generation of an action potential triggered by the neurotransmitter acetylcholine (ACh), leading to the depolarization of the sarcolemma. Depolarization of the sarcolemmal membrane is key as it precedes the depolarization of the T-tubules, allowing the action potential to reach deep into the muscle fiber.

As the action potential travels along the T-tubules, it then causes the opening of calcium channels in the sarcoplasmic reticulum (SR). Calcium ions (Ca2+) are thus released into the sarcoplasm, enabling them to bind to troponin on the actin filament, which initiates muscle fiber contraction. Therefore, the correct sequence would be: Depolarization of the sarcolemmal membrane, followed by depolarization of the T-tubules, then the opening of Ca2+ release channels on the SR, uptake of Ca2+ into the SR by calcium ATPase, binding of Ca2+ to troponin C, and finally the binding of actin and myosin.

The event that occurs before depolarization of T-tubules is depolarization of the sarcolemmal membrane, corresponding to option 1).