Excitation–Contraction Coupling II

What starts excitation–contraction coupling?

An action potential travels down a motor neuron to its axon terminal at the neuromuscular junction.

What happens when the action potential reaches the motor-neuron terminal?

Voltage-gated Ca²⁺ channels open, allowing Ca²⁺ to enter the neuron.

What does Ca²⁺ do in the motor-neuron terminal?

It causes acetylcholine-containing vesicles to fuse with the presynaptic membrane.

What is released when the vesicles fuse with the presynaptic membrane?

Acetylcholine, or ACh, is released into the synaptic cleft.

What does ACh bind to on the muscle fiber?

ACh binds to nicotinic acetylcholine receptors, or nAChRs, on the motor end plate.

What type of receptor is an nAChR?

A ligand-gated ion channel that opens when ACh binds.

What happens when nAChRs open?

Mainly Na⁺ enters the muscle fiber, depolarizing the muscle membrane.

What happens when muscle-membrane depolarization reaches threshold?

Voltage-gated Na⁺ channels open and create a muscle action potential.

Where does the muscle action potential travel?

Across the sarcolemma and down the T-tubules.

What does T-tubule depolarization do?

It causes the dihydropyridine receptor, DHPR, to change shape.

Where is DHPR located?

In the T-tubule membrane.

What is RyR?

The ryanodine receptor, a Ca²⁺ release channel in the sarcoplasmic reticulum membrane.

How does DHPR cause RyR to open?

In skeletal muscle, DHPR is mechanically coupled to RyR. DHPR’s conformational change opens RyR.

What happens when RyR opens?

Ca²⁺ leaves the sarcoplasmic reticulum and enters the cytoplasm of the muscle fiber.