A&P FLix Video Transcript

This is a transcript of the video on the Student Features page.

Muscle Contraction

Part 2: Excitation-Contraction Coupling

Typically, a single motor neuron arising in the brain or spinal cord conducts action potentials that travel to hundreds of skeletal muscle fibers within a muscle.

The sequence of events that converts action potentials in a muscle fiber to a contraction is known as excitation-contraction coupling.

If we look at a single muscle fiber, we see that an action potential travels across the entire sarcolemma, and is rapidly conducted into the interior of the muscle fiber by structures called transverse tubules.

Transverse, or t-tubules, are regularly spaced in folding's of the sarcolemma that branch extensively throughout the muscle fiber.

At numerous junctions, the t-tubules make contact with the calcium storing membranous network known as the sarcoplasmic reticulum, or SR.

Where it abuts the t-tubule, the SR forms sac-like bulges called terminal cisternae.

One portion of the t-tubule plus two adjacent terminal cisternae is known as a triad.

The membranes of the t-tubule and terminal cisternae are linked by a series of proteins that control calcium release.

As an action potential travels down the t-tubule, it causes a voltage-sensitive protein to change shape. This shape change opens a calcium release channel in the SR, allowing calcium ions to flood the sarcoplasm.

This rapid influx of calcium triggers a contraction of the skeletal muscle fiber. Thus, calcium ions are responsible for the coupling of excitation to the contraction of skeletal muscle fibers.