T-tubules are unique to the muscle fiber and are actually “folds” or invaginations of the sarcolemma which penetrate into the center of the muscle fiber.

The t-tubules are also surrounded by the sarcoplasmic reticulum, which is responsible for storing and pumping calcium ions.

The pictures below show the t-tubules, running perpendicular to the axis of the muscle fibers, as well as the sarcoplasmic reticulum surrounding it.

For a much more elaborate portrait of the muscle cell, here is an interesting graphic. This is a cross-sectional view, i.e. if you were to cut a fiber in half and look at the inside from head-on, this is the perspective you would have:

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A few things:

• Ignore all the oddly shaped trans-membrane proteins at the top of the fiber. Those are outside the scope of this introduction.
• Notice the lipid bi-layer, i.e. sarcolemma, surrounding the cell, with the sarcomere running down the center.
• Also, the light blue space enclosed, denoting the sarcoplasm.

Again, a sarcomere is more or less just the arrangement of one set of thick and thin filaments.

Greatest physiology rap ever…

http://youtu.be/xhgDbjrrmFg

First, it’s important to understand that the mechanism of skeletal muscle contraction is still not completely understood, or all the components identified.

Still, the sliding filament theory is the most well-supported model. In simplest terms, myosin acts like a sort of ratchet, bonding to actin and pulling itself along the actin chain, which creates tension in the muscle. This is believed to be what occurs when the muscle is contracting.

Here is a more biological representation of the “sliding filaments”

Here’s what scientists figure it looks like in there:
http://youtu.be/Ct8AbZn_A8A

However, a major difference here is that muscle fiber cells have more than one nucleus per cell.

The top left graphic shows a rough outline of the muscle fiber and all its nuclei.

This microscopic slide of a patch of muscle fibers shows all the seemingly random nuclei, stained purple.

Even with this rough overview, you can see there are a lot of different things going on in the muscle fiber for it to function properly.

Cations are positively charged ions, i.e. molecules which have lost an electron. In this case we are talking about sodium and potassium.

Here is a picture of ACh bonding to receptors on the motor end plate.

Skeletal has the capacity to stretch beyond it’s natural length (extensibility), but in the absence of any resistance, it will quickly resume its original position. This is the property of elasticity.

However, just like a rubberband, if a muscle is stretched to the point of altering its natural position, it could come at the cost of producing force.