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Intro to Anatomy 6: Tissues, Membranes, Organs
Intro to Anatomy 6: Tissues, Membranes, Organs
The Lone Ranger
Published by The Lone Ranger
Default Muscle Tissue

Muscle Tissue:
Muscle tissue is specialized for contraction. Contraction of muscle tissues moves parts of the body and also moves substances within the body. Contraction of muscles also helps to generate body heat.

The cells of muscle tissues are usually elongated in shape, and contain large amounts of the proteins actin and myosin. We’ll discuss how these proteins interact to cause muscle contraction when we consider the muscular system.

There are three different kinds of muscle tissue found in humans and other vertebrates. These are skeletal (voluntary) muscle, smooth muscle, and cardiac muscle.

Skeletal Muscle Tissue:
Skeletal muscles, as the name implies, are typically attached to bones. When they contract, they move the bones to which they’re attached. Since the bones form the framework of the body, when a bone moves, the body part for which it forms the framework moves as well. Skeletal muscles are the only muscles that are under our voluntary control.

Skeletal Muscle Fibers
Note that each cell has many nuclei.
The striations are caused by alternating bands
of actin and myosin. The dark bands
(A bands) are thick filaments of
myosin with thinner fibers of actin between
them, and the light bands (I bands) are
actin fibers extending beyond the A bands.

Under a microscope, the cells of skeletal muscles are elongated and cylinder-shaped. Each cell has several nuclei (not just one nucleus per cell, as in most other tissues), and the arrangement of actin and myosin fibers gives the cell a distinctly striated appearance. (This is why skeletal muscles are sometimes known as striated muscles.)

Skeletal muscle cells can contract quickly and with a great deal of force, but they don’t relax completely between contractions. (The fact that your skeletal muscles are always partially contracted contributes to muscle tone.) Because they don’t relax completely between contractions, skeletal muscles can experience fatigue if forced to contract to often or for too long. This happens because the cells can use up their available oxygen and so lactic acid levels begin to build up. If lactic acid levels become too high, the muscle fatigues and stops functioning.

Skeletal muscles actually contain two different types of fibers. The proportion of the two fiber types depends upon the kinds of exercise you do.

Red fibers (“slow-twitch fibers”) contain more mitochondria, store oxygen in the protein myoglobin (myoglobin is very similar to hemoglobin as you might imagine, and, like hemoglobin, is red in color), and rely on aerobic respiration to produce energy. They metabolize ATP relatively slowly and contract relatively slowly, but have great endurance. Marathon runners tend to have lots of red fibers in their skeletal muscles.

White fibers (“fast-twitch fibers”) have fewer mitochondria, metabolize ATP more quickly, and contract more quickly and more forcefully. On the other hand, they're more likely to exhaust their oxygen supplies and so produce lactic acid, leading to muscle fatigue. Weight-lifters and sprinters tend to have lots of white fibers in their skeletal muscles.

The cylindrical cells of skeletal muscles form muscle fibers that are bound together by connective tissues to form bundles of muscle fibers. In turn, the muscle fiber bundles are bound together to form muscles that are attached to bones by tendons.

Skeletal Muscle Structure

Smooth Muscle Tissue:
Smooth muscles don’t have obvious striations when observed under a microscope. These muscles are involuntary, meaning that they aren’t under your conscious control. They line the digestive tract and other hollow body tubes, and their contractions help to move substances through these tubes.

Smooth muscle tends to be laid down in sheets, and the cells are spindle-shaped instead of elongated and cylindrical. Each cell has only a single nucleus.

Smooth muscles contract slowly and with relatively little force, but unlike skeletal muscles, they don’t fatigue. The cells within a sheet of smooth muscle tend to coordinate their contractions, so “waves” of contraction move through a sheet of smooth muscle, which helps to push substances through body tubes efficiently, if not very quickly.

Smooth Muscle of the Intestinal Wall
The outermost layer of smooth muscle is oriented longitudinally, meaning that the cells run down the length of the intestine. When these cells contract, they tend to shorten the intestine. The inner layer of smooth muscle is oriented circularly -- that is, around the intestine. When these circular muscles contract, they cause the intestine to become narrower. Coordinated contraction of the longitudinal and circular muscles squeezes material through the intestine in much the same way you squeeze toothpaste out of the tube.

Cardiac Muscle Tissue:
Cardiac muscle makes up the bulk of the heart, and its properties are rather like a blending of the properties of skeletal muscles and smooth muscles. Like skeletal muscle, it is striated and contracts quickly and forcefully; like smooth muscle, it relaxes completely between contractions and is involuntary. (It’s fortunate for us that it is! Imagine what would happen if you had to lie awake all night remembering to make your heart beat!)

The cells of cardiac muscle are cylindrical in shape and striated, like those of skeletal muscles. Like the cells of smooth muscle, each cardiac muscle cell has only a single nucleus. Unlike the cells of either skeletal muscle or smooth muscle, cardiac muscle cells are branched – this makes them look “Y” shaped under a microscope.

Because cardiac muscle relaxes completely between contractions, it does not fatigue as skeletal muscle does. (Imagine what would happen if your heart simply stopped working if you demanded too much of it!) Individual cells in cardiac muscle are connected by thin structures called intercalated disks, which appear to transmit impulses between the cells so that the heart muscle contracts as a unit. [This is sometimes misinterpreted. According to the very bad movie The Amazing Colossal Man (I saw it on Mystery Science Theater 3000), the entire heart muscle is made up of just one cell. That’s complete nonsense! A man growing to 50 feet in height is physiologically impossible for many reasons, but not because the heart contains only a single cell!]

Like all muscle cells, cardiac muscle cells are surrounded by a network of collagen fibers that knit the cells together. Cardiac muscle cells aren’t so tightly bound together as are skeletal muscle cells, however.

Cardiac Muscle Cells
Note the striations, similar to those in skeletal muscle cells. A cardiac muscle cell has only a single nucleus, however. You can see that the cardiac muscle cells are branched and that cells are connected to those above and below by thin intercalated disks. Collagen fibers between adjacent cells bind them together.


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