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Muscle Functions

Muscle plays four important roles in the body:

1. Producing Movement

Just about all movements of the human body are a result of muscle contraction. Mobility of the body as a whole reflects the activity of skeletal muscles, which are responsible for all locomotion and manipulation. These are distinct from the smooth muscle of blood vessel walls and cardiac muscle of the heart, which work together to circulate blood and main­tain blood pressure, and the smooth muscle of other hollow organs, which forces fluids and other substances through internal body channels.

2. Maintaining Posture

We are rarely aware of the workings of the skeletal muscles that maintain body posture. Yet, they func­tion almost continuously, making one tiny adjust­ment after another so that we can maintain an erect or seated posture despite the never-ending down­ward pull of gravity.

3. Stabilizing Joints

As the skeletal muscles pull on bones to cause movements, they also stabilize the joints of the skeleton. Indeed, muscle tendons are extremely important in reinforcing and stabilizing joints that have poorly fitting articulating surfaces (the shoul­der joint, for example).

4. Generating Heat

The fourth function of muscle, generation of body heat, is a by-product of muscle activity. As ATP is used to power muscle contraction, nearly three-quarters of its energy escapes as heat. This heat is vital in maintaining normal body temperature. Since skeletal muscle accounts for at least 40 per­cent of body mass, it is the muscle type most responsible for heat generation.

Skeletal Muscle Activity

Muscle cells have some special functional properties that enable them to perform their function. The first of these is stimulation, the ability tot receive and respond to a stimulus. The second, contractility, is the ability to shorten when an adequate stimulus is received.

The Nerve Stimulus and the Action Potential

Skeletal muscle cells must be stimulated by nerve impulses to contract. One motor neuron (nerve cell) may stimulate a few muscle cells or hundreds of them, depending on the particular muscle and the work it does. One neuron and all the skeletal muscle cells it stimulates are a motor unit. When a long threadlike extension of the neu­ron, called the nerve fiber or axon, reaches the muscle, it branches into a number of axon terminals, each of which forms junctions with the sarcolemma of a different muscle cell. These junctions are called neuromuscular junctions. Although the nerve endings and the muscle cells membranes are very close, they never touch. The gap between them, the synaptic cleft, is filled with tissue (interstitial) fluid.

When the nerve impulse reaches the axon terminals, a chemical referred to as a neurotransmitter is released. The specific neurotransmitter that stimulates skeletal muscle cells is acetylcholine or ACh. Acetylcholine diffuses across the synaptic cleft and attaches to receptors (membrane proteins) that are part of the sarcolemma. If enough acetylcholine is released, the sarcolemma at that point becomes temporarily more permeable to sodium ions (Na⁺), which rush into the muscle cell and to potassium ions (K⁺) which diffuse out of the cell. However, more Na⁺ enters than K⁺ leaves. This gives the cell interior an excess of positive ions, which reverses the electrical conditions of the sar­colemma and opens more channels that allow Na⁺ entry only. This generates an electri­cal current called an action potential. Once begun, the action potential is unstoppable; it trav­els over the entire surface of the sarcolemma, con­ducting the electrical impulse from one end of the cell to the other. The result is contraction of the muscle cell.

It should be mentioned that while the action potential is occurring, acetylcholine, which began the process, is broken down to acetic acid and choline by enzymes (acetylcholinesterase, or AchE) present on the sarcolemma. For this reason, a single nerve impulse produces only one contraction. This prevents continued contrac­tion of the muscle cell in the absence of additional nerve impulses. The muscle cell relaxes until stim­ulated by the next round of acetylcholine release.