The Anatomy & Physiology of a Muscle Contraction

Muscle contractions are the result of small filaments compressing.

There are approximately 650 skeletal muscles in the human body. A muscle contraction is the act by which muscles generate tension and force. Each posture of the body may be the result of both a contraction and relaxation of different muscles.

Anatomy of a Muscle

Muscles are a conglomeration of fibers wrapped together into large bundles. Each fiber is made up of thread-like myofibrils, and spanning the length of myofibrils are sarcomeres, which are the basic units of contraction. Sarcomeres are linked to each other by proteins called a Z band.

Filaments

A series of thick filaments called myosin and thin filaments called actin contract within the sarcomeres. Myosin is a motor protein that works upon actin, a globular protein. Myosin has small heads that extend from the filament and rest only nanometers (a billionth of a meter) away from the actin.

Sacromeres

Myosin and actin alternate positions along the sacromeres. There is a small gap between the myosin and the Z band at the end of the sarcomere. This is called an I band, and it is necessary because a contraction will actually pull the Z band closer to the myosin. Actin is attached to both ends of the Z band but has a little gap in the middle called an H zone. Once again, the gap ensures that a muscle contraction will draw actin close to each other without touching. The areas where myosin and actin overlap are called the A band.

Contraction

Muscle contraction begins when muscle cells receive signals from motor neurons, which trigger the release of calcium from a part of the cell that is called the sarcoplasmic reticulum. The increase of calcium exposes sites on the actin to which myosin heads can now bind. These heads tilt toward the center, drawing in the actin. This contracts the sarcomere and, as a consequence, the entire muscle. This is called the sliding-filament model.

Reset

The myosin heads only release their grip on the actin when they bind with ATP, or adenosine triphosphate. ATP is a carrier for chemical energy that's produced in the metabolic process. The sarcomere is now relaxed and free to contract once more.

References

• University of Tennessee-Knoxville: Muscle Contraction