Muscles

There are three different types of muscle tissue. These include the skeletal muscles which are under voluntary control and are made of striated muscle tissue, the visceral muscles which are under involuntary control and are made of smooth muscle tissue, and cardiac muscle tissue which is found only in the heart. Cardiac muscle tissue has properties in common with each of the other two types. An interesting exception to the visceral muscles being under involuntary control is the diaphragm, which controls breathing. This muscle is normally under involuntary control (we don’t usually have to think about breathing), but a person can exert a limited amount of voluntary control also (for example, purposely holding one’s breath or breathing quickly or deeply).

Muscle cells contain filaments of two kinds of proteins, actin and myosin, which slide past each other as the muscle contracts. After a muscle contracts, ATP (produced in the muscle cells’ mitochondria) is needed to relax the muscle and return the actin and myosin filaments to their normal positions. When a person (or other animal) dies and the mitochondria are no longer producing ATP, the muscles cannot relax. This stiffening of the muscles is called rigor mortis. Muscle contraction is initiated when an electrical impulse from a nerve cell reaches its associated muscle cell(s), causing positively- and negatively-charged ions to switch places all along the muscle cell (fiber). Movement of Ca⁺⁺ ions in/out of the muscle cell (fiber) is important in both contraction and relaxation of the muscle, so if a person doesn’t ingest enough calcium, some could be taken out of the bones to supply the muscles with what they need to contract and relax. Any given muscle fiber reacts in an “all or none” response — it is either relaxed or contracted, and the variability in contraction of the overall muscle is based on the number of fibers which contract.

It is important to remember that muscles can only pull or contract, not push. Thus, many muscles come in sets of antagonists that do opposite jobs. For example, the muscle on the “top” of your arm bends the arm at the elbow while the muscle “under” your arm straightens the arm.

Skeletal muscles can be further subdivided into two sub-types depending on their use or function, and the chemical composition of each is slightly different. Think of a Thanksgiving turkey: the first question asked is invariably, “Do you want white or dark meat?” These correspond to white and red muscle tissue, respectively. Think of where the white and dark meat are found: dark meat is found in the legs and other constantly-used posture muscles, while white meat is found in the “breast,” or flight muscles. Turkeys aren’t known for flight, so these flight muscles get infrequent, quick bursts of use. Because the posture muscles are constantly used, they need a more constant, steady supply of oxygen. Thus red muscle tissue contains an extra chemical called myoglobin which is a special protein-type molecule for oxygen storage. The presence of myoglobin in posture muscles enables the sustained contractions necessary to maintain proper posture and walk, so in a turkey, red muscle tissue is found in the legs and other support muscles. The presence of myoglobin gives the muscle tissue its red or dark color. White muscles that are only used occasionally don’t have myoglobin in their tissue. Since turkey flight muscles are only used for short, quick flights, they don’t need as much oxygen, thus don’t need myoglobin to store it, and appear “white” in color.

Skeletal muscles can be classified as one of several different types including:

Flexor

A muscle which bends a joint (click to show animation)

Extensor

A muscle which straightens a joint (click to show animation)

Abductor

A muscle which moves a body part away from the midline of the body

Adductor

A muscle which moves a body part toward the midline of the body

Put your mouse over a muscle to see its name.

Unfortunately, your text has no good illustration of the locations of the main muscles in the human body. The main muscles include: