Muscular System

In recent years, Americans have become more muscle-conscious, with many of us working out in gyms, running, and swimming to build up muscles as well as to derive general health benefits.

Even for people with no interest in body-building, it is important to use the muscles daily. Inactive people can develop stiff muscles, setting up a cycle of even less activity, weight gain, and other health problems.

Types of Muscle Cells

All muscle cells are specialized for contraction. Those in striated muscle tissue make up the skeletal muscles, which are attached to bones and make voluntary movements possible. Those in smooth muscle tissue are found in the muscular walls of internal organs that contract involuntarily, such as the uterus, blood vessels, and organs of the digestive system. The cardiac

(heart) muscle is made up of a special type of striated muscle; though it is striated, its contractions are involuntary.

Striated muscles are usually attached to tough connective tissue called tendons, which are attached to bones. Most muscles are in opposing pairs, with one of the pair contracting while the other relaxes. For instance, if you reach out toward something, the triceps muscle on the back of your upper arm contracts. If you then lift your hand to touch your shoulder, the biceps muscle on the front of your upper arm contracts, while the triceps relaxes. Both muscles are attached by tendons to bones in the forearm, and the bones' movements are controlled by those muscles.

Smooth muscle cells are spindle-shaped, have one nucleus, and contain the proteins actin and myosin. These molecules are arranged irregularly. Many of the smooth muscle cells are in two layers of tissue, with an inner circular sheet surrounded by an outer longitudinal sheet. The two layers work together in alternation to squeeze an organ's contents and move them along—as needed during digestion or childbirth, for instance.

The muscle cells in striated muscles show a regular pattern of striations when a cross-section is examined microscopically. Each cell is a fiber; thousands of fibers, bound by connective tissue, make up an entire skeletal muscle.

Sliding Filaments

Muscle fibers themselves are made up of thick (myosin) and thin (actin) filaments that are arranged in parallel and slide past each other. The striated pattern depends on the alternation of thick and thin filaments. The two types of filaments are connected by smaller cross-bridges of myosin. Each cell also contains several nuclei, which are pushed to the outside of the cytoplasm by the filaments.

The sliding filament theory explains the contraction of skeletal muscles. According to the theory, when a muscle contracts the filaments slide past each other, and the cross-bridges are rearranged. When the filaments slide apart again, the muscle relaxes.

Actin and myosin filaments slide past each other in smooth muscle cells also, but the fibrils are interwoven in a mass of fibers, not arranged in parallel. The actin and myosin filaments slide back and forth, but contraction is slower than in skeletal muscle and continues for a longer time.

Cardiac muscle appears striated, but the cells are shorter than those in skeletal muscle and have only one nucleus per cell. The heartbeat that continues from before birth to death is made possible by the involuntary contractions of cardiac muscle resulting from the movements of actin and myosin.

The Nerve-Muscle Connection

When a nerve impulse reaches a muscle, calcium is released at the neuromuscu-lar junction. The calcium enters the muscle cells and activates the myosin bridges. Using energy from ATP molecules (see Chapter 1), the muscle filaments slide together, and the muscle contracts. Each fiber twitches (contracts) individually, with the contraction of the entire muscle being made up of individual twitches of many fibers. The twitch may be as fast as 7.5 milliseconds in skeletal muscle or as slow as 100 milliseconds in a smooth muscle.

Skeletal muscles and smooth muscles both respond to signals from nerves; smooth muscles may also contract in response to hormones. Adrenaline in the blood, for instance, can cause the smooth muscles in arteries to contract, narrowing the diameter of the vessels. Other hormones can aid in digestion, respiration, and other functions by causing various smooth muscles to contract.

Like smooth muscles, cardiac muscle responds to signals from both nerves and hormones, but the strength and rate of the heartbeat are controlled by the heart's pacemaker tissue.

Energy for Contraction

The contractions of muscles are fueled by the energy stored in ATP molecules during cellular respiration. Muscles also depend on a supply of oxygen and food and give off waste products that must be removed. All of these needs are supplied by the circulating blood. The color of a muscle indicates whether it has a good blood supply. Dark red beef, for instance, is well supplied, but the white muscle of a chicken breast is not.

Muscle fatigue results when muscle cells continue working too long. Too little oxygen is being delivered to the muscle for ordinary respiration to continue. Instead, the cells use fermentation to make ATP, and the end product (lactic acid) accumulates in the cells. This lowers the pH and reduces the muscles' ability to contract. Muscle fatigue is reversible; after the exercise stops, fermentation is no longer needed, and the lactic acid is broken down.

Muscle fatigue is common in anyone who is out of condition, but less so in those who exercise regularly. Exercise increases muscle size and benefits the circulatory system, which makes more nutrients available to the muscle cells and postpones the need for fermentation during exercise. In contrast, lack of exercise—such as that enforced curing prolonged bed rest—allows muscles to become shrunken, or atrophied.

The still-mysterious disorder called fibromyalgia is characterized by general, chronic pain in the muscles and skeleton. The pain and disability associated with the disorder causes patients to refrain more and more from physical activity, causing their muscles to become increasingly decondi-tioned. Treatment includes stretching and mild exercise to recondition muscles and reduce pain.6


Distant parts of the body communicate with each other in two ways—chemical and electrical. The chemical secretions from glands act relatively slowly, the electrical and chemical signals from nerve cells very quickly. Each method has advantages for the organism, and glands and nerves work together to keep the body functioning and in chemical balance.

Many glands secrete their products through ducts that pass directly to their target cells. The eyes' tear glands and the skin's sweat glands are examples of these glands, called exocrine glands.

The endocrine glands, in contrast, secrete hormones—chemical messengers that act at a distance from their sources— into the bloodstream, which carries them to their destinations. Neurosecretory cells are neurons (nerve cells) that both carry electrical signals and secrete hormones into the bloodstream.

Though cells' possible functions are determined by their DNA, their actual functions occur partly in response to their environment. Cell functions are regulated by the neuroendocrine system (the nervous and endocrine systems). Neural and endocrine tissues work together to control and coordinate cell growth, activities, and division.

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