Human Anatomy

Muscular System of the Human Body

Invertebrates have muscles attached to exoskeletons. Insects are examples. In animals without exoskeletons, such as squids, muscles are not affixed. [4] Vertebrates, animals with backbones, have three types of muscle tissue -- cardiac, smooth, and skeletal. Muscle tissue in a healthy average human accounts for approximately 50 percent of total body weight.

Cardiac muscle is found only in the heart, thus its name. Smooth muscle is found in the intestinal wall, in the stomach and in walls of blood vessels. Because smooth muscle is found in internal organs of the body, or viscera, it is sometimes called "visceral muscle." Smooth muscle is also found in the eye. Skeletal muscles are attached to the bones of the human skeleton by tendons.

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Myofibrils, special organelles in muscle cells, contain linear arrangements of sacromeres, units that produce contraction, made up of the proteins actin and myosin. Actin makes up the thin filaments in a sacromere. Myosin makes up the thick filaments. These protein-containing filaments lie parallel to each other. As contraction is triggered, myosin pulls actin, and the filaments slide past each other creating the contraction, i.e., shortening the muscle tissue and ultimately producing movement. Because muscle tissue contains such high densities of contractile protein filaments, it is sometimes referred to as "contractile tissue." [2, 3]


Posterior view of muscles from Gray's Anatomy.

Cardiac Muscle

Cardiac muscle is striated muscle, consisting of branched chains of interconnected cells in a lattice network. [2] Under a microscope, the cells of cardiac muscle appear to have transverse stripes. [1]

Cardiac muscle is under involuntary control. Intricately interconnected cells of cardiac muscle tissue rapidly transmit the signal to contract and drive the heartbeat. Specialized cells in cardiac muscle tissue -- pacemaker cells -- initiate spontaneous rhythmic contractions. These pacemaker cells generate the signal on their own, thus the heartbeat is said to be myogenic and do not depend on nerve stimulation. However, the rate can be modified by the autonomic nervous system. [3]

Cardiac muscle resists tearing, but cannot regenerate if destroyed. [3]

Smooth Muscle

Smooth muscle contracts involuntarily at the command of the autonomic nervous system. The shoots of long spindle-shaped cells appear smooth as opposed to striated under a microscope.

Among the functions of smooth muscle are to move food along the digestive tract, empty the urinary bladder, and control the flow of blood through vessels. [3]

Skeletal Muscle

Skeletal muscle tissue, like cardiac, is striated. Skeletal muscle is under voluntary control. It is skeletal muscle tissue that allows humans to move. For example, voluntary movements such as walking, talking, and riding a bicycle, are all powered by skeletal muscles. Skeletal muscles also generate the movements of respiration. [3]

Generally, skeletal muscles are attached to the bones by tendons. One attachment, the origin remains somewhat immobile during contraction. The insertion, the other end of the muscle, is attached to the bone that moves during a contraction. [1]

Voluntary movement of skeletal muscles occurs at the joints, or articulations -- where two bones meet -- through contraction and shortening of muscles. [1] Because muscles shorten during this contraction, they pull and cannot push. This necessitates working in antagonistic pairs with each member of a pair pulling in opposite directions. [2] For example, to raise (flex) the forearm, the biceps contracts; to lower or extend the forearm, the triceps contracts. [2] A contraction of the biceps causes the forearm, the site of insertion, to move toward the origin, in this case, the shoulder. [1]

Skeletal muscles cells are elongated, running along the length of the skeletal muscle tissue. [2] A simple action, such as picking up a pencil from a desk, requires the coordinated efforts of a number of muscles working together.

Fun Fact: Exercise improves muscle strength and endurance. [2] Specific exercise produce different results. For example, jogging or running increases endurance; weightlifting increases muscle strength. [3]

Continue on to naming and examples of muscles in the human body.


References
1 Biology, Seventh Edition; Peter H. Raven, George B. Johnson, Jonathan B. Losos, Susan R. Singer; McGraw-Hill, New York, New York; 2005.
2 Essentials in Biology; Sylvia S. Mader; McGraw-Hill, New York, New York; 2007.
3 Life: The Science of Biology, Seventh Edition; William K. Purves, David Sadava, Gordon H. Orians, H. Craig Heller; Sinauer Associates, Sunderland, Massachusetts; 2004.
4 Science 101: Biology; George Ochoa; HarperCollins, New York, New York; 2007.

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