- Be able to distinguish the three types of muscle.
- Be able to describe how they differ in structure and function.
Muscle is especially adapted for contractility and, to a lesser extent, for conductivity. Its function is performed by shortening, or attempting to shorten, its length. Muscle is composed of elongated cells that show a longitudinal polarity in the direction of contraction. Their shape and functional polarity render contraction maximally effective. The most important ability of these highly specialized cells is to transform chemically stored energy into mechanical work.
Muscles require a rich blood supply to provide nutrients and oxygen and to eliminate waste products. Blood vessels course in the associated connective tissue, which also serves to bind together the muscle cells and to provide a harness for them so that their pull may be exerted usefully. Nerves also accompany the blood vessels in the connective tissue.
The unit of muscle tissue is a cell, often referred to as a muscle fiber. The term "fiber" is used here in contrast to a connective tissue fiber, which is noncellular, and to a nerve fiber which is a cell process. Every muscle fiber is surrounded by a basal lamina. Its plasma membrane (which is not visible with the light microscope) is often called the sarcolemma and its cytoplasm is given a special name, sarcoplasm. Within the sarcoplasm are cytoplasmic contractile elements, the myofilaments.
Muscle tissue is almost exclusively of mesodermal origin, except the iris muscle of the eye and myoepithelial cells (modified smooth muscle cells, located between the glandular cells and the basement membrane of mammary, sweat, lacrimal, and some salivary glands), which are derived from ectoderm.
On both a structural and functional basis, muscle is classified as smooth, skeletal or cardiac. Skeletal and cardiac muscle fibers have a characteristic striated appearance due to the organization of myofilaments, which are aligned in register. In smooth muscle fibers the myofilaments are not arranged with regularity and so these cells are nonstriated.