Muscle
Learning objectives
- 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 with elongated cells arranged in parallel to the direction of contraction. This contraction is used to move the body or change the shape of certain organs. Blood vessels within the associated connective tissue supply a rich blood supply to provide nutrients and oxygen and to eliminate waste products. 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 non-cellular, and to a nerve fiber which is a cell process. Every muscle fiber is surrounded by a basal lamina called the endomysium. 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.
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. In smooth muscle fibers the myofilaments are not arranged with regularity and so these cells are nonstriated.