GENERAL PROPERTIES OF MUSCLE TISSUE

A. Terminology: Many special terms are applied to muscle. Most include the prefixes sarco- or myo- .
B. Specialization for Contraction: Muscle cells are structurally and functionally specialized for contraction, which requires 2 types of special protein filaments called myofilaments: thin filaments containing actin and thick filaments containing myosin.
C. Mesodermal Origin: Nearly all muscle cells arise from mesoderm. Mesenchymal cells differentiate into muscle cells through a process involving accumulations of myofilaments in the cytoplasm and development of special membranous channels and compartments. Exception: Smooth muscles of the iris arise from ectoderm.
D. Cell Shape: Muscle cells are typically longer than they are wide, sometimes reaching lengths of 4 cm. Muscle cells are therefore often called muscle fibers, or myofibers.
E. Organization: Muscle tissues are groups of muscle cells organized by connective tissue. This arrangement allows the groups to act together or separately, generating mechanical forces of varying strength. Named muscles of the body leg, biceps brachii) are organs made up of highly organized muscle tissue.
F, Types of Muscle Tissue: The main muscle tissue types are smooth muscle and the 2 types of striated muscle, skeletal and cardiac. Smooth muscle is found mainly in the walls of hollow organs leg, intestines and blood vessels; its contraction is slow, often in waves, and under involuntary control. In histologic section, it lacks the banding pattern, or striations, seen in the other 2 types. Skeletal muscle is found mainly in association with bones, which act as pulleys and levers to multiply the force of its quick, strong, voluntary contractions. Cardiac muscle is found exclusively in the walls of the heart; its contractions are quick, strong, rhythmic, and involuntary.