(3.7) Muscle Tissue
Muscular tissue
Consists of elongated cells called muscle fibers or myocytes that can use ATP to generate force. As a result, muscular tissue produces body movements, maintains posture, and generates heat. It also provides protection and is highly vascularized. Based on its location and certain structural and functional features, muscular tissue is classified into three types: skeletal, cardiac, and smooth .
Cardiac muscle tissue
Description: Are branched and usually have only one centrally located nucleus; an occasional cell has two nuclei. They attach end to end by transverse thickenings of the plasma membrane called intercalated discs, which contain both desmosomes and gap junctions. Intercalated discs are unique to cardiac muscle tissue. The desmosomes strengthen the tissue and hold the fibers together during their vigorous contractions. The gap junctions provide a route for quick conduction of electrical signals called muscle action potentials throughout the heart. Involuntary control, meaning it is not consciously controlled. Location: Heart wall. Function: Pumps blood to all parts of the body.
Skeletal muscle tissue
Description: Consists of long, cylindrical, striated fibers (alternating light and dark bands within the fibers called striations that are visible under a light microscope). Roughly cylindrical in shape, a muscle fiber is a multinucleated cell with the nuclei located at the cell's periphery. Skeletal muscle is considered voluntary because it can be made to contract or relax by conscious control. Location: Usually attached to bones by tendons. Function: Motion, posture, heat production, and protection.
Smooth muscle tissue
Description: Usually involuntary, and they are nonstriated (lack striations), hence the term smooth. A smooth muscle fiber is a small spindle‐shaped cell that is thickest in the middle and tapers at each end. It contains a single, centrally located nucleus. Gap junctions connect many individual fibers in some smooth muscle tissues, for example, in the wall of the intestines. Such muscle tissues can produce powerful contractions as many muscle fibers contract in unison. In other locations, such as the iris of the eye, smooth muscle fibers contract individually, like skeletal muscle fibers, because gap junctions are absent. Location: Iris of the eyes, walls of hollow internal structures such as blood vessels, airways to the lungs, stomach, intestines, gallbladder, urinary bladder, and uterus. Function: Motion (constriction of blood vessels and airways, propulsion of foods through gastrointestinal tract, contraction of urinary bladder and gallbladder).