Histology is the study of tissues. Tissues are groups of cells that are similar in structure and function. There are four tissue types associated with animals such as humans. They are:

epithelial_tissue,

connective_tissue (CT),

muscle_tissue, and

nerve_tissue

Each of these tissue categories has distinctive cell shapes and functions.

Muscle tissue is responsible for locomotion and for the movements of the various parts of the body with respect to one another. There are three categories of muscle tissue: skeletal, cardiac, and smooth_muscle. Another term for skeletal muscle is striated muscle. The term visceral is often used as a synonym for smooth muscle.

NOTE:

(1) Visceral (smooth) muscle is located in the walls of blood vessels and internal organs. It is also found in the walls of ducts associated with the digestive, respiratory, reproductive, and urinary systems.

(2) Smooth muscle cells are often elongated and tapered, therefore they are often called "spindle-shaped". These cells have a single nucleus. Smooth muscle is involuntary.