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) Cardiac muscle is associated only with the heart. It forms the bulk of the heart wall.

(2) The muscle of the heart wall is referred to as the myocardium of the heart.

(3) Cardiac muscle tissue has striations like skeletal muscle, but also has a distinctive specialization: intercalated_discs.

(4) Intercalated discs are clusters of gap_junctions and desmosomes that help regulate the passage of energy throughout the heart.

(5) Cardiac muscle is "ropy" in appearance, whereas skeletal muscle is tightly organized. The "ropy" appearance is caused by the arrangment of supporting bands of CT within the myocardium.

(6) Cardiac muscle is involuntary.