Harvard Human Anatomy - Muscle

Types of muscle: The three types of muscle in the human body are smooth, cardiac, and skeletal.

Smooth muscle: Smooth muscle is capable of slow, sustained contraction. It is found in hollow organs, the walls of blood vessels, and in the skin (which makes hairs stand on end). Smooth muscle cells are usually narrow and spindle-shaped. Their nuclei are single and centrally located, and their cytoplasm evidences no visible striations. 

Cardiac muscle: Capable of repeated contractions without fatiguing, cardiac muscle is found in the walls of the heart. Cardiac muscle cells are branching cells, with junctions called intercalated discs. They are arranged in a spiral in the heart to wring blood out the top.

Skeletal muscle: Skeletal muscle tissue forms the familiar muscles of the musculoskeletal system, such as the biceps and triceps. Skeletal muscle cells are capable of forceful contraction. These cells are arranged roughly in parallel to one another; they display conspicuous repeating striations in the cytoplasm and tend to be very long and mutlinucleated.

Unipennate muscle: These muscles have fibers that run the entire length of only one side of the muscle, like a quill.

Bipennate muscle: This consists of two rows of oblique muscle fibers, facing in opposite diagonal directions and converging on a tendon.

Deep fascia: Layers of fibrous connective tissue which surround individual muscles. They divide muscles into compartments.

Fusiform muscle: This type of muscle is cylindrically shaped in the center and tapers at the ends.

Contraction: Muscle contractions are accomplished through the interaction of actin and myosin. There are three main types: Isometric, isotonic, and tonic.

Tonic contraction: A slight but sustained muscle contraction, e.g., maintaining one's posture.

Isotonic contraction: In this contraction, the muscle shortens against a constant load to produce movement.

Isometric contraction: In this contraction, the muscle tension increases as the muscle maintains a fixed overall length.