Striations (muscle tissue) (stry-AY-shunz): Alternating light and dark transverse bands visible in skeletal and cardiac muscle tissue; these bands are created by the highly organized, repeating arrangement of the thick (myosin) and thin (actin) filaments.
Endomysium (en-doh-MY-see-um): A delicate layer of reticular connective tissue that immediately surrounds and ensheaths an individual muscle cell (myofiber or muscle fiber); it contains capillaries and nerves.
Myofiber (Muscle Fiber) (my-oh-FY-ber): The term for a single, elongated muscle cell; in skeletal muscle, it is a large, multinucleated cell, while in cardiac and smooth muscle, it is a smaller cell (cardiomyocyte or smooth muscle cell).
Epimysium (ep-ih-MY-see-um): The thick sheath of dense, irregular connective tissue that surrounds the entire gross muscle organ; it is continuous with the fascia that separates muscles from other tissues.
Perimysium (per-ih-MY-see-um): A sheath of connective tissue that surrounds a bundle of muscle fibers (fascicle); it contains larger blood vessels and nerves that supply the muscle.
Fascicle/fiber bundle (FAS-ih-kul): A discrete bundle of muscle cells (myofibers) within a muscle, surrounded by the perimysium; the organization into bundles allows for independent neural control of portions of the muscle.
Sarcolemma (sar-koh-LEM-uh): The specialized plasma membrane of a muscle cell (myofiber); it is responsible for receiving the nerve impulse and propagating the action potential that leads to muscle contraction.
Sarcoplasmic reticulum (sar-koh-PLAZ-mik ruh-TIK-yuh-lum): The specialized smooth endoplasmic reticulum of a muscle cell; its primary function is to store and release calcium ions (Ca2+) necessary to initiate muscle contraction.
Myosin (MY-oh-sin): The major component protein of the myosin (thick) filament; it is an ATPase with projecting heads (myosin heads) that bind to and pull on the actin (thin) filament to generate force.
Actin (AK-tin): The major component protein of the actin (thin) filament; it is the structural filament onto which the myosin heads attach and pull during muscle contraction.
Myofibril (my-oh-FY-bril): A long, cylindrical bundle of contractile proteins found within the cytoplasm of a myofiber; it is composed of highly ordered repeating units called sarcomeres.
Sarcomere (SAR-kuh-meer): The basic contractile unit of striated muscle (skeletal and cardiac); it is defined as the segment of a myofibril between two successive Z lines.
A band (ay band): The dark-staining transverse band in a sarcomere; this band represents the full length of the myosin (thick) filaments and includes the region where actin (thin) filaments overlap with them.
I band (eye band): The light-staining transverse band in a sarcomere; this band is bisected by the Z line and contains only actin (thin) filaments.
Z line (Z disc) – The dense protein structure that anchors the actin (thin) filaments and bisects the I band; it defines the lateral boundary of the sarcomere.
H band (H zone) – The central, light-staining region within the A band of a relaxed sarcomere; it contains only myosin (thick) filaments (no overlap with actin).
M line (em line): The dense, narrow protein line located precisely in the center of the H band; it serves to anchor and stabilize the myosin (thick) filaments.
Myosin head (MY-oh-sin hed): The globular, projecting portion of the myosin molecule that contains the ATPase activity and the binding site for actin; it is the physical structure that executes the power stroke.
Myosin (thick) filament (MY-oh-sin FIL-uh-ment): A large filament composed primarily of myosin molecules; it occupies the central region of the sarcomere (A band).
Actin (thin) filament (AK-tin FIL-uh-ment): A small filament composed primarily of actin molecules; it is anchored to the Z line and extends into the A band. Also contains troponin, tropomyosin, and nebulin proteins.
Titin (TY-tin): A massive, elastic protein filament that extends from the Z line to the M line; it functions to anchor the myosin (thick) filaments and provides the elasticity and passive tension of the sarcomere.
Cardiomyocyte (Cardiac muscle cell) (kar-dee-oh-MY-oh-syt): The individual muscle cell of the heart; it is typically shorter, branched, and possesses one or two central nuclei, and it connects to adjacent cells via intercalated discs.
Intercalated disc (in-ter-KAL-ay-ted disk): A complex, specialized junctional complex found at the ends of adjacent cardiomyocytes; it contains gap junctions (for electrical coupling) and strong adherence junctions (to anchor actin filaments and resist tension).
Smooth muscle cell (smooth MUS-ul sell): A spindle-shaped muscle cell (fusiform) lacking striations; it possesses a single, central nucleus and is responsible for slow, sustained, involuntary contractions in the walls of hollow organs and blood vessels.