AN67.1-3 | Muscle histology — Glossary

The basic contractile unit of striated muscle, extending from one Z-line to the next, containing interdigitating thick (myosin) and thin (actin) filaments

The dark band of the sarcomere containing the full length of thick (myosin) filaments; remains constant in length during contraction

The light band of the sarcomere containing only thin (actin) filaments; shortens during contraction

The central lighter region within the A-band containing only thick (myosin) filaments; disappears during full contraction

The boundary of each sarcomere; anchoring point for thin (actin) filaments; T-tubules located here in cardiac muscle

Specialised junctions between cardiac muscle cells containing fascia adherens, desmosomes, and gap junctions; the diagnostic feature of cardiac muscle

Connexin-based channels at intercalated discs that allow ion flow between cardiac cells, creating a functional syncytium for coordinated contraction

Transverse tubular invagination of the sarcolemma that transmits action potentials to the interior of the muscle fibre; located at A-I junction (skeletal) or Z-line (cardiac)

Specialised smooth endoplasmic reticulum of muscle cells that stores and releases calcium ions for excitation-contraction coupling

The structural unit in skeletal muscle consisting of one T-tubule flanked by two terminal cisternae of sarcoplasmic reticulum

A quiescent stem cell located between the sarcolemma and basal lamina of skeletal muscle fibres; activated after injury to regenerate muscle

A subsarcolemmal protein linking the actin cytoskeleton to the extracellular matrix via the dystrophin-associated glycoprotein complex; absent in Duchenne muscular dystrophy

The thin connective tissue layer surrounding individual muscle fibres; contains capillaries and satellite cells

The connective tissue sheath surrounding a bundle (fascicle) of muscle fibres

The dense connective tissue layer surrounding an entire muscle

A regulatory protein complex on actin filaments with three subunits: TnC (binds calcium), TnI (inhibits actin-myosin binding), TnT (binds tropomyosin)

A regulatory protein that covers myosin-binding sites on actin filaments at rest; displaced by the troponin-calcium complex to allow cross-bridge cycling

A yellow-brown wear-and-tear pigment accumulating in ageing cardiac muscle cells; composed of oxidised lipid and protein residues in lysosomes

The predominant pattern of cell death in myocardial infarction where the cell outline is preserved but the nucleus is lost; replaced by scar tissue

Dense transverse bands of hypercontracted sarcomeres seen in early MI and reperfusion injury; caused by calcium influx into dying cells

A benign smooth muscle tumour; most commonly found in the uterus (fibroid); the most common benign tumour in Indian women

Breakdown of skeletal muscle releasing myoglobin, CK, and potassium into the blood; causes include crush injury, statins, and marathon running in heat

A calcium-binding protein in smooth muscle that activates myosin light chain kinase (MLCK) to initiate contraction, replacing the troponin system of striated muscle

Cytoplasmic and membrane-associated structures in smooth muscle analogous to Z-lines; anchor actin filaments and transmit contractile force