Cellular Physiology of Muscle Vocabulary

Vocabulary terms and definitions on skeletal muscle tissue microscopic anatomy, connective tissue coverings, myofilaments, sliding filament theory, and cellular excitation-contraction structures.

Skeletal Muscle

Striated, voluntary muscle tissue attached to the skeleton that contains long fibers, contracts rapidly, and requires rest after tiring.

Cardiac Muscle

Striated, involuntary muscle tissue forming the structural walls of the heart, whose contraction rate is established by a pacemaker and modified by neural input.

Smooth Muscle

Nonstriated, involuntary muscle tissue located in the walls of hollow visceral organs that carries out slow, sustained contractions.

Excitability (Responsiveness)

The ability of a muscle cell to receive and respond to chemical stimuli, such as neurotransmitters or hormones, by producing an action potential along its sarcolemma.

Contractility

The ability of muscle tissue to shorten forcibly when adequately stimulated.

Extensibility

The ability of muscle tissue to be stretched or extended beyond its resting length.

Elasticity

The ability of muscle fibers to recoil and resume their resting length after being stretched.

Sarcolemma

 The plasma membrane surrounding a muscle fiber.

Sarcoplasm

The cytoplasm of a muscle cell, containing substantial amounts of stored glycogen and oxygen-binding myoglobin.

Syncytium

A multinucleated structure formed by fused cells, characteristic of skeletal muscle fibers that range from $10\text{--}100\text{ }\boldsymbol{\rm \breve{\textbf u}m}$ in diameter.

Endomysium

A thin layer of connective tissue surrounding individual muscle cells.

Perimysium

Connective tissue wrapper that bundles individual muscle fibers into a fascicle.

Epimysium

An external sheath of connective tissue surrounding the entire muscle organ.

Fascicle

A discrete bundle of muscle cells separated from other bundles by perimysium.

Tendon

A cordlike extension of connective tissue beyond the muscle that attaches it to a bone.

Myofibril

A rod-like contractile structure running parallel within a muscle cell that occupies approximately $80\%$ of the cell volume.

Sarcomere

The functional contractile unit of a myofibril, bounded at each end by a Z disc.

Thick Filament

A myofilament (~$16\,\text{nm}$ diameter) consisting of roughly 200 myosin molecules extending across the full length of the A band.

Thin Filament

A myofilament (~$7\text{--}8\,\text{nm}$ diameter) composed of helical F-actin strands, tropomyosin, and troponin, extending across the I band and into the A band.

Z Disc

A coin-shaped protein sheet composed mostly of alpha-actinin that anchors thin filaments and connects to thick filaments via titin.

H Zone

The lighter region in the center of the A band where thick filaments do not overlap with thin filaments in a relaxed muscle fiber.

M Line

A dark line in the middle of the H zone consisting of fine strands that cross-connect adjacent thick filaments.

Titin

An elastic protein filament with corkscrew properties that anchors thick filaments to Z discs and assists muscle recoil after stretching.

Tropomyosin

A regulatory protein strand on thin filaments that stiffens the actin helix and covers crossbridge binding sites when muscle is relaxed.

Troponin

A 3-polypeptide complex (TnI, TnT, TnC) attached to actin that binds calcium ions to move tropomyosin away from actin binding sites.

Sliding Filament Mechanism

The process of contraction where thin filaments slide past thick filaments, pulling Z discs closer together and shortening sarcomeres without altering individual filament lengths.

Sarcoplasmic Reticulum

An interconnected network of smooth endoplasmic reticulum wrapping each myofibril that stores and releases intracellular $\text{Ca}^{2+}$ during muscle excitation.

T-tubule (Transverse Tubule)

An invagination of the sarcolemma into the interior of the muscle fiber that conducts action potentials deep into the cell cytoplasm.

Triad

A three-part membrane grouping formed by one central T-tubule flanked on either side by two terminal cisternae of the sarcoplasmic reticulum.