Muscle Tissue and Contraction Vocabulary

Vocabulary based on lecture notes covering muscle tissue types, gross and microscopic anatomy, characteristics, and the sliding filament model of contraction.

Muscle Tissue

A type of tissue that can transform chemical energy ($ATP$) into directed mechanical energy capable of exerting force.

Myo-, Mys-, and Sarco-

Prefixes commonly used in terminology referring to muscle.

Skeletal Muscle

A type of muscle tissue characterized by single, very long, cylindrical, multinucleate cells with obvious striations; attached to bones or skin.

Cardiac Muscle

Striated muscle tissue found in the walls of the heart consisting of branching chains of cells that are uni- or binucleate.

Smooth Muscle

Single, fusiform, uninucleate muscle cells with no striations found in the walls of hollow visceral organs, airways, and large arteries.

Muscle Fibers

A term used to refer specifically to elongated skeletal and smooth muscle cells.

Excitability

The ability to respond to a stimulus by changing the electrical membrane potential.

Conductivity

The characteristic involving the transmission of an electrical change down the length of the cell membrane.

Contractility

Exhibited when filaments slide past each other, enabling muscle to cause movement.

Extensibility

The ability of muscle tissue to be stretched.

Elasticity

The ability of muscle tissue to return to its original length following a lengthening or shortening.

Epimysium

Dense irregular connective tissue that wraps the whole muscle organ.

Perimysium

Dense irregular connective tissue that wraps a fascicle and houses blood vessels and nerves.

Endomysium

Areolar connective tissue wrapping an individual muscle fiber, providing electrical insulation and capillary support.

Fascicle

A discrete bundle of muscle cells (muscle fibers) segregated from the rest of the muscle by a connective tissue sheath.

Sarcolemma

The plasma membrane of a muscle fiber, which contains voltage-gated ion channels for electrical signal conduction.

Sarcoplasm

The muscle cell cytoplasm containing typical organelles plus contractile proteins and specializations.

T-tubules (transverse tubules)

Invaginations of the sarcolemma that extend deep into the cell and contain voltage-sensitive calcium channels.

Sarcoplasmic Reticulum (SR)

A specialized organelle in muscle fibers used to store calcium ($Ca^{2+}$).

Triad

The structural relationship formed by one T-tubule and two flanking terminal cisterns of the sarcoplasmic reticulum.

Myofibril

Rod-like contractile elements that make up most of the muscle fiber volume, composed of chains of sarcomeres.

Sarcomere

The smallest contractile unit (functional unit) of a muscle fiber, extending from one Z disc to the next.

A band

The dark band in a myofibril consisting of thick filaments and overlapping thin filaments.

I band

The light band in a myofibril containing only thin (actin) filaments.

H zone

The central region of an A band that contains only thick filaments; this zone disappears during full contraction.

M line

A line in the center of the H zone where thick filaments are linked by accessory proteins.

Thick Filament

Composed of many myosin molecules, featuring heads with binding sites for $ATP$ and actin.

Thin Filament

Consists of two strands of actin subunits twisted into a helix plus regulatory proteins troponin and tropomyosin.

Tropomyosin

A regulatory protein in thin filaments that blocks the active sites for myosin attachment on actin when the muscle is relaxed.

Troponin

A regulatory protein in thin filaments that possesses a binding site for calcium ($Ca^{2+}$).

Sliding Filament Model of Contraction

The theory stating that during contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more without changing filament length.

Motor Unit

A single motor neuron and all the muscle fibers it controls.

Neuromuscular Junction

The site where a motor neuron's synaptic knob meets the motor end plate of a skeletal muscle fiber.

Acetylcholine (ACh)

The neurotransmitter released from synaptic vesicles that triggers muscle excitation by binding to receptors on the sarcolemma.

End-plate potential (EPP)

A local depolarization at the motor end plate caused by $Na^+$ rapidly entering and $K^+$ slowly exiting the muscle fiber.

Power Stroke

The motion of the myosin head that pulls the thin filament toward the center of the sarcomere.