Muscle Physiology and Cellular Structure

Vocabulary flashcards reviewing skeletal, smooth, and cardiac muscle characteristics, cellular growth/repair mechanisms, connective tissue organization, sarcomere anatomy, and internal membrane structures.

Skeletal Muscle

Striated muscle tissue mostly attached to bone whose contraction is initiated by action potentials in somatic motor neurons and is usually under voluntary control.

Smooth Muscle

Non-striated muscle tissue surrounding hollow organs and tubes that operates autonomously or under autonomic nervous system, hormonal, and autocrine/paracrine control.

Cardiac Muscle

Striated muscle tissue of the heart regulated by autonomic nervous system, hormones, and paracrine/autocrine signals, capable of undergoing spontaneous contractions.

Muscle Fiber

An elongated, cylindrical, multinucleated skeletal muscle cell formed by the fusion of myoblasts, with diameters ranging from $10\,\mu\text{m}$ to $100\,\mu\text{m}$ and lengths up to $20\,\text{cm}$.

Myoblasts

Undifferentiated, mononucleated cells that fuse during embryonic development to form multinucleated skeletal muscle fibers.

Satellite Cells

Quiescent stem cells located between the sarcolemma and basement membrane that proliferate and differentiate into myoblasts to repair or reinforce damaged muscle fibers.

Hypertrophy

An increase in muscle fiber size occurring in response to heavy exercise or damage through fiber enlargement, fiber splitting, and satellite cell proliferation and fusion.

Tendons

Bundles of connective tissue composed mainly of collagen fibers that attach skeletal muscles to bones.

Connective Tissue Layers of Muscle

The structural sheath system of skeletal muscle, consisting of epimysium surrounding the entire muscle, perimysium surrounding fascicles, and endomysium surrounding individual muscle fibers.

Myofibrils

Cylindrical bundles of cytosolic filaments approximately $1\,\mu\text{m}$ to $2\,\mu\text{m}$ in diameter that extend throughout the muscle fiber and link to tendons at both ends.

Thick Filaments

Cytosolic filaments composed mainly of the protein myosin, organized with a long tail of two heavy chains and two globular heads containing light and heavy chains.

Myosin-ATPase

An enzymatic binding site located on each globular head of a myosin molecule that hydrolyzes bound ATP to harness energy for muscle contraction.

Thin Filaments

Cytosolic filaments composed principally of actin along with regulatory proteins troponin, tropomyosin, and nebulin.

Actin

A globular monomeric protein that polymerizes into two intertwined helical chains forming the core of thin filaments, each monomer containing a binding site for myosin.

Sarcomere

The repeating functional unit of thick and thin filaments in a myofibril, bounded at both ends by Z lines.

A Band

The wide, dark central band of a sarcomere created by the presence and full length of thick filaments.

I Band

A light band bisected by the Z line, containing those portions of thin filaments that do not overlap with thick filaments.

H Zone

A narrow light space in the center of the A band corresponding to the gap between opposing ends of thin filaments in a sarcomere.

M Line

A dark band in the center of the H zone composed of proteins that link together central regions of adjacent thick filaments.

Titin

An elastic protein extending from the Z line to the M line that links to thick filaments and M-line proteins to maintain thick filament alignment.

Filament Overlap Geometry

The cross-sectional arrangement in the overlap region of the A band where each thick filament is surrounded by six thin filaments in a hexagonal array and each thin filament is surrounded by three thick filaments in a triangular array.

Sarcoplasmic Reticulum

A homologous endomembrane network in muscle fibers that forms sleevelike segments around each myofibril to manage $\text{Ca}^{2+}$ storage and release.

Terminal Cisternae

Enlarged regions at the ends of sarcoplasmic reticulum segments that store high quantities of $Ls\text{Ca}^{2+}$$ bound to calsequestrin.

Calsequestrin

A $\text{Ca}^{2+}$-binding protein located inside the terminal cisternae allowing large amounts of $\text{Ca}^{2+}$ to be stored without requiring transport against an extreme concentration gradient.

Transverse Tubules (T-tubules)

Membranous tubules continuous with the sarcolemma that conduct action potentials from the surface deep into the muscle fiber between terminal cisternae.

Sarcolemma

The specialized plasma membrane surrounding a skeletal muscle fiber.