ATPase: enzyme that hydrolyzes ATP to ADP

acetylcholine (ACh): neurotransmitter that binds at a motor end-plate to trigger depolarization

actin: protein that makes up most of the thin myofilaments in a sarcomere muscle fiber

action potential: change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibers

aerobic respiration: production of ATP in the presence of oxygen

angiogenesis: formation of blood capillary networks

aponeurosis: broad, tendon-like sheet of connective tissue that attaches a skeletal muscle to another skeletal muscle or to a bone

atrophy: loss of structural proteins from muscle fibers

autorhythmicity: heart’s ability to control its own contractions

calmodulin: regulatory protein that facilitates contraction in smooth muscles

cardiac muscle: striated muscle found in the heart; joined to one another at intercalated discs and under the regulation of pacemaker cells, which contract as one unit to pump blood through the circulatory system. Cardiac muscle is under involuntary control.

concentric contraction: muscle contraction that shortens the muscle to move a load

contractility: ability to shorten (contract) forcibly

contraction phase: twitch contraction phase when tension increases

creatine phosphate: phosphagen used to store energy from ATP and transfer it to muscle

dense body: sarcoplasmic structure that attaches to the sarcolemma and shortens the muscle as thin filaments slide past thick filaments

depolarize: to reduce the voltage difference between the inside and outside of a cell’s plasma membrane (the sarcolemma for a muscle fiber), making the inside less negative than at rest

desmosome: cell structure that anchors the ends of cardiac muscle fibers to allow contraction to occur

elasticity: ability to stretch and rebound

endomysium: loose, and well-hydrated connective tissue covering each muscle fiber in a skeletal muscle

epimysium: outer layer of connective tissue around a skeletal muscle

excitability: ability to undergo neural stimulation

extensibility: ability to lengthen (extend)

fascicle: bundle of muscle fibers within a skeletal muscle

fast glycolytic (FG): muscle fiber that primarily uses anaerobic glycolysis

fast oxidative (FO): intermediate muscle fiber that is between slow oxidative and fast glycolytic fibers

fibrosis: replacement of muscle fibers by scar tissue

glycolysis: anaerobic breakdown of glucose to ATP

hyperplasia: process in which one cell splits to produce new cells

hypertonia: abnormally high muscle tone

hypertrophy: addition of structural proteins to muscle fibers

hypotonia: abnormally low muscle tone caused by the absence of low-level contractions

intercalated disc: part of the sarcolemma that connects cardiac tissue, and contains gap junctions and desmosomes

lactic acid: product of anaerobic glycolysis

latch-bridges: subset of a cross-bridge in which actin and myosin remain locked together

latent period: the time when a twitch does not produce contraction

motor end-plate: sarcolemma of muscle fiber at the neuromuscular junction, with receptors for the neurotransmitter acetylcholine

motor unit: motor neuron and the group of muscle fibers it innervates

muscle tension: force generated by the contraction of the muscle; tension generated during isotonic contractions and isometric contractions

muscle tone: low levels of muscle contraction that occur when a muscle is not producing movement

myoblast: muscle-forming stem cell

myofibril: long, cylindrical organelle that runs parallel within the muscle fiber and contains the sarcomeres

myoglobin: a protein that binds to oxygen when it is abundant, and releases it when oxygen becomes scarce.

myosin: protein that makes up most of the thick cylindrical myofilament within a sarcomere muscle fiber

myotube: fusion of many myoblast cells

neuromuscular junction (NMJ): synapse between the axon terminal of a motor neuron and the section of the membrane of a muscle fiber with receptors for the acetylcholine released by the terminal

neurotransmitter: signaling chemical released by nerve terminals that bind to and activate receptors on target cells

pacesetter cell: cell that triggers action potentials in smooth muscle

pericyte: stem cell that regenerates smooth muscle cells

perimysium: connective tissue that bundles skeletal muscle fibers into fascicles within a skeletal muscle

sarcolemma: plasma membrane of a skeletal muscle fiber

sarcomere: longitudinally, repeating functional unit of skeletal muscle, with all of the contractile and associated proteins involved in contraction

sarcopenia: age-related muscle atrophy

sarcoplasmic reticulum (SR): specialized smooth endoplasmic reticulum, which stores, releases, and retrieves Ca⁺²

sarcoplasm: cytoplasm of a muscle cell

satellite cell: stem cell that helps to repair muscle cells

skeletal muscle: striated, multinucleated muscle that requires signaling from the nervous system to trigger contraction; most skeletal muscles are referred to as voluntary muscles that move bones and produce movement

slow oxidative (SO): muscle fiber that primarily uses aerobic respiration

smooth muscle: nonstriated, mononucleated muscle in the skin that is associated with hair follicles; assists in moving materials in the walls of internal organs, blood vessels, and internal passageways

somites: blocks of paraxial mesoderm cells

stress-relaxation response: relaxation of smooth muscle tissue after being stretched

synaptic cleft: space between a nerve (axon) terminal and a motor end-plate

terminal cistern: swelling of the sarcoplasmic reticulum where it meets the transverse tubule.

thick filament: the thick myosin strands and their multiple heads projecting from the center of the sarcomere toward, but not all to way to, the Z-discs

thin filament: thin strands of actin and its troponin-tropomyosin complex projecting from the Z-discs toward the center of the sarcomere

transverse (T) tubule: projection of the sarcolemma into the interior of the cell

triad: the grouping of one T-tubule and two terminal cisternae

tropomyosin: regulatory protein that covers myosin-binding sites to prevent actin from binding to myosin

troponin: regulatory protein that binds to actin, tropomyosin, and calcium

varicosity: enlargement of neurons that release neurotransmitters into synaptic clefts

visceral muscle: smooth muscle found in the walls of visceral organs

voltage-gated sodium channels: membrane proteins that open sodium channels in response to a sufficient voltage change, and initiate and transmit the action potential as Na⁺ enters through the channel