Skeletal Muscle Structure and Function Vocabulary

Vocabulary flashcards defining key anatomical and physiological terms of the musculoskeletal system, excitation-contraction coupling, and muscle mechanics.

actin

A thin protein filament that interacts with myosin to facilitate muscle contraction.

action potential

A rapid electrical impulse that travels along the membrane of a nerve or muscle cell to initiate contraction.

adenosine triphosphatase (ATPase)

An enzyme that cleaves adenosine triphosphate (ATP) to yield adenosine diphosphate (ADP), inorganic phosphate, and energy.

adenosine triphosphate (ATP)

A high-energy molecule that serves as the primary energy currency for muscle contractions and cellular processes.

a-motor neuron

A motor neuron that innervates extrafusal skeletal muscle fibers to initiate muscle action.

apoptosis

The process of programmed cell death.

concentric contraction

A dynamic muscle contraction in which the muscle shortens while producing force.

dynamic contraction

A muscle contraction that produces movement at a joint, comprising concentric and eccentric contractions.

eccentric contraction

A dynamic muscle contraction in which the muscle lengthens while producing force.

muscle fibers

Individual muscle cells that comprise skeletal muscle tissue.

musculoskeletal system

The integrated anatomical system consisting of bones, joints, and skeletal muscles that produces movement.

myofibrils

Elongated, threadlike structures within muscle fibers containing repeating contractile units called sarcomeres.

myosin

A thick protein filament that interacts with actin to produce muscle force and movement.

myosin cross-bridges

Projections from myosin filaments that attach to binding sites on actin filaments during muscle contraction.

myosin heavy chain (MHC)

The primary structural component of the myosin head that determines contraction speed and fiber type characteristics.

nebulin

A structural protein that extends along the length of actin filaments to regulate thin filament length and stability.

perimysium

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

plasmalemma

The plasma membrane surrounding an individual muscle fiber.

satellite cells

Muscle stem cells located between the basal lamina and plasmalemma that play a key role in muscle growth, repair, and adaptation.

single-fiber contractile velocity (V)

The speed at which an isolated individual muscle fiber contracts and shortens under a given load.

size principle

The principle stating that motor units are recruited in order of increasing motor neuron size, starting with smaller, low-threshold units.

sliding filament theory

The mechanism of muscle contraction wherein actin filaments slide over myosin filaments, shortening the sarcomere.

static (isometric) muscle contraction

A muscle contraction in which force is developed without changing muscle length or joint angle.

summation

The additive effect of successive muscle twitches produced by high-frequency electrical stimulation.

tetanus

A continuous, maximal muscle contraction resulting from rapid, high-frequency stimulation.

endomysium

A fine layer of connective tissue that wraps individual muscle fibers.

epimysium

The outer layer of fibrous connective tissue surrounding an entire skeletal muscle.

excitation-contraction coupling

The sequence of events by which an action potential on the sarcolemma triggers calcium release and subsequent muscle contraction.

fascicle

A bundle of individual muscle fibers enclosed by perimysium.

force-velocity relation

The physiological relationship describing how force production decreases as the velocity of concentric shortening increases.

length-tension relation

The relationship describing how maximum force generation depends on the length of the muscle prior to stimulation.

motor unit

A single a-motor neuron and all the individual muscle fibers it innervates.

power stroke

The tilting movement of the myosin head that pulls thin actin filaments toward the center of the sarcomere.

principle of orderly recruitment

The fixed order in which motor units are activated according to their size and threshold during force production.

rate coding

The variation in firing frequency of motor neurons used to regulate the force output of a muscle.

titin

A giant elastic protein that anchors myosin filaments to the Z-disc and maintains sarcomere alignment during contraction and stretch.

transverse tubules (T-tubules)

Invaginations of the sarcolemma that run deep into the muscle fiber to rapidly conduct action potentials.

tropomyosin

A regulatory protein wrapped around actin filaments that physically blocks myosin-binding sites when the muscle is at rest.

sarcolemma

The cell membrane of a muscle fiber, consisting of the plasmalemma and basement membrane.

sarcomere

The fundamental functional and contractile unit of a myofibril, bounded by Z-discs.

sarcoplasm

The fluid cytoplasm of a muscle fiber containing dissolved proteins, glycogen, enzymes, and organelles.

sarcoplasmic reticulum (SR)

A membrane-bound network of tubules inside muscle fibers that stores and releases calcium ions ($\text{Ca}^{2+}$).

troponin

A regulatory protein attached to tropomyosin and actin that binds calcium ions to initiate cross-bridge cycling.

twitch

A single brief contraction and relaxation cycle in a muscle fiber in response to a single electrical stimulus.

type I fibers

Slow-twitch muscle fibers characterized by high oxidative capacity, high fatigue resistance, and slow contraction speed.

type II fibers

Fast-twitch muscle fibers characterized by high glycolytic capacity, rapid force development, high power output, and lower fatigue resistance.