The Muscular System: Anatomy, Physiology, and Muscle Actions

Vocabulary practice flashcards covering muscle histology, microscopic anatomy, sliding filament theory, neuromuscular junction transmission, muscle energy pathways, and origins, insertions, and actions of principal human skeletal muscles.

Endomysium

The delicate connective tissue sheath that encloses an individual skeletal muscle fiber.

Perimysium

A fibrous connective tissue membrane that wraps around several muscle fibers to form a bundle known as a fascicle.

Fascicle

A bundle of skeletal muscle fibers bundled together and surrounded by perimysium.

Epimysium

A tough connective tissue overcoat that wraps around the entire skeletal muscle and blends into tendons or aponeuroses.

Tendon

A cordlike connective tissue structure made mostly of collagen that attaches skeletal muscle directly to bone.

Aponeurosis

A sheetlike fibrous connective tissue structure that attaches muscles indirectly to bones, cartilage, or connective tissue coverings.

Varicosities

Bulbous swellings of autonomic nerve fibers that release neurotransmitters over smooth muscle cells instead of conventional neuromuscular junctions.

Intercalated Discs

Specialized gap junctions that unite branching cardiac muscle cells, allowing ions to pass freely and synchronizing cardiac contraction.

Sarcolemma

The specialized plasma membrane of a muscle cell (fiber).

Myofibrils

Long, ribbonlike organelles packed inside a muscle cell consisting of aligned bundles of myofilaments whose light and dark bands produce striations.

Sarcomere

The fundamental contractile unit of a skeletal muscle fiber, extending along a myofibril from one Z disc to the next adjacent Z disc.

I Band (Light Band)

The pale region of a myofibril containing only thin (actin) myofilaments anchored to a central Z disc.

A Band (Dark Band)

The dark region across a myofibril containing thick (myosin) myofilaments, including areas where actin and myosin overlap.

Sarcoplasmic Reticulum (SR)

A specialized smooth endoplasmic reticulum that encases each myofibril and functions to store and release calcium ions ($Ca^{2+}$) for muscle contraction.

Transverse (T) Tubules

Tubular invaginations of the sarcolemma that penetrate deeply into the muscle cell to conduct electrical action potentials to the sarcoplasmic reticulum.

Motor Unit

A functional unit consisting of a single motor neuron and all the skeletal muscle fibers it stimulates.

Neuromuscular Junction

The specific region of close association between the axon terminal of a motor neuron and the sarcolemma of a skeletal muscle fiber.

Synaptic Cleft

The microscopic, fluid-filled space between the axon terminal of a motor neuron and the folded sarcolemma of the muscle fiber.

Acetylcholine (ACh)

The neurotransmitter released by motor neurons via exocytosis that diffuses across the synaptic cleft to stimulate skeletal muscle fibers.

Acetylcholinesterase (AChE)

The enzyme residing in the synaptic cleft that rapidly degrades acetylcholine into acetic acid and choline, terminating muscle stimulation.

Cross Bridge

The structural link formed when an energized myosin head attaches to an exposed binding site on an actin thin filament.

Power Stroke

The mechanical step of contraction where ADP and inorganic phosphate ($P_i$) are released from the myosin head, causing it to pivot and drag the actin filament toward the M line.

Irritability (Responsiveness)

The functional characteristic of muscle tissue that allows it to receive and respond to stimuli.

Contractility

The unique property of muscle tissue enabling it to shorten forcibly when appropriately stimulated.

Extensibility

The ability of muscle fibers to stretch or elongate beyond their resting length.

Elasticity

The physical capacity of a muscle fiber to recoil and resume its original resting length after being stretched.

Latent Period

The brief period immediately following stimulation during which excitation-contraction coupling occurs before measurable tension is generated.

Unfused (Incomplete) Tetanus

A sustained, wavering muscle contraction that occurs when stimuli arrive in rapid succession before complete relaxation has occurred.

Fused (Complete) Tetanus

A smooth, sustained contraction exhibiting no relaxation whatsoever due to very rapid successive stimulation.

Direct Phosphorylation

The coupled metabolic reaction in which creatine phosphate (CP) transfers a high-energy phosphate group to ADP, yielding $1\text{ ATP}$ per CP for up to approximately $15\text{ seconds}$.

Aerobic Pathway

Mitochondrial cellular respiration requiring oxygen to break down glucose, pyruvic acid, fatty acids, or amino acids, producing about $32\text{ ATP}$ per glucose along with $CO_2$ and $H_2O$.

Anaerobic Glycolysis

An anaerobic metabolic pathway in the cytosol that converts glucose to pyruvic acid and subsequently lactic acid, producing a net gain of $2\text{ ATP}$ per glucose for $30\text{--}40\text{ seconds}$ of activity.

Isotonic Contraction

A muscle contraction where tension remains constant while muscle length changes and a load is moved.

Isometric Contraction

A muscle contraction in which tension rises significantly but muscle length remains unchanged because the load is immovable.

Muscle Tone

A sustained state of continuous partial muscle contraction mediated by asynchronous motor unit activation that maintains firmness and posture.

Hypertrophy

An increase in total skeletal muscle size and strength driven by the enlargement of individual muscle fiber diameters rather than an increase in cell number.

Slow Oxidative (SO) Fibers

Fatigue-resistant, red skeletal muscle fibers rich in myoglobin, mitochondria, and capillaries that contract slowly and primarily utilize aerobic respiration (prominent in postural muscles).

Fast Glycolytic (FG) Fibers

Large-diameter, easily fatigued white skeletal muscle fibers that have low myoglobin content, few mitochondria, and high glycogen stores, generating ATP anaerobically for rapid, powerful contractions.

Dystrophin

A structural protein that anchors muscle fiber microfilaments to the surrounding fascia; its absence or defect causes muscular dystrophy.

Origin

The anatomical attachment point of a muscle tendon to an immovable or less movable bone.

Insertion

The anatomical attachment point of a muscle tendon to a movable bone that moves toward the origin during contraction.

Prime Mover (Agonist)

The muscle that bears the principal responsibility for causing a specific movement.

Antagonist

A muscle that opposes or reverses the action of a prime mover.

Synergist

A muscle that assists a prime mover by producing the same movement or reducing undesirable movements.

Fixator

A specialized synergist that immobilizes a bone or muscle origin to provide a stable anchor for the prime mover.

Circular Fascicle Arrangement

A pattern where muscle fascicles are arranged in concentric rings, forming sphincters that surround external body openings (e.g., orbicularis oris).

Convergent Fascicle Arrangement

A pattern where fascicles converge from a broad origin toward a single, central insertion tendon (e.g., pectoralis major).

Fusiform Fascicle Arrangement

A modified parallel fascicle arrangement that produces a spindle-shaped muscle with an expanded central belly (e.g., biceps brachii).

Pennate Fascicle Arrangement

A muscle architecture where short fascicles attach obliquely to a central tendon, which can be unipennate, bipennate, or multipennate.