Muscular System and Muscle Tissue

Comprehensive practice vocabulary flashcards covering the structural hierarchy, molecular physiology, neuromuscular control, biomechanics, compartments, and embryology of muscle tissue and muscles of the human body.

Skeletal Muscle

voluntary, striated muscle fibers controlled by the central nervous system, numbering greater than 650 distinct muscles in the human body.

Cardiac Muscle

An involuntary striated muscle tissue located exclusively in the walls of the heart.

Smooth Muscle

An involuntary non-striated muscle tissue found predominantly in the walls of hollow visceral organs such as the stomach, respiratory tubes, bladder, blood vessels, and uterus.

Excitability

The functional property of all muscle tissues to receive and respond to chemical or electrical stimuli.

Contractility

The ability of muscle cells to shorten forcefully and produce tension upon stimulation.

Extensibility

The ability of muscle tissue to be stretched beyond its resting length without sustaining damage.

Elasticity

The ability of muscle tissue to passively recoil back to its normal resting length after being stretched.

Sarcoplasmic Reticulum (SR/SER)

The specialized, membranous smooth endoplasmic reticulum that surrounds each myofibril and functions to store and release calcium ions ($Ca^{2+}$) to regulate contraction.

Nutrient Reserve Function of Muscle

The storage function of skeletal muscle referring to contractile proteins that are catabolized for energy during prolonged starvation only after adipose tissue reserves are exhausted.

Epimysium

An exterior dense irregular connective tissue layer composed primarily of type I collagen fibers that encloses the entire muscle, separates it from adjacent structures, and connects to deep fascia.

Perimysium

A dense irregular connective tissue sheath composed of type I and III collagen fibers that groups individual muscle fibers into bundles called fascicles and carries blood vessels and nerves.

Endomysium

A delicate connective tissue sheath composed of reticular fibers (collagen type III) containing capillaries and nerve fibers that surrounds each individual muscle fiber.

Muscle Fascicle

A discrete bundle of skeletal muscle fibers enclosed by the perimysium.

Tendon

A dense regular rope-like or cord-like bundle formed by the convergence of the epimysium, perimysium, and endomysium that attaches a muscle to the bone matrix.

Aponeurosis

A broad, flat sheet of dense regular connective tissue formed by muscle sheaths that anchors a muscle to bone, cartilage, or adjacent muscles.

Myotendinous Junction

The anatomical transition zone where the connective tissue sheaths of a muscle blend directly into the collagen matrix of a tendon or aponeurosis.

Fleshy Attachment

A direct muscle attachment characterized by very short connective tissue fibers where the epimysium appears fused directly to the periosteum of a bone.

Indirect Attachment

A muscle attachment in which connective tissue layers extend beyond the muscle belly to form a distinct tendon or broad aponeurosis.

Myoblasts

Embryonic mesodermal progenitor cells that fuse by the hundreds to thousands to generate a single multinucleated skeletal muscle fiber.

Myosatellite Cells

Unfused embryonic myoblasts retained in adult muscle between the sarcolemma and basement membrane that divide and fuse with existing fibers for hypertrophy and repair.

Muscle Hypertrophy

An increase in skeletal muscle size driven by increased protein synthesis, myofibril formation, and satellite cell fusion, occurring without de novo generation of new muscle fibers (hyperplasia).

Myofibril

A cylindrical, lengthwise subdivision within a muscle fiber ($1\text{ to }2\,\mu m$ in diameter) consisting of bundled actin and myosin filaments, numbering from ~50 per cell in a fetus to ~2000 in an untrained adult.

Myofilaments

Cytoplasmic protein filaments composed of actin (thin filaments) and myosin (thick filaments) that occupy up to $80\%$ of a muscle fiber's volume and drive contraction.

Terminal Cisternae

Expanded, sac-like chambers of the sarcoplasmic reticulum situated adjacent to T tubules that store and concentrate calcium ions ($Ca^{2+}$).

Calsequestrin

A calcium-binding protein located within the terminal cisternae of the sarcoplasmic reticulum that binds $Ca^{2+}$ to facilitate its accumulation against a concentration gradient.

SERCA

The sarco/endoplasmic reticulum calcium ATPase pump located on the SR membrane that actively transports $Ca^{2+}$ from the sarcoplasm into the SR lumen against a 1000-fold concentration gradient.

Triad

A three-part structural unit comprising one central transverse (T) tubule flanked on either side by two terminal cisternae of the sarcoplasmic reticulum.

Sarcomere

The smallest functional contractile unit of a myofibril, defined as the repeating segment between two successive Z lines (discs), numbering approximately 10,000 per myofibril.

A Band

The dark anisotropic band of a sarcomere corresponding to the entire length of the thick myosin filaments, which polarizes light.

I Band

The light isotropic band of a sarcomere containing only thin actin filaments that does not polarize light, located outside the zone of overlap.

H Band (H Zone)

The pale central region (Helles) of the A band on either side of the M line containing thick myosin filaments only, without overlapping thin filaments.

M Line

The central structural line (Mittel) at the middle of the A band composed of myomesin and M-proteins that attaches and cross-links neighboring thick filaments.

Hexagonal Filament Arrangement

The spatial organization within the zone of overlap where each thick myosin filament is surrounded symmetrically by exactly 6 thin actin filaments, and each thin filament is surrounded by 3 thick filaments.

Z Disc (Z Line)

The boundary structure (Zwischen) located in the center of each I band constructed of actinins and titin that anchors thin actin filaments and bounds the sarcomere.

Actinin

A structural protein within the Z disc that anchors thin actin filaments and binds adjacent myofibrillar sarcomeres together.

Titin

A giant elastic protein spanning from the Z disc to the thick filament that centers myosin, prevents overstretching, and provides passive tension and elastic recoil.

Concentric Contraction

An isotonic contraction during which a muscle generates tension and shortens to accomplish physical work (e.g., the upward phase of a bicep curl).

Eccentric Contraction

An isotonic contraction during which a muscle generates force while actively lengthening, serving as a brake to decelerate motion (e.g., the lowering phase of a pushup).

Isometric Contraction

A contraction in which tension is developed within the muscle without any change in overall muscle length (e.g., maintaining a plank position).

Sliding Filament Mechanism

The model of muscle contraction where thin filaments slide past thick filaments toward the M line, narrowing the I band and H zone while the A band remains unchanged in length.

Neuromuscular Junction (NMJ)

The specialized intercellular synapse where the terminal bouton of a somatic motor neuron meets the motor end plate of a skeletal muscle fiber.

Terminal Bouton

The expanded bulbous ending of a motor neuron axon that contains synaptic vesicles packed with the neurotransmitter acetylcholine (ACh).

Synaptic Cleft

The narrow extracellular fluid-filled gap (approximately 20 to 50 nm wide) separating the terminal bouton of a motor neuron from the motor end plate.

Acetylcholine (ACh)

The neurotransmitter released via exocytosis from somatic motor neuron terminals to initiate muscle membrane depolarization.

Nicotinic Acetylcholine Receptor (nAChR)

A ligand-gated, non-specific monovalent cation channel on the motor end plate that opens upon binding ACh, permitting rapid $Na^+$ influx and $K^+$ efflux.

Acetylcholinesterase (AChE)

An enzyme embedded within the synaptic cleft that rapidly hydrolyzes acetylcholine into acetate and choline to limit stimulation to a single twitch.

Dihydropyridine Receptor (DHPR)

A voltage-sensitive protein located within the T-tubule membrane that detects action potential depolarization and mechanically unlatches RyR1.

Ryanodine Receptor (RyR1)

A calcium release channel located on the sarcoplasmic reticulum membrane that is mechanically linked to DHPR and opens to discharge $Ca^{2+}$ into the sarcoplasm.

Rigor State

The tightly bound, static state where the myosin head remains locked onto the G-actin binding site in the absence of ATP, as observed in rigor mortis.

Cocking Phase

The conformational change in which myosin ATPase hydrolyzes bound ATP into ADP and $P_i$, utilizing the released energy to rotate the head into a high-energy cocked position.

Power Stroke

The mechanical swivel of the myosin head toward the M line upon release of inorganic phosphate ($P_i$), dragging the actin filament inward by approximately 10 nm.

Length-Tension Relationship / evidence for sliding model

Active muscle tension is maximal when sarcomeres are at intermediate resting lengths, declining when overstretched or overly shortened.

Optimal Resting Length

The physiological range of sarcomere length ($75\%$ to $130\%$ of optimal, between $1.6\,\mu m$ and $2.6\,\mu m$) that permits maximal overlap of cross-bridges without filament collision.

Passive Force in Muscle

The non-contractile resistance to stretching exerted primarily by the elastic protein titin and connective tissue sheaths as a muscle is elongated beyond its resting length.

Motor Unit

A functional neuromuscular entity consisting of a single somatic motor neuron and all the skeletal muscle fibers innervated by its terminal axon branches.

Fine Control Motor Unit

A motor unit configuration containing very few muscle fibers (e.g., 4 fibers per unit in extraocular eye muscles) enabling precise, delicate movements.

Gross Control Motor Unit

A motor unit configuration containing large numbers of muscle fibers (e.g., up to 2000 fibers per unit in large weight-bearing leg muscles) designed for powerful contractions.

Henneman's Size Principle

The orderly recruitment rule where smaller, weaker, fatigue-resistant motor units are activated first, with progressively larger, more powerful, and fatigable units added as needed.

Rate Coding

The neuromuscular mechanism of increasing contractile force by elevating the firing frequency of action potentials across already recruited motor units until tetanus is reached.

Muscle Power

The quantitative measure of mechanical muscle performance defined as the amount of tension produced per unit time.

Muscle Endurance

The capacity of a skeletal muscle to sustain physical activity over an extended duration of time.

Slow Oxidative (SO / Type I) Fibers

Small-diameter, red skeletal muscle fibers equipped with slow myosin ATPase, abundant mitochondria, rich myoglobin, and dense capillaries, specialized for fatigue-resistant aerobic contractions.

Fast Glycolytic (FG / Type IIx) Fibers

Large-diameter, pale skeletal muscle fibers characterized by fast myosin ATPase, high glycogen stores, few mitochondria, and reliance on anaerobic glycolysis, producing rapid, powerful, fatigable contractions.

Fast Oxidative-Glycolytic (FOG / Type IIa) Fibers

Intermediate-diameter muscle fibers with fast-acting myosin ATPases and aerobic metabolic machinery, conferring intermediate speed, strength, and fatigue resistance.

Myoglobin

A red, iron-containing intracellular oxygen-binding pigment found abundantly in slow oxidative muscle fibers that facilitates oxygen diffusion from blood capillaries.

Sarcopenia

The progressive, age-related loss of skeletal muscle mass and functional strength, declining at a rate of approximately $10\%$ per decade after age 30 and reaching up to $50\%$ by age 80.

Intercalated Discs

Specialized cell junctions connecting adjacent branching cardiac muscle cells, containing desmosomes for structural integrity and gap junctions for electrical coupling.

Cardiac Muscle T Tubule Organization

The structural arrangement in heart muscle characterized by a single, large-diameter T tubule located at each Z disc per sarcomere, rather than at the A-I junctions.

Multi-Unit Smooth Muscle

Smooth muscle tissue lacking extensive gap junctions where individual cells are innervated by autonomic motor terminals, permitting finely graded, independent contraction.

Somitomeres

Paired, whorled mesodermal segments in the developing embryonic head; the 1st through 3rd form extrinsic eye muscles, and the 4th through 7th form pharyngeal arch muscles.

Myotomes

The dorsolateral region of embryonic somites that differentiates into the skeletal musculature of the axial body wall, limbs, and tongue.

Splanchnic Mesoderm

The embryonic mesodermal layer that develops into visceral involuntary musculature, giving rise to both cardiac muscle of the heart and smooth muscle of internal organs.

Extrinsic Eye Muscles Embryonic Origin

The developmental grouping of six voluntary oculomotor muscles derived specifically from the 1st, 2nd, and 3rd somitomeres of the embryonic head.

Pharyngeal Arch (Branchiomeric) Muscles

The skeletal muscles of facial expression, mastication, and pharyngeal constriction that originate embryologically from the 4th through 7th somitomeres.

Embryonic Limb Rotation

The developmental rotation occurring in weeks 6 to 8 where upper limbs rotate laterally ($90^\circ$ outward, elbow pointing caudally) and lower limbs rotate medially ($90^\circ$ inward, knee pointing cranially).

Muscle Origin

The relatively fixed or less movable point of attachment of a skeletal muscle, typically located proximal to the insertion.

Muscle Insertion

The movable point of attachment of a skeletal muscle that is pulled toward the origin during contraction, typically located distal to the origin.

Agonist (Prime Mover)

A skeletal muscle whose contraction is directly and primarily responsible for producing a specific movement at a joint.

Antagonist

A muscle that opposes or reverses the action produced by a particular agonist, relaxing while the agonist contracts.

Synergist

A muscle that assists a prime mover by generating complementary force, stabilizing an intermediate joint, or preventing unwanted motions.

Fixator

A specialized synergist that contracts isometrically to anchor or immobilize a bone or the origin of a prime mover, providing a stable foundation.

Circular Muscle

A muscle architecture whose fascicles are arranged concentrically around an opening or orifice to act as an anatomical sphincter guarding body entrances and exits.

Convergent Muscle

A muscle pattern where fascicles spread over a broad origin area and converge onto a single slender tendon, aponeurosis, or raphe, permitting versatile pull directions.

Parallel Muscle

The most common fascicle arrangement in which fibers run parallel to the long axis, shortening by approximately $30\%$ and designed for high displacement speed.

Fusiform Muscle

A modified parallel muscle with an expanded central belly that tapers toward both ends at its tendons of attachment (e.g., biceps brachii).

Parallel Interrupted Muscle

A parallel muscle divided into discrete longitudinal segments by transverse fibrous bands or tendinous inscriptions (e.g., rectus abdominis).

Wrapping Muscle

A parallel muscle whose fibers curve or twist around an underlying bone to redirect its angle of pull during joint rotation (e.g., supinator).

Parallel Muscle Cross-Sectional Force Rule

The physiological relationship stating that $1\,\text{in.}^2\,(6.45\,\text{cm}^2)$ of parallel muscle cross-sectional area develops approximately $50\,\text{lb}\,(23\,\text{kg})$ of tension.

Pennate Muscle

A feather-like muscle pattern where fascicles attach obliquely to a central tendon, packing more fibers per unit volume to generate high contractile tension.

Unipennate Muscle

A pennate muscle whose fascicles insert obliquely into only one side of the central tendon (e.g., extensor digitorum longus).

Bipennate Muscle

A common pennate muscle whose fascicles insert obliquely into both sides of a central tendon like the barbs of a quill feather (e.g., rectus femoris).

Multipennate Muscle

A pennate muscle featuring a tendon that branches internally into multiple tendinous plates, with fascicles converging on all sides (e.g., deltoid).

Tendon Tension Formula for Pennate Muscles

The mathematical formula $F_T = F_m \times \cos(\theta)$ where tendon tension ($F_T$) equals total muscle fiber force ($F_m$) multiplied by the cosine of the pinnation angle ($\theta$).

Extensor Digitorum Pinnation Tension Loss

The specific pennate force reduction observed in the extensor digitorum where an angle of $\theta = 32^\circ$ ($\cos(32^\circ) = 0.85$) reduces transmitted tendon force by $15\%$ relative to fiber force.

Fulcrum

The fixed pivot point around which a rigid lever rotates, represented anatomically in the human body by a joint.

Applied Force (Effort)

The muscular force exerted at the site of muscle insertion that moves a bone lever against an opposing resistance.

Load (Resistance)

The gravitational and physical weight of the bone, surrounding tissues, and any external objects that must be moved by a lever system.

First-Class Lever

A lever system arranged in a load-fulcrum-effort sequence where the fulcrum lies between the applied force and the load (e.g., scissors; posterior neck muscles extending the head at the atlanto-occipital joint).

Second-Class Lever

A lever system arranged in a fulcrum-load-effort sequence where a small force can move a massive load slowly over a short distance (e.g., wheelbarrow; standing on tiptoe via gastrocnemius).

Third-Class Lever

The most common lever system in the body arranged in a load-effort-fulcrum sequence, operating at a mechanical disadvantage to maximize displacement speed and distance (e.g., biceps brachii flexing the forearm).