Knes 191a: The Muscular System

Comprehensive vocabulary flashcards covering the structural hierarchy, connective tissue coverings, intracellular organelles, sarcomere anatomy, neuromuscular junction mechanisms, excitation-contraction coupling, energy systems, and muscular clinical pathologies from Knes 191a: The Muscle System.

Excitability

The ability of a muscle to respond to stimulation, such as a nerve signal.

Contractility

The ability of a muscle to contract and develop tension or force.

Extensibility

The ability of a muscle to be stretched or lengthened.

Elasticity

The ability of a muscle to return toward its original length after stretching.

Whole Muscle

The complete muscle organ, composed of many bundles of muscle fibers along with supporting connective tissues, nervous tissue, and blood vessels.

Fascicle

A bundle containing multiple individual muscle fibers surrounded by perimysium.

Muscle Fiber

One long, cylindrical skeletal muscle cell containing multiple nuclei formed from the fusion of myoblasts.

Myofibril

A long, rodlike structure running parallel inside a muscle fiber that contains repeating contractile units called sarcomeres.

Sarcomere

The repeating functional contractile unit of a myofibril, extending from one Z line (Z disc) to the next.

Myofilaments

The protein filaments found inside sarcomeres, principally thin actin filaments and thick myosin filaments.

Myoblasts

Precursor cells that fuse together during embryonic development to produce multinucleated skeletal muscle fibers.

Fascia

Connective tissue sheet that surrounds and separates muscles from nearby structures, supporting and holding them in position.

Epimysium

Connective tissue layer that surrounds the entire skeletal muscle, protecting and supporting the whole organ.

Perimysium

Connective tissue sheath that surrounds each individual fascicle, separating muscle fibers into bundles.

Endomysium

Delicate connective tissue layer that surrounds each individual muscle fiber.

Tendon

A cordlike or rope-shaped band of dense connective tissue that commonly attaches muscle to bone and transmits contraction force.

Aponeurosis

A broad, flat, sheetlike connective tissue structure that can attach muscle to bone or other connective tissue coverings to transmit contractile force.

Sarcolemma

The plasma membrane surrounding the skeletal muscle fiber that encloses the cell and conducts electrical impulses across its surface.

Sarcoplasm

The cytoplasm of a muscle fiber containing internal structures, organelles, nutrients, and substances required for cellular activity.

Transverse Tubules (T-tubules)

Inward tubular invaginations of the sarcolemma that project deep into the muscle fiber to carry action potentials into the cell interior.

Sarcoplasmic Reticulum (SR)

A network of membranous channels surrounding myofibrils that stores calcium ions ($\text{Ca}^{2+}$) and releases them to initiate contraction.

Terminal Cisternae

Enlarged sac-like regions of the sarcoplasmic reticulum positioned adjacent to T-tubules that act as calcium storage reservoirs.

Triad

A three-unit anatomical structure in a muscle fiber consisting of one central T-tubule flanked on either side by a terminal cisterna of the sarcoplasmic reticulum.

Z Disc (Z Line)

The structural boundary of a sarcomere that anchors the thin actin filaments.

I Band (Light Band)

The region of the sarcomere containing only thin actin filaments, bisected by the Z disc.

A Band (Dark Band)

The region spanning the entire length of the thick myosin filaments, including the zones of overlap with thin actin filaments.

H Zone

The lighter central region of the A band that contains exclusively thick myosin filaments without thin actin overlap.

M Line

A protein structure located at the exact center of the sarcomere within the H zone that helps organize and stabilize thick myosin filaments.

Actin

A contractile protein that forms the backbone of thin filaments and possesses specific binding sites for myosin heads.

Myosin

A contractile motor protein forming the thick filaments whose heads attach to actin binding sites to pull actin toward the center of the sarcomere.

Troponin

A regulatory protein complex that binds calcium ions ($\text{Ca}^{2+}$), causing a conformational change that shifts tropomyosin away from actin binding sites.

Tropomyosin

A regulatory protein that covers the myosin-binding sites on actin in a relaxed muscle fiber to block cross-bridge formation.

Titin

A large structural protein that anchors thick filaments, stabilizes the sarcomere, and contributes to the passive elasticity of muscle tissue.

Alpha-actinin

A structural protein that anchors and organizes the thin actin filaments at the Z disc.

Cross-Bridge

The physical attachment formed when an energized myosin head binds to an exposed active site on an actin filament.

Power Stroke

The pivoting action of the myosin head that pulls the thin actin filament toward the center of the sarcomere, releasing ADP and inorganic phosphate ($\text{P}_i$).

Neuromuscular Junction (NMJ)

The specialized synapse or communication site where the axon terminal of a motor neuron meets the motor end plate of a skeletal muscle fiber.

Motor Neuron

A nerve cell that conducts action potentials from the central nervous system to peripheral muscle fibers to elicit contraction.

Axon Terminal (Synaptic Bulb)

The distal expanded end of a motor neuron arborization where neurotransmitters are stored and secreted into the synaptic cleft.

Synaptic Vesicles

Membrane-bound spheres inside the axon terminal that package and store the neurotransmitter acetylcholine.

Acetylcholine (ACh)

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

Synaptic Cleft

The microscopic, fluid-filled space between the presynaptic motor neuron terminal and the postsynaptic motor end plate.

Motor End Plate

The specialized, folded region of the sarcolemma directly opposite the axon terminal that contains high densities of acetylcholine receptors.

ACh Receptors

Chemically gated ion channel proteins on the motor end plate that bind acetylcholine to allow sodium ($\text{Na}^+$) influx.

Acetylcholinesterase (AChE)

An enzyme located in the synaptic cleft and motor end plate that rapidly hydrolyzes acetylcholine into acetate and choline to terminate stimulation.

Polarization

The resting electrical state of a cell membrane where the inside is negative relative to the outside, characterized by high extracellular $\text{Na}^+$ and high intracellular $\text{K}^+$.

Depolarization

The reduction or reversal of membrane polarity caused by the inward flow of sodium ions ($\text{Na}^+$), generating a positive internal charge and propagating an action potential.

Repolarization

The restoration of the negative resting membrane potential driven by the movement of potassium ions ($\text{K}^+$) out of the cell.

Action Potential

A rapid, self-propagating electrical impulse that travels along the excitable membrane of a neuron or sarcolemma.

Creatine Phosphate (CP)

A high-energy storage molecule in muscle tissue that rapidly donates a high-energy phosphate group to ADP to resynthesize ATP during the first $3\text{--}15\text{ seconds}$ of maximal exertion.

Creatine Kinase

The intracellular enzyme that catalyzes the reversible transfer of a phosphate group between creatine phosphate and ADP to form ATP.

Glycolysis

A 10-step anaerobic metabolic pathway occurring in the cytosol/sarcoplasm that converts 1 molecule of glucose into 2 molecules of pyruvic acid, yielding a net of $2\text{ ATP}$ and $2\text{ NADH}$.

Lactate Shuttle

The physiological mechanism by which lactate produced in active muscle fibers diffuses into the bloodstream to be taken up and used as an energy substrate by the heart and other skeletal muscles or converted to glucose by the liver.

Cori Cycle

The metabolic pathway in which lactate produced by anaerobic glycolysis in muscles travels via the blood to the liver, where gluconeogenesis converts it back into glucose for muscle use.

Oxidation

A chemical reaction that involves the loss of electrons or hydrogen atoms from a molecule.

Reduction

A chemical reaction that involves the gain of electrons or hydrogen atoms by a molecule.

Excess Post-Exercise Oxygen Consumption (EPOC)

The volume of oxygen consumed above resting levels following exercise (also termed oxygen debt) used to resynthesize creatine phosphate, clear lactate, metabolize heat, and replenish oxygen stores in myoglobin and blood.

Hemoglobin

An iron-containing, oxygen-transport protein located in red blood cells that transports oxygen from pulmonary capillaries to peripheral tissues.

Myoglobin

An oxygen-binding globular protein found exclusively in muscle sarcoplasm that stores oxygen and facilitates its intracellular diffusion to mitochondria.

All-or-None Principle

The principle stating that an individual muscle fiber will either fire a complete, maximal action potential when stimulated to threshold or produce no action potential at all.

Threshold

The minimal stimulus voltage or electrical charge required to initiate an action potential along an excitable cell membrane.

Motor Unit

A single somatic motor neuron together with all the individual skeletal muscle fibers that it innervates.

Muscle Twitch

A single, brief cycle of contraction and relaxation exhibited by a muscle fiber in response to an isolated stimulus, consisting of a latent period, contraction period, and relaxation period.

Latent Period

The brief time interval between stimulus application and the initial measurable development of muscle tension.

Summation

The additive mechanical effect in which a second stimulus arrives before complete muscle relaxation occurs, generating greater cumulative tension.

Tetanus

A smooth, sustained maximal contraction produced when a muscle fiber is stimulated at a frequency high enough to eliminate or minimize relaxation intervals.

Recruitment

The process of activating additional motor units within a muscle to progressively grade and increase total contractile force.

Hypertrophy

An enlargement in total muscle mass and cross-sectional diameter resulting from an increase in the size of existing muscle fibers and synthesis of new contractile proteins.

Atrophy

A reduction in the size, mass, and contractile protein content of muscle fibers caused by disuse, denervation, immobilization, disease, or aging.

Muscle Tone

A sustained, partial state of involuntary resting tension maintained within skeletal muscles via low-level autonomic/spinal motor unit firing that supports posture and joint stability.

Type I Muscle Fibers (Slow Oxidative)

Slow-contracting, fatigue-resistant skeletal muscle fibers containing abundant mitochondria, myoglobin, and vascular capillary networks, specialized for aerobic endurance activities.

Type IIa Muscle Fibers (Fast Oxidative-Glycolytic)

Fast-contracting muscle fibers possessing intermediate fatigue resistance that produce relatively high force using both aerobic oxidative phosphorylation and anaerobic glycolysis.

Type IIx Muscle Fibers (Fast Glycolytic)

Rapidly contracting, highly fatiguable fibers rich in glycogen that produce high power and force primarily through anaerobic glycolysis.

Delayed-Onset Muscle Soreness (DOMS)

Pain, stiffness, and inflammation emerging hours to days after strenuous or unaccustomed muscular work—especially eccentric loading—caused by microscopic mechanical disruption of sarcomeres and surrounding connective tissues.

Eccentric Contraction

A mechanical contraction mode during which a muscle generates active tension while concurrently being elongated by an external load.

Ischemia

A condition of insufficient arterial blood supply to a tissue that impairs local oxygen and nutrient delivery and limits ATP regeneration.

Muscle Fatigue

A reversible, temporary reduction in the physiological capacity of a muscle to generate force or maintain mechanical power output.

Myasthenia Gravis

A chronic autoimmune disorder characterized by autoantibodies that bind and destroy acetylcholine receptors on the motor end plate, causing progressive skeletal muscle weakness and fatiguability.

Duchenne Muscular Dystrophy (DMD)

An inherited X-linked recessive muscular dystrophy caused by a deficiency in the cytoskeletal protein dystrophin, rendering the sarcolemma fragile and susceptible to contraction-induced necrosis.

Dystrophin

A critical structural protein that links the internal cytoskeleton of a muscle fiber to the extracellular matrix, mechanically stabilizing and reinforcing the sarcolemma during contraction.

Spasm

A sudden, involuntary, abnormal contraction of a muscle or muscle group.

Cramp

A prolonged and painful involuntary muscle spasm that may be triggered by dehydration, muscle fatigue, inadequate circulation, or electrolyte deficits.

Tic

A spasmodic, involuntary, repetitive twitching movement of muscles normally under voluntary control.

Tremor

An involuntary, rhythmic, oscillatory movement produced by alternating or irregular contractions of opposing muscle groups.

Fasciculation

A localized, brief, involuntary contraction or twitch of an entire single motor unit that is visible beneath the skin.

Fibrillation

An uncoordinated, spontaneous, rapid contraction of individual muscle fibers that is not visible through the skin and must be detected by electromyography (EMG).

Anabolic-Androgenic Steroids

Synthetic derivatives of testosterone that stimulate muscle protein synthesis to induce skeletal muscle hypertrophy and strength gains, but can produce adverse hepatic, cardiovascular, and hormonal effects.