Muscle & Cardiovascular Physiology

Comprehensive vocabulary flashcards covering skeletal muscle contraction, cross-bridge cycling, mechanics, fiber types, pathology, and complete cardiovascular physiology including cardiac electrophysiology, hemodynamics, and pressure regulation.

Skeletal Muscle Tissue

Striated, multinucleated muscle tissue under voluntary control, featuring cylindrical fibers with peripherally located nuclei and regular transverse banding patterns.

Cardiac Muscle Tissue

Striated, involuntary muscle tissue found exclusively in the heart wall, characterized by branching fibers, single central nuclei, and intercalated discs connecting adjacent cardiomyocytes.

Smooth Muscle Tissue

Non-striated, involuntary muscle tissue composed of spindle-shaped cells with a single centrally located nucleus, found in the walls of hollow organs and blood vessels.

Sarcomere

The repeating functional unit of muscle contraction and shortening, delimited by two Z-lines, consisting of aligned thick (myosin) and thin (actin) myofilaments.

Tropomyosin

A thread-like regulatory protein that lies along the groove of actin filaments and physically blocks cross-bridge binding sites on actin when cytosolic $Ca^{2+}$ is low.

Troponin

A three-subunit regulatory protein complex bound to actin and tropomyosin that binds cytosolic $Ca^{2+}$, undergoing a conformational shift that pulls tropomyosin off the myosin-binding sites.

DHP Receptor (Dihydropyridine Receptor)

A voltage-sensing protein embedded in the transverse tubule (T-tubule) membrane that changes conformation in response to action potentials to open adjacent ryanodine receptor channels.

Ryanodine Receptor

A calcium-release channel situated in the terminal cisternae membrane of the sarcoplasmic reticulum that releases stored $Ca^{2+}$ into the cytosol upon activation by DHP receptors.

SERCA Pump

A $Ca^{2+}$-ATPase transporter located in the sarcoplasmic reticulum membrane that actively pumps cytosolic $Ca^{2+}$ back into the sarcoplasmic reticulum lumen using ATP, inducing muscle relaxation.

Cross-Bridge Cycle

The four-step molecular mechanism of muscle contraction wherein energized myosin binds actin, executes a power stroke upon releasing $ADP$ and $P_i$, unbinds upon binding a new $ATP$, and re-energizes via ATP hydrolysis.

Rigor Mortis

The state of persistent muscle rigidity that occurs after death due to ATP depletion, preventing ATP from binding myosin to break the cross-bridge linkage with actin.

Motor Unit

A single somatic motor neuron and all of the individual skeletal muscle fibers that it innervates, which contract in an all-or-none fashion when stimulated.

Muscle Tetanus

A sustained, maximal muscle contraction resulting from repetitive, high-frequency stimulation; it presents as unfused tetanus when stimuli occur during partial relaxation, and fused tetanus when stimuli arrive before any relaxation occurs.

Physiological Cross-Sectional Area (PCSA)

A structural parameter of muscle architecture reflecting the total number of sarcomeres aligned in parallel, which is directly proportional to maximum active tension production.

Creatine Phosphate

A high-energy phosphate molecule that rapidly regenerates $ATP$ from $ADP$ via the enzyme creatine kinase during the initial seconds of muscle activity ($CP + ADP \rightleftharpoons C + ATP$).

Slow-Oxidative Fibers (Type 1)

Fatigue-resistant skeletal muscle fibers characterized by high myoglobin content, abundant mitochondria, low myosin-ATPase activity, and slow contraction velocity, recruited first during movement.

Fast-Glycolytic Fibers (Type 2X)

Easily fatigable skeletal muscle fibers characterized by high glycolytic enzyme activity, low myoglobin content, high myosin-ATPase activity, and fast contraction velocities, organized into large motor units for burst power.

Duchenne Muscular Dystrophy (DMD)

An X-linked recessive genetic disorder caused by mutations preventing the production of functional dystrophin, leading to progressive muscle cell membrane instability, damage, and severe muscle wasting.

Gowers' Sign

A clinical sign of proximal muscle weakness wherein a patient uses their hands and arms to physically 'walk up' their lower limbs in order to stand from a supine or seated floor position.

Hematocrit

The fraction of total whole blood volume occupied by packed erythrocytes following centrifugation, normally approximating $45\%$.

Erythropoietin (EPO)

A peptide hormone produced primarily by the kidneys that stimulates the proliferation and differentiation of erythrocyte progenitor cells in the bone marrow.

Intercalated Discs

Specialized transverse junctions connecting adjacent cardiac muscle cells end-to-end, containing desmosomes for structural anchoring and gap junctions for electrical coupling.

Contractile Cardiomyocyte Plateau Phase (Phase 2)

The sustained depolarization phase of a ventricular action potential produced by an influx of $Ca^{2+}$ through voltage-gated L-type $Ca^{2+}$ channels balanced by an efflux of $K^+$.

Cardiac Refractory Period

A prolonged period lasting approximately $250\,\text{ms}$ during which a contractile cardiomyocyte cannot be re-excited, preventing tetanus and allowing rhythmic chamber relaxation and ventricular filling.

Pacemaker Potential

The spontaneous, slow diastolic depolarization ($\sim -60\,\text{mV}$ to $-40\,\text{mV}$) of autorhythmic cardiac cells driven by inward $Na^+$ via F-type channels, inward $Ca^{2+}$ via T-type channels, and declining outward $K^+$ current.

Sinoatrial (SA) Node

A specialized collection of autorhythmic pacemaker cells in the right atrial wall near the superior vena cava that sets the intrinsic resting heart rhythm at approximately $70\,\text{action potentials per minute}$.

Electrocardiogram (ECG)

A non-invasive clinical tracing of electrical potential differences recorded at the body surface, reflecting the summation of all electrical dipole currents generated by cardiac action potentials.

P Wave

The initial deflection on an electrocardiogram tracing that represents the spread of electrical depolarization through the atria.

QRS Complex

The prominent electrocardiographic waveform representing ventricular depolarization, during which atrial repolarization also occurs simultaneously.

T Wave

The electrocardiographic wave representing the repolarization of the ventricles.

Heart Murmur

An abnormal sound heard over the heart produced by turbulent blood flow, resulting either from a stenotic valve that fails to open fully or an insufficient valve that permits backward leakage.

Isovolumetric Ventricular Contraction

The early phase of ventricular systole in which ventricles contract with all valves closed, causing ventricular pressure to spike rapidly without changes in ventricular blood volume.

Stroke Volume (SV)

The volume of blood pumped out by each ventricle with a single heartbeat, defined as end-diastolic volume minus end-systolic volume ($SV = EDV - ESV$).

Cardiac Output (CO)

The total volume of blood pumped out by each ventricle per unit time, calculated as the product of heart rate and stroke volume ($CO = HR \times SV$).

Ejection Fraction (EF)

The percentage of end-diastolic blood volume ejected from a ventricle during a single beat, defined as $EF = \frac{SV}{EDV}$, used clinically to assess cardiac contractility.

Preload

The degree of stretch exerted on ventricular cardiomyocytes at the end of diastole, determined by end-diastolic volume (EDV) and operating through the Frank-Starling mechanism to increase stroke volume.

Afterload

The combined arterial pressure and vascular resistance that the ventricles must overcome to open the semilunar valves and eject blood.

Inotropy

The intrinsic contractility of cardiac muscle cells at any given end-diastolic volume, augmented extrinsically by sympathetic stimulation of $\beta_1$-adrenergic receptors via cAMP and protein kinase A.

Elastic Arteries

Thick-walled, large-diameter conduit vessels located close to the heart (such as the aorta) that distend during ventricular ejection and recoil during diastole to act as pressure reservoirs.

Mean Arterial Pressure (MAP)

The average pressure driving blood through the systemic capillary beds, calculated as diastolic pressure plus one-third pulse pressure ($MAP = DP + \frac{1}{3} \times PP$) or as $MAP = CO \times TPR$.

Pulse Pressure (PP)

The difference between systolic pressure and diastolic pressure ($PP = SP - DP$), reflecting stroke volume and arterial compliance.

Active Hyperemia

An intrinsic local regulatory response where elevated tissue metabolic rate causes local decrease in $O_2$ and accumulation of metabolites, triggering arteriolar vasodilation and increased organ blood flow.

Capillary Bulk Flow

The passive, non-diffusional volume filtration and absorption of protein-free plasma across capillary walls driven by the balance of hydrostatic and colloid osmotic Starling pressures.

Skeletal Muscle Pump

A peripheral circulatory mechanism where contracting skeletal muscles compress deep veins, elevating venous pressure and driving blood through one-way venous valves toward the heart.

Arterial Baroreceptors

Sensory stretch receptors situated in the carotid sinuses and aortic arch that monitor systemic blood pressure and adjust afferent firing rates to the medullary cardiovascular control center.