Human Physiology: Autonomic Nervous System, Muscle Physiology, Cardiovascular Function, and Blood

Comprehensive vocabulary flashcards covering the Autonomic Nervous System, Skeletal and Smooth Muscle Excitation-Contraction Coupling, Cardiac Electrophysiology, Hemodynamics, and Blood Physiology.

Somatic Nervous System

The voluntary motor division of the peripheral nervous system consisting of a single heavily myelinated neuron originating in the central nervous system and releasing acetylcholine ($\text{ACh}$) directly onto skeletal muscle effectors.

Autonomic Nervous System (ANS)

The involuntary visceral motor division of the peripheral nervous system that innervates cardiac muscle, smooth muscle, and glands via a two-neuron efferent chain containing preganglionic and postganglionic fibers.

Preganglionic Neuron

The first neuron in the autonomic efferent pathway whose cell body resides in the central nervous system (brain stem or spinal cord) and whose lightly myelinated axon projects to an autonomic ganglion.

Postganglionic Neuron

The second neuron in the autonomic efferent pathway whose cell body lies in an autonomic ganglion and whose unmyelinated axon extends directly to the visceral effector organ.

Sympathetic Division

The thoracolumbar division ($T_1\text{--}L_2$) of the autonomic nervous system responsible for 'fight, flight, freeze' responses, acute challenge arousal, and mobilizing body energy.

Parasympathetic Division

The craniosacral division of the autonomic nervous system that mediates 'rest, restore, regulate' activities, visceral homeostasis, and energy conservation.

Spinal Cord Horns (Sensory vs. Motor)

The dorsal horn contains interneurons receiving afferent somatic and visceral sensory signals, while the lateral and ventral horns house cell bodies that send efferent autonomic (visceral motor) and somatic motor impulses outward.

Dual Innervation

The innervation of a visceral organ by both sympathetic and parasympathetic divisions, producing either antagonistic effects (e.g., heart rate regulation) or complementary effects.

Mono-innervated Organs

Structures supplied by only one division of the autonomic nervous system (predominantly sympathetic), including blood vessels, the adrenal medulla, liver, adipose tissue, and cellular metabolism.

Nicotinic Receptors

Cholinergic receptors located on postganglionic autonomic cell bodies and skeletal muscle motor end plates that bind acetylcholine ($\text{ACh}$) to produce rapid, always stimulatory responses.

Muscarinic Receptors

G-protein-coupled acetylcholine ($\text{ACh}$) receptors found on parasympathetic target effectors and sympathetic cholinergic targets (such as skeletal muscle vessels during exercise).

Adrenergic Receptors

G-protein-coupled membrane receptors divided into alpha ($\alpha$) and beta ($\beta$) classes that bind norepinephrine ($\text{NE}$) or epinephrine to elicit excitatory or inhibitory responses.

Varicosities

Swollen, bulbous release sites along autonomic axons that discharge neurotransmitters over wide synaptic areas rather than forming focal neuromuscular junctions.

Neuromuscular Junction (NMJ)

The specialized synapse between the axon terminal of a somatic motor neuron and the folded motor end plate of a skeletal muscle fiber.

Acetylcholinesterase (AChE)

An enzyme residing in the synaptic cleft of the neuromuscular junction that degrades acetylcholine ($\text{ACh}$) to terminate skeletal muscle depolarization.

Sarcoplasmic Reticulum (SR)

A specialized network of endoplasmic reticulum in muscle cells that sequesters and releases calcium ions ($\text{Ca}^{2+}$) via enlarged regions termed terminal cisternae.

Transverse Tubule (T-tubule)

Tubular invaginations of the muscle sarcolemma that conduct electrical action potentials deep into the cell interior toward the myofibrils.

Triad

A specific skeletal muscle micro-architecture comprising one central transverse tubule flanked on either side by two terminal cisternae of the sarcoplasmic reticulum.

Sliding Filament Model

The mechanism of muscle contraction wherein thin filaments slide inward past thick filaments toward the $M$ line, causing the $I$ band and $H$ zone to shorten while the $A$ band remains unchanged.

Crossbridge

The mechanical physical link formed when an activated myosin head binds directly to an exposed myosin-binding site on an actin filament.

Power Stroke

The mechanical step in the crossbridge cycle where the release of inorganic phosphate ($\text{P}_i$) causes the myosin head to pivot, pulling actin toward the center of the sarcomere.

Rigor State

The low-energy state occurring at the completion of the power stroke when $\text{ADP}$ is released, leaving the myosin head tightly locked to actin until a new $\text{ATP}$ binds.

Motor Unit

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

Size Principle

The recruitment pattern where motor units with small motor neurons and few muscle fibers are activated first for small forces, while larger motor units are recruited progressively as force demands increase.

Latent Period

The time delay between the initiation of an action potential on the sarcolemma and the measurable generation of mechanical muscle tension.

Isometric Contraction

A contraction in which tension increases up to the applied load but the muscle length does not change and no joint movement occurs.

Isotonic Contraction

A contraction in which muscle tension overcomes the load, creating movement via shortening (concentric) or controlled lengthening (eccentric).

Treppe

A staircase-like increase in peak tension observed when a muscle is stimulated repeatedly at low frequencies allowing complete relaxation between twitches.

Unfused Tetanus (Incomplete Tetanus)

A state of sustained, fluctuating muscle tension produced by intermediate stimulation frequencies that allow brief, partial relaxation periods between successive stimuli.

Fused Tetanus (Complete Tetanus)

A smooth, continuous, maximal muscle contraction with no relaxation between stimuli, elicited by high-frequency action potential firing.

Slow Oxidative Fibers (Type I)

Red skeletal muscle fibers possessing high oxidative capacity, rich mitochondrial and capillary densities, high myoglobin, low myosin ATPase activity, small diameters, and high fatigue resistance.

Fast Glycolytic Fibers (Type IIb)

White skeletal muscle fibers featuring high glycolytic capacity, high myosin ATPase activity, large diameters, high force generation, and low resistance to fatigue.

Muscle Fatigue

A reversible decline in a muscle's ability to maintain a constant force of contraction in the face of long-term, repetitive stimulation.

Calmodulin

The intracellular calcium-binding regulatory protein in smooth muscle that binds cytosolic $\text{Ca}^{2+}$ to form a complex that activates myosin light chain kinase ($\text{MLCK}$).

Myosin Light Chain Kinase (MLCK)

An enzyme activated by the $\text{Ca}^{2+}$-calmodulin complex in smooth muscle that phosphorylates the myosin regulatory light chain to initiate crossbridge cycling.

Myosin Light Chain Phosphatase (MLCP)

An enzyme in smooth muscle that dephosphorylates the myosin light chain, preventing crossbridge interaction and promoting muscle relaxation.

Single-Unit Smooth Muscle

Smooth muscle composed of cells electrically coupled by gap junctions that contract rhythmically and synchronously as a single functional syncytium, found in the gastrointestinal tract and uterus.

Multi-Unit Smooth Muscle

Smooth muscle composed of discrete, independently innervated cells lacking extensive gap junctions, allowing fine, graded, high-precision control, found in the iris, large airways, and large arteries.

Pacemaker Potential (Smooth Muscle)

A regular, spontaneous slow depolarization of the cell membrane that regularly reaches threshold ($-50\,\text{mV}$) to generate action potentials without neural command.

Slow-Wave Potential

Cyclical, oscillating subthreshold fluctuations in membrane potential that do not generate action potentials on their own unless further depolarized by external stimuli.

Atrioventricular (AV) Valves

Heart valves that prevent backflow from ventricles into atria during systole, consisting of the tricuspid valve on the right side and the bicuspid (mitral) valve on the left side, tethered by chordae tendineae and papillary muscles.

Semilunar Valves

Three-cusp heart valves located at the bases of major arterial trunks (pulmonary valve on the right and aortic valve on the left) that open during ventricular ejection and prevent diastolic backflow.

Sinoatrial (SA) Node

The primary physiological pacemaker of the heart situated in the superior right atrium that spontaneously fires action potentials at the fastest intrinsic rate.

Atrioventricular (AV) Node Delay

The intrinsic delay in electrical impulse conduction through the $AV$ node ($\approx 0.1\,\text{sec}$) that ensures atria completely finish contracting and filling the ventricles before ventricular systole begins.

P Wave

The first electrocardiogram ($\text{ECG}$) deflection, caused by electrical depolarization spreading through the atria from the sinoatrial node.

QRS Complex

The tall multi-peaked $\text{ECG}$ waveform representing rapid ventricular depolarization while electrically masking atrial repolarization.

T Wave

The dome-shaped $\text{ECG}$ deflection reflecting ventricular repolarization, which starts at the ventricular apex and moves toward the base.

P-R Interval (P-Q Interval)

The segment on an $\text{ECG}$ measured from the start of the $P$ wave to the onset of the $QRS$ complex representing the transit time through the atria and the $AV$ node.

Cardiac Contractile Cell Action Potential (Plateau Phase)

Phase $2$ of a ventricular myocyte action potential, marked by sustained membrane depolarization resulting from inward $\text{Ca}^{2+}$ flux through $L$-type voltage-gated calcium channels balancing outward $\text{K}^+$ flux.

Funny Channels

Hyperpolarization-activated cyclic nucleotide-gated channels in cardiac pacemaker cells that open at negative potentials to conduct an inward $\text{Na}^+$ current, initiating the pacemaker prepotential.

Isovolumetric Contraction

Phase $2$ of the cardiac cycle occurring during early systole when ventricular pressure rises sharply while both the atrioventricular and semilunar valves remain completely closed, keeping blood volume constant.

Isovolumetric Relaxation

Phase $4$ of the cardiac cycle occurring during early diastole when the ventricles relax and pressure plummets behind closed semilunar and closed atrioventricular valves with no volume change.

End-Diastolic Volume (EDV)

The maximum volume of blood contained within a ventricle at the conclusion of diastole, typically measuring approximately $130\,\text{mL}$ in resting ventricles.

End-Systolic Volume (ESV)

The residual volume of blood left inside a ventricle at the conclusion of ventricular ejection (systole), typically measuring approximately $60\,\text{mL}$ at rest.

Stroke Volume (SV)

The volume of blood pumped out by each ventricle per heartbeat, defined by the formula $\text{SV} = \text{EDV} - \text{ESV}$, averaging roughly $70\,\text{mL/beat}$.

Ejection Fraction (EF)

The fraction or percentage of end-diastolic blood volume ejected during one systolic stroke, calculated as $\text{EF} = \frac{\text{SV}}{\text{EDV}}$.

Cardiac Output (CO)

The total volume of blood pumped out by each ventricle per minute, calculated as $\text{CO} = \text{HR} \times \text{SV}$, typically equal to approximately $5.0\,\text{L/min}$ at rest.

Frank-Starling Law of the Heart

The intrinsic principle stating that increased end-diastolic volume stretches cardiac myocytes closer to their optimal length, increasing troponin affinity for $\text{Ca}^{2+}$ and boosting stroke volume proportionally to venous return.

Preload

The degree of mechanical stretch experienced by ventricular myocardium just prior to contraction, directly determined by end-diastolic volume ($\text{EDV}$).

Afterload

The resistance or arterial pressure that the ventricles must overcome to force open the semilunar valves and eject blood into the systemic or pulmonary circuits.

Contractility (Inotropy)

The intrinsic contractile force and velocity of myocardial fibers independent of preload, enhanced by sympathetic stimulation via cyclic $\text{AMP}$ and protein kinase $A$.

Pressure Reservoir

The physiological role of elastic conduit arteries (such as the aorta) that expand to store blood pressure during ventricular systole and elastically recoil during diastole to drive continuous peripheral blood flow.

Vascular Resistance Equation

The mathematical relationship determining resistance to fluid flow: $R \propto \frac{\eta \times l}{r^4}$, demonstrating that vascular resistance is inversely proportional to the fourth power of vessel radius ($r$).

Pulse Pressure (PP)

The difference between systolic blood pressure ($\text{SP}$) and diastolic blood pressure ($\text{DP}$), expressed as $\text{PP} = \text{SP} - \text{DP}$.

Mean Arterial Pressure (MAP)

The average perfusion pressure driving blood through the systemic capillary beds, calculated as $\text{MAP} = \text{DP} + \frac{1}{3}\text{PP} = \frac{\text{SP} + 2\,\text{DP}}{3}$, or hemodynamically as $\text{MAP} = \text{CO} \times \text{TPR}$.

Korotkoff Sounds

Audible tapping sounds heard through a stethoscope during sphygmomanometry caused by turbulent blood flow passing through an artery partially compressed by a blood pressure cuff.

Active Hyperemia

The intrinsic autoregulatory increase in organ blood flow triggered by elevated local metabolic activity, which lowers local $\text{O}_2$ and elevates local $\text{CO}_2$, causing arteriolar smooth muscle relaxation.

Reactive Hyperemia

The transient surge in local tissue blood flow that occurs immediately following the removal of a temporary mechanical occlusion or blockage of blood supply.

Myogenic Response

The intrinsic vascular reflex wherein arteriolar smooth muscle mechanically contracts in response to passive stretch (elevated perfusion pressure) and relaxes in response to reduced pressure.

Renin-Angiotensin-Aldosterone System (RAAS)

A hormonal cascade triggered by renal renin secretion that converts angiotensinogen to angiotensin $I$, which $ACE$ converts to angiotensin $II$, inducing systemic vasoconstriction, thirst, and renal $\text{Na}^+$ and water reabsorption to increase $\text{MAP}$.

Starling Forces

The four physical pressures governing transcapillary fluid exchange: capillary hydrostatic pressure ($P_{\text{CAP}}$), interstitial fluid hydrostatic pressure ($P_{\text{IF}}$), capillary colloid osmotic pressure ($\pi_{\text{CAP}}$), and interstitial colloid osmotic pressure ($\pi_{\text{IF}}$).

Skeletal Muscle Pump

A venous return mechanism in the limbs where contraction of surrounding skeletal muscle squeezes deep veins, driving blood through opened proximal valves while distal valves prevent retrograde pooling.

Albumin

The most abundant plasma protein, comprising approximately $60\%$ of total plasma protein, synthesized by the liver to maintain plasma colloid osmotic pressure and act as a carrier.

Erythrocyte (RBC)

A biconcave, anucleated formed element lacking organelles, packed with approximately $250\,\text{million}$ hemoglobin molecules ($97\%$ of dry mass) that generates $\text{ATP}$ solely via anaerobic respiration.

Diapedesis

The active migration of leukocytes out of the circulatory bloodstream by squeezing between adjacent capillary endothelial cells into inflamed tissues.

Chemotaxis

The directed migration of white blood cells toward an inflammatory site along a concentration gradient of foreign or endogenous chemical attractants.

Neutrophil

The most abundant granulocyte ($50\text{--}70\%$ of leukocytes) containing a $4\text{--}6$ lobed nucleus and lysosome-like granules that acts as the first responder to phagocytize and destroy bacteria.

Eosinophil

A granulocyte ($2\text{--}4\%$ of leukocytes) possessing a bilobed nucleus and large reddish granules with digestive enzymes that combat parasitic worms and degrade histamine to end allergic responses.

Basophil

The rarest granulocyte ($0.5\text{--}1\%$ of leukocytes) with a bilobed nucleus and dark coarse granules containing histamine that initiates allergic and inflammatory reactions.

Lymphocyte

An agranular leukocyte ($25\text{--}45\%$ of leukocytes) characterized by a large spherical nucleus filling most cell volume, classified into antibody-producing $B$-cells and cell-destroying $T$-cells.

Monocyte

A large agranular leukocyte ($3\text{--}8\%$ of leukocytes) with abundant cytoplasm and a kidney-shaped nucleus that migrates into tissues to differentiate into a phagocytic macrophage.

Hemostasis

The three-step physiological cascade that stops bleeding after blood vessel rupture, consisting of vascular spasm, platelet plug formation, and blood coagulation.

Von Willebrand Factor (vWF)

A plasma protein that adheres to exposed subendothelial collagen fibers at sites of vascular trauma, binding to surface receptors on passing platelets to activate them.

Thrombin

A proteolytic coagulation enzyme activated during the clotting cascade that cleaves soluble fibrinogen into insoluble fibrin monomers.

Factor XIIIa (FXIIIa)

A cross-linking coagulation enzyme activated by thrombin that covalently stabilizes loose fibrin mesh polymers into a permanent, insoluble cross-linked fibrin clot.

Reticulocyte

An immature red blood cell formed after ejection of the normoblast nucleus that circulates in peripheral blood for $1\text{--}2\,\text{days}$ while synthesizing residual hemoglobin before maturing into an erythrocyte.

Agglutination

The clumping of donor red blood cells that occurs when plasma antibodies (agglutinins) bind to matching foreign antigens (agglutinogens) on erythrocyte surfaces, potentially causing hemolysis.