USMLE Step 1 key terms

A rapid, transient change in membrane voltage in which depolarization opens voltage-gated sodium channels and repolarization restores the resting potential. It travels along excitable cells to transmit signals.

The steady electrical charge difference across a cell membrane at rest, maintained mainly by potassium leak channels and the sodium-potassium pump. Typical neurons sit near minus seventy millivolts.

Jumping of action potentials from one node of Ranvier to the next along a myelinated axon. It speeds impulse transmission and reduces energy cost compared with unmyelinated fibers.

The substrate concentration at which an enzyme works at half its maximum rate. A low value indicates high substrate affinity; changes in the constant suggest competitive inhibition.

A seven-transmembrane receptor that activates an intracellular heterotrimeric G protein when bound by ligand, altering second-messenger production such as cAMP or inositol trisphosphate.

A pattern in which one altered copy of a gene on a non-sex chromosome is enough to cause disease. Affected people usually have an affected parent, and males and females are affected equally.

The condition in which allele and genotype frequencies in a large, randomly mating population stay constant across generations absent selection, mutation, migration, or genetic drift.

Programmed cell death in which caspases dismantle the cell into membrane-bound bodies that are cleared without inflammation. It removes damaged or surplus cells during development and tissue turnover.

A gene whose normal product restrains cell division or repairs DNA. Losing both functional copies, as in retinoblastoma or Li-Fraumeni syndrome, permits uncontrolled growth.

A lasting increase in synaptic strength after repeated high-frequency stimulation, mediated by increased calcium entry through NMDA receptors and more AMPA receptors at the synapse. It underlies learning and memory.

A persistent weakening of synaptic transmission after low-frequency stimulation, often through calcium-dependent removal of AMPA receptors. It contributes to memory refinement and cerebellar motor learning.

An acute, fluctuating disturbance of attention and awareness, usually caused by medical illness, drugs, or metabolic derangement. It develops over hours to days and differs from the chronic decline of dementia.

Findings after damage to corticospinal pathways above the anterior horn, including spasticity, hyperreflexia, extensor plantar responses, and minimal muscle atrophy. Strength loss tends to affect groups of muscles.

Clear fluid produced mainly by choroid plexus cells, filling the ventricles and subarachnoid space. It cushions the brain, removes waste, and its composition helps diagnose meningitis or hemorrhage.

Coating of a pathogen with antibodies or complement fragments so phagocytes can grip it through Fc or complement receptors. It greatly increases the efficiency of phagocytosis and clearance.

A group of plasma proteins that activate in sequence to tag microbes, recruit inflammatory cells, and assemble a membrane attack complex that lyses cells. It links innate and adaptive immunity.

A surface molecule on nearly all nucleated cells that displays short peptides from internal proteins to CD8-positive cytotoxic T cells. Cells showing foreign peptide are killed.

The coordinated process that stops bleeding, involving vascular constriction, platelet plug formation, and fibrin clot stabilization through the coagulation cascade. It is balanced by natural anticoagulants.

The rule that stretching cardiac muscle with greater filling raises the force of contraction, so stroke volume increases as end-diastolic volume rises within physiologic limits.

The volume of blood the heart pumps per minute, equal to heart rate times stroke volume. It is a key measure of circulatory adequacy and is regulated by preload, afterload, and contractility.

A negative-feedback loop in which stretch receptors in the carotid sinus and aortic arch sense blood pressure and adjust sympathetic and vagal outflow to keep pressure stable.

A control mechanism in which rising hormone or metabolite levels suppress further release from the source. It keeps endocrine axes such as thyroid and adrenal systems within a stable range.

Reduced responsiveness of muscle, fat, and liver to insulin, causing impaired glucose uptake and increased hepatic glucose output. It drives hyperglycemia and compensatory hyperinsulinemia in type 2 diabetes.

An enzyme in thyroid follicular cells that attaches iodine to thyroglobulin and links tyrosine residues, forming thyroid hormone precursors. It is inhibited by thioamides used in hyperthyroidism.

A glucocorticoid made in the adrenal cortex that raises blood glucose, suppresses inflammation and immune responses, and maintains vascular tone. Its release is controlled by ACTH from the pituitary.

Recycling of bile acids, drugs, or metabolites from the intestine back to the liver after absorption in the ileum or colon. It conserves bile salts and can prolong drug effects.

The hydrogen-potassium ATPase on gastric parietal cell membranes that secretes acid into the stomach lumen in exchange for potassium. It is blocked irreversibly by proton pump inhibitors.

A small aggregate of bile salts and phospholipids that surrounds fat digestion products in the intestinal lumen, allowing them to be transported to the brush border for absorption.

The model in which myosin heads bond to actin, pivot, and release in repeated cycles, pulling thin filaments toward the sarcomere center and shortening the muscle fiber.

A large multinucleated bone-resorbing cell derived from monocyte precursors. It dissolves mineral and collagen, releasing calcium, and is stimulated by parathyroid hormone and RANK ligand.

The outermost epidermal layer of flattened, keratinized dead cells that limits water loss and blocks entry of microbes and toxins. Its lipid-rich matrix is essential for barrier function.

An immediate allergic reaction in which IgE bound to mast cells triggers histamine release after allergen exposure, producing urticaria, bronchospasm, or anaphylaxis.

The volume of fluid filtered by the renal glomeruli per unit time, reflecting total functioning nephron mass. It is estimated clinically from creatinine and falls in chronic kidney disease.

The process in the loop of Henle that builds a concentrated medullary interstitium, letting the collecting duct reabsorb water under ADH control and concentrate urine.

An endocrine cascade in which kidney renin starts production of angiotensin II, which constricts vessels and stimulates aldosterone release, raising blood pressure and sodium retention.

A sharp midcycle rise in luteinizing hormone triggered by rising estradiol, causing ovulation and formation of the corpus luteum. It is a key marker of the fertile window.

The sequence in seminiferous tubules in which spermatogonia divide and mature into sperm, supported by Sertoli cells and driven by testosterone and FSH.

A phospholipid mixture secreted by type II pneumocytes that lowers alveolar surface tension, preventing collapse at low lung volumes and reducing the work of breathing.

The relationship between air reaching alveoli and blood flowing through pulmonary capillaries. Mismatch, such as shunt or dead space, impairs gas exchange and lowers arterial oxygen.

The change in lung volume produced by a given change in transpulmonary pressure. High compliance means easy inflation; low compliance, as in fibrosis, increases the work of breathing.