Medical terms, defined.

The tentorium cerebelli is a tent-shaped dural fold that separates the occipital lobes (supratentorial) from the cerebellum (infratentorial) and forms the roof of the posterior cranial fossa. Its free edge surrounds the tentorial notch (...

The posterior communicating artery (PCom) connects the internal carotid artery (anterior circulation) to the posterior cerebral artery (posterior circulation), forming the lateral side of the Circle of Willis. It is a classic site for be...

The anterior inferior cerebellar artery (AICA) is a branch of the basilar artery (proximal portion) that supplies the anterior-inferior cerebellum, the lateral pons (caudal), and the middle cerebellar peduncle. It also gives rise to the...

The posterior inferior cerebellar artery (PICA) is the largest branch of the intracranial vertebral artery. It supplies the lateral medulla, the inferior cerebellar vermis, and the posterior-inferior cerebellar hemisphere. Occlusion caus...

The superior sagittal sinus (SSS) is an unpaired dural venous sinus running along the superior attached margin of the falx cerebri in the midline. It receives blood from superior cerebral veins and drains CSF via arachnoid granulations,...

The superior cerebellar artery (SCA) arises from the distal basilar artery just below the bifurcation into the posterior cerebral arteries, separated from them by the oculomotor nerve (CN III). It supplies the superior cerebellum, dentat...

The posterior spinal arteries (paired) supply the posterior one-third of the spinal cord, including the dorsal columns (fasciculus gracilis and cuneatus) and part of the dorsal horns. They arise from the vertebral arteries (or PICA) and...

The basilar artery is formed by the union of the two vertebral arteries at the pontomedullary junction and ascends along the ventral pons in the basilar groove. It supplies the pons, cerebellum, midbrain, and (via the posterior cerebral...

The left anterior fascicle (LAF) is one of two subdivisions of the left bundle branch, along with the left posterior fascicle. It conducts the impulse to the anterior-superior left ventricle (anterolateral papillary muscle region). It is...

The left posterior fascicle is one of two divisions of the left bundle branch (the other is the left anterior fascicle). It is a short, broad band that conducts impulses to the posteroinferior left ventricle, including the inferior wall...

The cardiac skeleton is a dense fibrous connective tissue framework at the base of the heart that surrounds the four valve orifices, anchors the valve leaflets, and electrically separates the atria from the ventricles. Its only normal co...

LAD most commonly = Left Anterior Descending artery, a branch of the left main coronary artery supplying the anterior LV wall, anterior 2/3 of the interventricular septum, and the apex. It is the most commonly occluded coronary artery ("...

The cardiac fibrous skeleton is a dense connective tissue framework (collagen) surrounding the four heart valve rings (annuli fibrosi), the aortic root, and the membranous septum. It provides structural support and anchors valve leaflets...

The renal papilla is the apex of a renal pyramid in the medulla, projecting into a minor calyx. It's where collecting ducts (of Bellini) empty urine into the calyx, and it's a watershed area of low blood flow that is vulnerable to ischem...

The renal cortex is the outer region of the kidney, lying beneath the capsule and between the medullary pyramids (as renal columns of Bertin). It contains the glomeruli, Bowman capsules, proximal and distal convoluted tubules, and cortic...

Renal pyramids are cone-shaped structures of the renal medulla (typically 8–18 per kidney) composed of straight tubules: loops of Henle, collecting ducts, and vasa recta. Their apices (renal papillae) drain urine into minor calyces, and...

Bowman capsule is the cup-shaped, double-walled structure at the start of the nephron that surrounds the glomerular capillaries and collects the ultrafiltrate (Bowman space) that flows into the proximal convoluted tubule. Together, the g...

The vasa recta are long, straight peritubular capillaries that arise from the efferent arterioles of juxtamedullary nephrons and run parallel to the loops of Henle in the renal medulla. They act as countercurrent exchangers, removing rea...

The renal corpuscle is the initial filtering unit of the nephron, consisting of the glomerulus (capillary tuft) surrounded by Bowman capsule. It filters plasma into Bowman space via the glomerular filtration barrier (fenestrated endothel...

Juxtaglomerular (JG) cells are modified smooth muscle cells in the wall of the afferent arteriole that synthesize, store, and secrete renin. They act as intrarenal baroreceptors, and renin release is stimulated by decreased renal perfusi...

The muscularis mucosae is a thin layer of smooth muscle forming the deepest layer of the GI tract mucosa (lamina propria → muscularis mucosae), separating the mucosa from the submucosa. Its contraction moves the mucosa to form folds and...

Haustra are the sacculations (pouches) of the colon wall formed by the tonic contraction of the taeniae coli, which are shorter than the colon itself. They are a radiographic hallmark of the normal colon, and their loss ("lead pipe" colo...

The serosa (serous membrane) is the outermost layer of intraperitoneal/intrathoracic organs, made of simple squamous mesothelium over a thin layer of connective tissue. It secretes lubricating serous fluid to reduce friction and correspo...

The pyloric sphincter is a thick ring of circular smooth muscle at the gastroduodenal junction that regulates gastric emptying and prevents duodenogastric (bile) reflux. It is tonically contracted, relaxes in response to peristaltic wave...

The muscularis propria (muscularis externa) is the main smooth muscle layer of the GI tract wall, lying deep to the submucosa. It usually has an inner circular and an outer longitudinal layer and drives peristalsis and segmentation. Inva...

Taeniae coli are three distinct longitudinal ribbons of smooth muscle (mesocolic, omental, free) that run along the cecum and colon, formed by condensation of the outer longitudinal muscle layer. They create the haustra (sacculations) an...

The hepatic ducts are the bile ducts that carry bile out of the liver. The right and left hepatic ducts drain the corresponding liver lobes and join at the porta hepatis to form the common hepatic duct. The common hepatic duct then joins...

Epiploic appendages are small, fat-filled peritoneal pouches along the antimesenteric surface of the colon (cecum through sigmoid; absent in the rectum). They are normally asymptomatic, but torsion or spontaneous venous thrombosis causes...

The internal anal sphincter (IAS) is a thickened continuation of the inner circular smooth muscle layer of the rectum/anal canal. It is involuntary, innervated by the autonomic nervous system, and provides about 70–85% of resting anal to...

The right main bronchus is wider, shorter (~2.5 cm), and more vertical (~25° from vertical) than the left main bronchus. Because of this, aspirated foreign bodies, food, and gastric contents most commonly lodge in the right lung, and end...

The canals of Hering are the short anatomic channels that connect bile canaliculi (formed between hepatocytes) to the interlobular bile ducts in the portal tract. They are lined by both hepatocytes and cholangiocytes and are a niche for...

Portal-systemic (portocaval) anastomoses are collateral venous connections between the portal and systemic circulations that open up in portal hypertension, shunting blood around the liver. Key sites: esophageal (left gastric → azygos; e...

The alveolar duct is a respiratory zone airway that arises from a respiratory bronchiole and leads to alveolar sacs. Its walls are almost entirely composed of alveoli (outpouchings), so it participates in gas exchange. It is lined by sim...

A bile canaliculus is a tiny channel formed by the apical (canalicular) membranes of adjacent hepatocytes into which bile is secreted. Canaliculi drain toward the portal triad into the canals of Hering and then bile ductules, and bile fl...

The external anal sphincter (EAS) is a striated (skeletal) muscle ring surrounding the anal canal, under voluntary control, innervated by the inferior rectal branch of the pudendal nerve (S2–S4). It provides voluntary continence and cont...

Type I pneumocytes (alveolar type I cells) are thin, flat squamous epithelial cells that cover ~95% of the alveolar surface area and form the thin blood-air barrier for gas exchange. They are highly susceptible to injury and cannot divid...

The terminal bronchiole is the last purely conducting airway of the respiratory tree (generation ~16). It is lined by simple cuboidal ciliated epithelium and Club (Clara) cells, has no cartilage or goblet cells, and has no alveoli, so it...

Club cells (formerly Clara cells) are non-ciliated, dome-shaped secretory cells of the bronchiolar epithelium (especially terminal bronchioles). They secrete surfactant-like components and club cell secretory protein (CC16), detoxify inh...

Hepatic sinusoids are low-pressure, highly permeable fenestrated capillaries that carry mixed blood from the portal vein (nutrient-rich, ~75%) and hepatic artery (oxygen-rich, ~25%) past hepatocytes to the central vein. They lack a basem...

The neural groove is the longitudinal midline depression in the neural plate that forms during neurulation (week 3 of development). Its lateral edges elevate as neural folds and fuse to form the neural tube, the precursor of the CNS. Fai...

The costophrenic angle (CPA) is the sharp, acute angle on a chest X-ray formed where the diaphragm meets the lateral chest wall (costal margin). Blunting of the CPA is a classic sign of pleural effusion (≥~200-300 mL on upright PA film;...

Somites are paired blocks of paraxial mesoderm that form along the neural tube starting in week 3 of development (about 42-44 pairs total). They differentiate into sclerotome (vertebrae and ribs), myotome (skeletal muscle of the trunk),...

The visceral (pulmonary) pleura is the thin serous membrane that directly covers the lung surface and extends into the fissures. It is supplied by autonomic (visceral) nerves, so it is insensitive to pain, unlike the parietal pleura. Blo...

A myotome is the group of skeletal muscles innervated by a single spinal nerve root (one segmental level). Testing myotomes (muscle strength by key movements) helps localize radiculopathy or spinal cord injury level, e.g., C5 shoulder ab...

The pharyngeal (branchial) membranes are thin double-layered partitions of ectoderm (outside) and endoderm (inside) that separate the pharyngeal clefts from the pharyngeal pouches in the developing embryo. Mesoderm/neural crest usually g...

The sclerotome is the ventromedial portion of the embryonic somite that gives rise to the vertebrae and ribs (axial skeleton and intervertebral discs' annulus fibrosus). Its cells migrate to surround the notochord and neural tube, and di...

The first pharyngeal (mandibular) arch gives rise to the maxillary and mandibular prominences. It is derived from neural crest mesenchyme and is supplied by CN V (trigeminal: V2 and V3). It forms the muscles of mastication and Meckel car...

A dermatome is an area of skin supplied by sensory fibers from a single spinal nerve (dorsal root/dorsal root ganglion). Dermatomal maps localize spinal cord/nerve root lesions and explain patterns like herpes zoster ("shingles") eruptio...

The first pharyngeal pouch is an endodermal outpouching of the primitive pharynx that gives rise to the tympanic (middle ear) cavity, mastoid air cells, and eustachian (pharyngotympanic) tube. It also contributes to the inner lining of t...

The third pharyngeal arch gives rise to the stylopharyngeus muscle (the only muscle derived from it), innervated by cranial nerve IX (glossopharyngeal). Its skeletal derivatives are the greater horn and lower body of the hyoid, and its a...