2,701 articles
Glymphatic system
Glymphatic system: brain-wide perivascular CSF-ISF exchange network, described by Iliff et al. 2012, that clears amyloid-β and tau; sleep-enhanced, AQP4-dependent, measured by DTI-ALPS, and contested by diffusion and IPAD models.
Goatee
Goatee: a small, usually pointed beard grown only on the chin, also called circle beard or Van Dyke when connected to a mustache. Named for a goat's chin tuft (1842); revived by Jacob Elordi in 2025–2026.
God helmet
God helmet, or Koren helmet, a device by Michael Persinger applying weak temporal-lobe magnetic fields to induce a sensed presence; double-blind and placebo studies attribute reported experiences to suggestibility, not the fields.
Golgi tendon reflex
Golgi tendon reflex (inverse myotatic reflex, autogenic inhibition): spinal reflex where Golgi tendon organ Ib afferents disynaptically inhibit the contracting muscle's alpha motor neurons and excite antagonists; task-dependent, not just protective.
Golgi's method
Golgi's method (reazione nera), Camillo Golgi's 1873 silver impregnation technique, stains 1–5% of neurons fully black; basis of the neuron doctrine debate with Cajal and still used for single-neuron morphology.
Gonad
Gonad: the gamete-producing reproductive gland, ovary (eggs) or testis (sperm); also an endocrine gland making sex hormones. Develops bipotential from the urogenital ridge; SRY triggers testis, Foxl2/Wnt4 the ovary.
Gonadarche
Gonadarche: reactivation of the hypothalamic GnRH pulse generator at puberty onset, causing gonadal maturation and sex steroid secretion; first sign is thelarche in girls, testicular enlargement (≥4 mL) in boys; distinct from adrenarche.
Goose bumps
Goose bumps, or piloerection (cutis anserine, horripilation), are skin bumps from arrector pili muscle contraction under sympathetic control, triggered by cold, fear, and emotional chills; largely vestigial in humans but linked to hair follicle stem cells.
Gorham-Stout disease
Gorham-Stout disease (GSD), or vanishing bone disease, is a rare complex lymphatic malformation causing progressive osteolysis via lymphatic proliferation in bone; roughly 400 cases documented, mostly children, treated with surgery, bisphosphonates, sirolimus, and radiotherapy.
Gracilis muscle
Gracilis muscle (musculus gracilis): slender medial-thigh adductor crossing hip and knee, inserting via pes anserinus, obturator nerve L2-L3; used for ACL grafts and reconstructive flaps.
Granular corneal dystrophy type I
Granular corneal dystrophy type I (GCD1, Groenouw type I, classic GCD): rare autosomal dominant TGFBI p.Arg555Trp stromal corneal dystrophy with childhood-onset breadcrumb-like corneal deposits, glare, erosions, and progressive visual impairment.
Granular corneal dystrophy type II
Granular corneal dystrophy type II (GCD2, Avellino corneal dystrophy) is an autosomal dominant TGFBI (R124H) stromal corneal dystrophy with hyaline and amyloid deposits, common in East Asia, exacerbated by refractive surgery, treated with PTK and keratoplasty.
Granule cell
Granule cell: a very small vertebrate CNS neuron; covers cerebellar granule cells (parallel fibers, mossy fiber input), axonless GABAergic olfactory bulb granule cells, and dentate gyrus principal neurons doing pattern separation.
Gray's Anatomy
Gray's Anatomy is the classic English anatomy reference, first published in London in 1858 as Anatomy: Descriptive and Surgical by Henry Gray with Henry Vandyke Carter's illustrations; now in its 42nd edition (2020) from Elsevier, distinct from Gray's Anatomy for Students and the TV series.
Grayson-Wilbrandt corneal dystrophy
Grayson-Wilbrandt corneal dystrophy (GWCD), also called Grayson-Wilbrandt-Hornhautdystrophie, is an extremely rare, questionable Bowman layer corneal dystrophy described in one 1966 family; autosomal dominant, removed from IC3D edition 3.
Great saphenous vein
Great saphenous vein (GSV), vena saphena magna: the largest superficial leg vein, from dorsal venous arch to femoral vein. Key for varicose vein reflux treatment and as the most-used coronary bypass conduit.
Greater omentum
Greater omentum (great epiploon): the largest peritoneal fold, a four-layer fatty apron from the stomach's greater curvature over the transverse colon; immune milky spots, gastroepiploic blood supply, omental caking in ovarian cancer, surgical flaps.
Greater trochanter
Greater trochanter (trochanter major): the quadrilateral bony eminence on the proximal lateral femur, attachment site for gluteus medius, minimus, piriformis, and obturator muscles; ossifies around age 4, linked to GTPS and fractures.
Groin
The groin, known in anatomy as the inguinal region, is the junction between the torso and the thigh, containing the inguinal canal through which hernias commonly occur.
Group C nerve fiber
Group C nerve fibers (C fibers, group IV) are unmyelinated, 0.2-1.5 micrometer peripheral axons conducting at 0.5-2 m/s, carrying slow burning pain, temperature, and itch, distinguished from myelinated A fibers.
Growth cone
Growth cone: the motile, sensory tip of a growing axon, first described by Cajal in 1890, that steers axon pathfinding via actin-driven filopodia and attractive or repulsive guidance cues such as netrins and semaphorins.
Growth hormone
Growth hormone (somatotropin, GH) is a pituitary peptide hormone acting via the GH receptor and IGF-1; treats growth hormone deficiency, banned in sport (WADA S2), and not supported as anti-aging or performance-enhancing therapy.
GUIDE-IT trial
GUIDE-IT (NCT01685840) was a 2013–2016 randomized trial in 894 high-risk HFrEF patients at 45 US and Canadian sites testing NT-proBNP-guided therapy targeting <1,000 pg/mL versus usual care; it was stopped for futility with no outcome difference.
Gums
Gums (gingivae) are the pink tissues surrounding the base of teeth, comprising marginal, attached, and interdental gingiva. Key diseases are reversible gingivitis and irreversible periodontitis, which affects about 4 in 10 U.S. adults over 30.
Gut–brain axis
Gut–brain axis (microbiota–gut–brain axis): bidirectional communication network linking the central and enteric nervous systems and gut microbiota via vagal, HPA-axis, immune, and metabolite signaling; studied in IBS and neuropsychiatric disease.
Habituation
Habituation: progressive decrease of a response to repeated stimulation, not caused by sensory adaptation or fatigue; distinguished by dishabituation, stimulus specificity, and spontaneous recovery; HPA axis habituates rapidly to repeated stress.
Haematopoiesis
Haematopoiesis, also spelled hematopoiesis and known as blood formation or haemogenesis, is the process by which blood cells form from stem cells, primarily in adult bone marrow.
Hair
Hair: keratin filaments grown from skin follicles, cycling through anagen, catagen, and telogen. Covers structure, thermoregulatory evolution of tightly curled scalp hair, variation, and hair-loss treatment.
Hair follicle
Hair follicle: tube-like skin organ producing hair; over 5 million at birth, 1 million on the scalp. Dermal papilla signals growth, bulge holds stem cells, cycles anagen, catagen, telogen, exogen.
Hamstring
Hamstrings: biceps femoris, semitendinosus, and semimembranosus, the posterior thigh muscles that flex the knee and extend the hip. Commonly strained during sprinting late swing; Nordic hamstring exercise cuts injury rates about 51%.