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Holoprosencephaly
Holoprosencephaly (HPE) is the most common human forebrain malformation, in which the cerebral hemispheres fail to separate. Alobar, semilobar, lobar, and MIH forms; SHH pathway disruption; about 1 in 10,000 live births.
Home Parenteral Nutrition
Home total parenteral nutrition (HPN) delivers complete intravenous nutrition at home via a central catheter for chronic intestinal failure. Pioneered in the late 1960s by Scribner, Shils, and Dudrick; about 32,000 US users annually; survival depends on underlying disease.
Homer family
Homer family (Homer1, Homer2, Homer3; Vesl) proteins: postsynaptic density adaptor proteins binding mGluR1/5, IP3 receptors, and Shank via EVH1 domains; long forms multimerize, short Homer1a is an activity-induced dominant negative.
Homosynaptic plasticity
Homosynaptic plasticity: activity-dependent change (LTP or LTD) at synapses that were themselves active during induction, Hebbian, input-specific, NMDA receptor and calcium dependent, distinct from heterosynaptic plasticity at unstimulated synapses.
Horner's syndrome
Horner's syndrome (oculosympathetic paresis) is the triad of miosis, ptosis, and anhidrosis from disruption of the three-neuron sympathetic pathway to the eye; described by Horner in 1869, confirmed with apraclonidine or cocaine testing, and localized with hydroxyamphetamine.
Hue
Hue: the color attribute ordering sensations around a color circle, independent of lightness and saturation. Rooted in cone-opponent retinal processing, formalized from Newton's color circle to CIECAM02 hue angle.
Human anatomy
Human anatomy is the scientific study of the structures of the human body, studied historically by dissection and today by imaging such as CT and MRI.
Human anus
Human anus: external opening at the end of the gastrointestinal tract, terminating the 2.5-4 cm anal canal. Two sphincters, involuntary internal and voluntary external, maintain fecal continence; the pectinate line divides endoderm from ectoderm.
Human back
Human back: posterior trunk region between neck and gluteal regions, built on a 33-vertebra spinal column with extrinsic and intrinsic muscle compartments; protects the spinal cord, maintains posture, and is a common site of back pain.
Human body
Human body: the physical structure of a human organism, studied via anatomy and physiology. Six levels of organization, eleven organ systems, homeostatic control, tens of trillions of cells, and modern cell atlas projects like HuBMAP and the Human Cell Atlas.
Human brain
Human brain: the three-pound central organ of the human nervous system, with about 100 billion nerve cells, an 80-percent-of-volume cortex, a restrictive blood-brain barrier, and a threefold size increase over two million years of hominin evolution.
Human brain mapping
Human brain mapping: measuring and charting human brain structure and function with PET, fMRI, EEG, MEG, and related methods; also a Wiley neuroimaging journal founded in 1993.
Human Brain Project
The Human Brain Project was Europe's largest brain research initiative, running from 2013 to 2023 with a goal of simulating the human brain on a supercomputer.
Human digestive system
Human digestive system: GI tract (mouth, esophagus, stomach, small and large intestines, anus) plus liver, pancreas, gallbladder; breaks food into amino acids, fatty acids, sugars; peristalsis, hormones, gut bacteria.
Human echolocation
Human echolocation: using reflected sound, especially self-generated mouth clicks, to sense silent objects. Practiced by some blind people and trainable by sighted people; term coined 1944; 10-week training studies show broad learning.
Human eye
The human eye is the paired sensory organ that focuses light onto the retina, converting it into electrical signals sent through the optic nerve to the brain.
Human feces
Human feces (stool): bowel waste, 75% water, formed as the colon absorbs ~90% of 1.5–2 L daily ileal effluent; classified by the Bristol Stool Chart, used in stool tests, FMT products, and sanitation.
Human hair growth
Human hair growth: the cyclical production of keratin fibers by hair follicles through anagen, catagen, telogen, and exogen phases. Scalp hair grows about 0.35 mm/day; anagen is roughly 90% of the cycle; androgens, DHT, minoxidil, and finasteride regulate it.
Human head
Human head: uppermost body region containing the brain, sensory organs, and the 22-bone skull (cranium), divided into neurocranium and viscerocranium; covers anatomy, development, variation, and head injury.
Human height
Human height (stature) is standing body length, about 80% heritable and shaped by nutrition and disease. It has risen for 150 years, ranges from 158 cm (Indonesia) to 183 cm (Dutch men), and serves as a population health indicator.
Human leg
Human leg: anatomically the knee-to-ankle region (crus), part of the 30-bone lower limb with femur, patella, tibia, fibula, and foot bones; weight-bearing structure central to bipedal walking and prosthetics.
Human mouth
Human mouth (oral cavity): first portion of the digestive system, containing lips, teeth, tongue, and palate; divided into vestibule and oral cavity proper; serves eating, speech, breathing, and taste; affected by oral diseases in about 3.7 billion people.
Human musculoskeletal system
Human musculoskeletal system: bones (206 in the adult skeleton), skeletal muscle, joints, tendons, ligaments, and cartilage; supports the body, protects organs, makes blood cells, and produces movement; aging and conditions like osteoarthritis and osteoporosis are its main disorders.
Human nose
Human nose: external nose and paired nasal cavities that warm, filter, and humidify air, enable olfaction, drain the paranasal sinuses, and show a nasal cycle alternating airflow between nostrils.
Human penis
Human penis: the external male organ of urination, reproduction, and sexual pleasure, built from two corpora cavernosa and a corpus spongiosum; average 8.9 cm flaccid, 13 cm erect; erection driven by nitric oxide and cGMP.
Human penis size
Human penis size: clinical measurements of length and girth. Pooled means: flaccid 8.70 cm, stretched 12.93 cm, erect 13.93 cm (55,761 men); erect length rose 38% over 29 years; micropenis is SPL 2.5 SD below mean.
Human physiology
Human physiology: study of the life-supporting functions and mechanisms of the human body, from Greek physis and logia. Distinguished by homeostasis (Cannon, 1929), organ-system integration, and clinical uses such as exercise testing.
Human reproductive system
The human reproductive system is the male and female organ system that produces gametes and sex hormones, centered on the testicles and ovaries, enabling fertilization, gestation, and birth of offspring.
Human skeleton
Human skeleton: the body's bony framework, 206 named adult bones in axial (80) and appendicular (126) divisions, about 270 at birth, with support, protection, movement, blood cell production, and mineral storage functions.
Human skin
Human skin, the body's largest organ (about 6 pounds, 18 square feet), has three layers: epidermis, dermis, and hypodermis; it provides barrier, sensory, thermoregulatory, and vitamin D functions and hosts a diverse microbiome.