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Maxillary nerve
Maxillary nerve (V2), middle purely sensory division of the trigeminal nerve; exits via foramen rotundum to the pterygopalatine fossa, ends as the infraorbital nerve, and supplies midface, upper teeth, palate, and nasal mucosa.
Maxillary sinus
Maxillary sinus (antrum of Highmore): the largest paranasal sinus, a pyramid-shaped air cavity in the maxilla draining via a superiorly placed ostium into the middle meatus; first to develop, closely related to upper teeth and sinus lift surgery.
Maximum heart rate
Maximum heart rate is the highest heart rate a person can reach during maximal exertion, which declines with age and is predicted by formulas like 220 minus age.
McBurney's point
McBurney's point is the abdominal surface landmark at the junction of the lateral and middle thirds of the umbilicus-to-right-ASIS line, described by Charles McBurney in 1889 as the site of maximal tenderness in acute appendicitis.
Mechanical lagophthalmos
Mechanical lagophthalmos is incomplete eyelid closure caused by physical obstruction such as proptosis, scarring, ectropion, or symblepharon, distinct from paralytic lagophthalmos; ICD-10 code H02.22, treated with lubricants, taping, tarsorrhaphy, and scar release.
Mechanoreceptor
Mechanoreceptors are sensory receptors converting touch, pressure, vibration, and stretch into neural signals. Key cutaneous types include Merkel, Meissner, Pacinian, and Ruffini receptors, with PIEZO2 as the principal mechanotransducer ion channel.
Meconium
Meconium, the newborn's first stool, is a thick greenish-black intestinal discharge of amniotic fluid, bile, and shed cells; passed in utero in 10-25% of births, it can cause meconium aspiration syndrome.
Medial collateral ligament
Medial collateral ligament (MCL), or tibial collateral ligament: broad knee ligament from the medial femoral epicondyle to the tibia, the primary restraint to valgus stress and the most commonly injured knee ligament, usually treated non-operatively.
Median arcuate ligament syndrome
Median arcuate ligament syndrome (MALS, Dunbar syndrome) is a rare disorder where the diaphragm's median arcuate ligament compresses the celiac artery and celiac plexus, causing postprandial abdominal pain, food fear, and weight loss; treated by surgical ligament release with 70-80% success.
Median nerve
Median nerve: terminal branch of the brachial plexus (lateral and medial cords, C5-T1) innervating most forearm flexors, thenar muscles, and the lateral 3.5 digits; compressed in carpal tunnel and pronator syndrome.
Median sacral artery
Median sacral artery (arteria sacralis mediana): small unpaired branch of the abdominal aorta just above its bifurcation, descending over L4–L5, sacrum, and coccyx; supplies sacrum, coccyx, and posterior rectum, with common-trunk origin variants relevant to spine and pelvic surgery.
Mediastinum
Mediastinum: central thoracic compartment between the pleural sacs, holding the heart, trachea, esophagus, and thymus; divided traditionally into four compartments and, by the ITMIG CT-based standard, into prevascular, visceral, and paravertebral compartments.
Medium spiny neuron
Medium spiny neuron (MSN, spiny projection neuron): inhibitory GABAergic projection neuron of the striatum, about 95% of striatal neurons; D1 direct-pathway and D2 indirect-pathway subtypes; lost in Huntington disease.
Medulla oblongata
The medulla oblongata is the lowermost part of the brainstem, continuous with the spinal cord, which controls breathing, heart rate, and blood pressure and houses nuclei for cranial nerves IX through XII.
Medullary thymic epithelial cell
Medullary thymic epithelial cell (mTEC): thymic medulla epithelial cell enforcing central tolerance via promiscuous gene expression of tissue-restricted antigens regulated by Aire and Fezf2; subsets mTEC I-IV include Aire+ mTEChi and tuft-like mTEC IV; AIRE mutations cause APS-1.
Medulloblastoma
Medulloblastoma is a grade 4 malignant embryonal brain tumor of the posterior fossa cerebellum, occurring mostly in children, classified into WNT, SHH, group 3, and group 4 molecular subgroups, treated with surgery, craniospinal radiation, and chemotherapy.
Meibomian gland
Meibomian gland (tarsal gland): large modified sebaceous gland in the eyelid tarsal plate, described by Heinrich Meibom in 1666, secreting meibum holocrinely to form the tear film's lipid layer; its dysfunction (MGD) causes most evaporative dry eye.
Melanin-concentrating hormone
Melanin-concentrating hormone (MCH) is a cyclic neuropeptide produced mainly in the hypothalamus that regulates feeding behavior, metabolism, and sleep-wake rhythms, and is a target for obesity drugs.
Melanocyte stimulating hormone secreting cell
Melanocyte stimulating hormone secreting cell, also called melanotrope or melanotroph, is the POMC-expressing endocrine cell of the pituitary intermediate lobe that secretes alpha-MSH and beta-endorphin under dopaminergic control.
Melanoma and neural system tumor syndrome
Melanoma and neural system tumor syndrome (melanoma-astrocytoma syndrome, ORPHA:252206, OMIM 155755) is an extremely rare autosomal dominant CDKN2A (9p21.3) predisposition syndrome combining cutaneous melanoma and neural tumors, typically astrocytoma.
Membrane potential
Membrane potential (Vmem, transmembrane voltage) is the inside-minus-outside voltage across a cell membrane, set mainly by K+ permeability and ion gradients, described by Nernst and Goldman-Hodgkin-Katz equations, measured by patch clamp and voltage imaging.
Menarche
Menarche, the first menstrual period, typically at ages 10–16 (mean ~12.4 y), 2–3 years after thelarche; driven by GnRH pulse reactivation; declining secular trend, contested plateau since 2000.
Ménière's disease
Ménière's disease, an inner ear disorder with episodic vertigo, fluctuating hearing loss, tinnitus, and aural fullness, linked to endolymphatic hydrops; cause unknown, treated with diet, drugs, injections, or surgery.
Meningeal lymphatic vessels
Meningeal lymphatic vessels (MLVs), dural lymphatics rediscovered in 2015 by Louveau and Aspelund, drain CSF, interstitial fluid, waste, and immune cells to deep cervical lymph nodes; dysfunction links to Alzheimer's, Parkinson's, stroke, and aging.
Meninges
Meninges: the dura mater, arachnoid mater, and pia mater, the three membranes covering the brain and spinal cord, with their spaces, CSF, blood supply, and related conditions like meningitis and hematomas.
Meningioma
Meningioma, the most common primary brain tumor (37.6% of adult intracranial tumors), a slow-growing tumor of the meninges graded WHO 1-3, with TERT/CDKN2A/B molecular grading and treatment by observation, surgery, or radiosurgery.
Meniscus (anatomy)
Meniscus (anatomy): the medial and lateral knee menisci, wedge-shaped fibrocartilage pads between femur and tibia that transmit load and absorb shock; vascular only peripherally, torn commonly, treated by repair or meniscectomy.
Menstrual cycle
Menstrual cycle: the monthly hormonal process preparing the body for pregnancy, 21-35 days, ovulation mid-cycle, menstruation if no pregnancy; also called period, menses.
Menstruation (mammal)
Menstruation (mammal): cyclic shedding of the uterine endometrium with bleeding, rare among mammals, occurring in higher primates, some bats, the elephant shrew, and the spiny mouse, driven by spontaneous decidualization and progesterone withdrawal.
Meralgia paraesthetica
Meralgia paraesthetica (Bernhardt-Roth syndrome), lateral femoral cutaneous nerve entrapment causing burning, tingling anterolateral thigh pain without motor loss; risk factors obesity, pregnancy, diabetes, tight clothing; treated conservatively first.