Anatomy Terms Starting With M
Anatomy Glossary: M
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Medulla
/ meh-DUL-ah / · Latin medulla, marrow or innermost part
Medulla is the innermost region of an organ, most notably the medulla oblongata of the brainstem, the renal medulla of the kidney, and the adrenal medulla, each performing distinct regulatory functions specific to its organ.
The medulla oblongata contains autonomic control centers for heart rate, blood pressure, breathing, swallowing, and vomiting; injury to this region is rapidly fatal because no other brain region can substitute for these functions. Housed within the renal medulla are the loops of Henle, vasa recta, and collecting ducts that generate and exploit the osmotic gradient required for urine concentration. The adrenal medulla, derived embryologically from neural crest cells rather than mesoderm, secretes epinephrine and norepinephrine directly into the bloodstream in response to sympathetic stimulation during stress.
This shared name across three anatomically unrelated structures reflects a consistent anatomical principle: the medulla is always the core, surrounded by a cortex or outer layer.
The medulla oblongata of the human brainstem contains the pre-Bötzinger complex, a small cluster of neurons first identified by Jack Feldman's group in 1991 that generates the basic rhythm of breathing; destroying this cluster in experimental animals abolishes spontaneous respiration entirely.
Urinary System Fun Facts →The word medulla always refers to the brainstem. In anatomy, medulla denotes the inner zone of any organ that has a distinct outer cortex, including the kidney, adrenal gland, lymph node, and thymus.
In the human kidney, the renal medulla is organized into 8 to 18 cone-shaped renal pyramids whose tips, called papillae, drain concentrated urine into the renal pelvis before it passes to the ureter.
How To Become A Nephrologist? →Meninges
/ meh-NIN-jeez / · Greek meninx, membrane
Meninges are the three concentric protective membranes, named the dura mater, arachnoid mater, and pia mater, that surround the brain and spinal cord and enclose the cerebrospinal fluid that cushions the central nervous system.
The dura mater is a tough, fibrous outer membrane adherent to the inner surface of the skull and spinal canal; the arachnoid mater is a delicate web-like middle layer separated from the dura by the thin subdural space; the pia mater is a thin, vascular membrane directly adherent to the brain and spinal cord surface. Cerebrospinal fluid circulates through the subarachnoid space between the arachnoid and pia at a production rate of approximately 500 milliliters per day, cushioning the central nervous system against mechanical trauma and providing immunological support. Blood vessels within the meninges nourish the outer portions of the brain, and rupture of a meningeal artery, as in epidural hematoma, can compress brain tissue within minutes.
Bacterial meningitis, caused by organisms such as Neisseria meningitidis, constitutes a medical emergency because the enclosed space allows rapidly rising pressure to compress the brainstem.
Sharks and other cartilaginous fishes possess only two meningeal layers, a primitive inner and outer membrane, rather than the three-layered arrangement found in mammals; the tripartite meninges appear to be a vertebrate innovation that became fully elaborated in amniotes.
Fun Facts About the Nervous System →The skull directly contacts the brain surface. Three distinct membrane layers and fluid-filled spaces intervene between the inner surface of the skull and the brain, providing both mechanical cushioning and immunological separation.
Fun Facts About the Skeletal System →In the Nile crocodile (Crocodylus niloticus), the meninges enclose a cerebrospinal fluid space that also distributes heat absorbed through the skull, contributing to selective brain cooling during basking, a thermoregulatory function not seen in mammals.
How To Become A Neurologist? →Muscle Fiber
/ MUS-ul FY-ber / · Latin musculus, little mouse; Latin fibra, thread
Muscle fiber is the individual cell of skeletal muscle, an elongated, multinucleate syncytium containing parallel myofibrils that generate contractile force when stimulated by a motor neuron.
Each fiber contains hundreds to thousands of myofibrils, each composed of sarcomeres, the repeating contractile units built from interdigitating actin thin filaments and myosin thick filaments. Type I fibers are rich in myoglobin and mitochondria, supporting sustained aerobic activity; type II fibers are larger and more glycolytic, producing brief, powerful contractions. Fiber type proportion is largely genetically determined but shifts toward type I with aerobic endurance training and toward type II with heavy resistance training.
A single human muscle fiber can reach lengths of 30 centimeters in large muscles such as the sartorius, making it one of the longest cell types in the body.
Elite sprinters carry a higher proportion of type IIx fibers, the fastest and most powerful subtype, than endurance athletes, and studies of Olympic-level sprinters suggest this distribution is fixed largely before birth, limiting how much training alone can alter peak sprint performance.
Muscular System Facts →A muscle fiber is the same as a whole muscle. Each whole muscle contains thousands of individual fibers bundled together with connective tissue sheaths, blood vessels, and motor nerve branches.
Fun Facts About the Nervous System →In the bluefin tuna (Thunnus thynnus), a thin layer of slow-twitch, aerobic red muscle fibers runs along the lateral line and sustains continuous cruising, while a large mass of fast-twitch white fibers powers the explosive bursts used during predatory strikes, a spatial separation of fiber types rarely seen so clearly in other vertebrates.
Muscle Tissue
/ MUS-ul TISH-yoo / · Latin musculus, little mouse; Old French tissu
Muscle tissue is one of the four basic animal tissue types, composed of contractile cells that generate the mechanical forces required for movement, posture, circulation, and the propulsion of materials through hollow organs.
The three types, skeletal, cardiac, and smooth, differ in structure, location, and neural control: skeletal muscle is voluntary and striated, cardiac muscle is involuntary and striated, and smooth muscle is involuntary and non-striated. All three types use actin-myosin cross-bridge cycling as the molecular basis of contraction, though the regulatory proteins and calcium-signaling pathways differ among them. Skeletal muscle constitutes approximately 40 percent of body mass in lean adults, making it the largest tissue by mass and a major site of glucose uptake and metabolism.
Cardiac muscle cells, unlike skeletal fibers, are connected by intercalated discs containing gap junctions that propagate electrical signals directly from cell to cell, allowing the heart to contract as a coordinated unit.
Jellyfish (phylum Medusozoa) contract using epitheliomuscular cells, a primitive cell type in which the same cell performs both epithelial and contractile functions, suggesting that dedicated muscle tissue evolved later from this dual-purpose ancestral cell.
Muscular System Facts →All muscle tissue is under voluntary control. Cardiac muscle maintains the heartbeat and smooth muscle regulates blood vessel diameter, gut motility, and airway caliber entirely without conscious input, and neither can be voluntarily stopped.
In the common octopus (Octopus vulgaris), the arms contain no rigid skeleton; instead, oblique, transverse, and longitudinal smooth muscle layers work against an incompressible fluid core to produce the precise, flexible movements used for prey capture and manipulation.
Myocardium
/ my-oh-KAR-dee-um / · Greek mys, muscle; kardia, heart
Myocardium is the thick muscular middle layer of the heart wall, composed of cardiomyocytes that generate the contractile force needed to pump blood through the circulatory system.
Left ventricular myocardium reaches up to 11 millimeters in thickness because it must generate enough pressure to drive blood through the entire systemic circulation, while atrial myocardium is only 2 to 3 millimeters thick, reflecting the lower pressures required for chamber filling. Cardiomyocytes demand a continuous oxygen supply, so the myocardium contains one capillary per cell, one of the densest vascular networks in the body. Ischemic heart disease, the leading cause of death globally, results from inadequate myocardial oxygen delivery when coronary arteries are narrowed by atherosclerotic plaques.
Unlike skeletal muscle, cardiomyocytes are electrically coupled through intercalated discs, allowing the entire myocardium to contract as a coordinated unit.
Cardiomyocytes are among the least regenerative cells in the human body, renewing at roughly 1 percent per year in young adults and declining to 0.45 percent per year by age 75. A person who survives to old age will have replaced fewer than half of the cardiomyocytes present at birth, meaning most heart muscle cells must last a lifetime.
How To Become A Heart Surgeon? →The heart wall is one uniform layer. Myocardium is the muscular middle layer sandwiched between the inner endocardium and the outer epicardium, and its thickness varies considerably from chamber to chamber.
In the Burmese python (Python bivittatus), the myocardium undergoes dramatic remodeling within 24 to 48 hours of consuming a large meal, with the heart increasing in mass by up to 40 percent to meet the metabolic demands of digestion, then returning to baseline after the meal is processed.