Mucous membrane A membrane that lines a body cavity that opens
to the exterior. Also called the mucosa (mu-KO-sa).
Mucus The thick fluid secretion of goblet cells, mucous cells,
mQcoQs glands, and mQcoQs membranes.
Muscle An organ composed of one of three types of muscular tissue
(skeletal, cardiac, or smooth), specialized for contraction to pro-
duce voluntary or involuntary movement of parts of the body.
Muscle action potential A stimulating impulse that propagates along
the sarcolemma and transverse tubules; in skeletal muscle, it is
generated by acetylcholine, which increases the permeability of
the sarcolemma to sodium ions (Na+).
Muscle fatigue (fa-TEG) Inability of a muscle to maintain its strength
of contraction or tension; may be related to insufficient oxygen,
depletion of glycogen, and/or lactic acid buildup.
Muscular tissue A tissue specialized to produce motion in response
to muscle action potentials by its qualities of contractility, exten-
sibility, elasticity, and excitability; types include skeletal, cardiac,
Muscle tone A sustained, partial contraction of portions of a skeletal
or smooth muscle in response to activation of stretch receptors
or a baseline level of action potentials in the innervating motor
Muscular dystrophies (DIS-tro-fez') Inherited muscle-destroying
diseases, characterized by degeneration of muscle fibers (cells),
which causes progressive atrophy of the skeletal muscle.
Muscularis (MUS-ku-la'-ris) A muscular layer (coat or tunic) of an organ.
Muscularis mucosae (mu-KO-se) A thin layer of smooth muscle fibers
that underlie the lamina propria of the mucosa of the gastrointes-
Mutation (mu-TA-shun) Any change in the sequence of bases in a
DNA molecule resulting in a permanent alteration in some inherit-
Myasthenia gravis (mT-as-THE-ne-a) Weakness and fatigue of skeletal
muscles caused by antibodies directed against acetylcholine
Myelin sheath (MT-e-lin) Multilayered lipid and protein covering,
formed by Schwann cells and oligodendrocytes, around axons of
many peripheral and central nervous system neurons.
Myocardial infarction (MI) (mT'-o-KAR-de-al in-FARK-shun) Gross
necrosis of myocardial tissue due to interrupted blood supply.
Also called a heart attack.
Myocardium (mT'-o-KAR-de-um) The middle layer of the heart wall,
made up of cardiac muscle tissue, lying between the epicardium
and the endocardium and constituting the bulk of the heart.
Myofibril (mT-o-FT-bril) A threadlike structure, extending longitudinal-
ly through a muscle fiber (cell) consisting mainly of thick filaments
(myosin) and thin filaments (actin, troponin, and tropomyosin).
Myoglobin (mT-o-GLO-bin) The oxygen-binding, iron-containing pro-
tein present in the sarcoplasm of muscle fibers (cells); contributes
the red color to muscle.
Myogram (MT-o-gram) The record or tracing produced by a myo-
graph, an apparatus that measures and records the force of
Myology (mT-OL-o-je) The study of muscles.
Myometrium (mT'-o-ME-tre-um) The smooth muscle layer of the
Myopathy (mT-OP-a-the) Any abnormal condition or disease of
Myopia (mT-O-pe-a) Defect in vision in which objects can be seen
distinctly only when close to the eyes; nearsightedness.
Myosin (MT-o-sin) The contractile protein that makes up the thick
filaments of muscle fibers.
Nail A hard plate, composed largely of keratin, that develops from
the epidermis of the skin to form a protective covering on the
dorsal surface of the distal phalanges of the fingers and toes.
Nail matrix (MA-triks) The part of the nail beneath the body and root
from which the nail is produced.
Nasal cavity (NA-zal) A mucosa-lined cavity on either side of the
nasal septum that opens onto the face at the external nares and
into the nasopharynx at the internal nares.
Nasal septum (SEP-tum) A vertical partition composed of bone (per-
pendicular plate of ethmoid and vomer) and cartilage, covered
with a mucous membrane, separating the nasal cavity into left
and right sides.
Nasolacrimal duct (na'-zo-LAK-ri-mal) A canal that transports the
lacrimal secretion (tears) from the nasolacrimal sac into the nose.
Nasopharynx (na'-zo-FAR-inks) The superior portion of the pharynx,
lying posterior to the nose and extending inferiorly to the soft
Neck The part of the body connecting the head and the trunk. A
constricted portion of an organ such as the neck of the femur or
Necrosis (ne-KRO-sis) A pathological type of cell death that results
from disease, injury, or lack of blood supply in which many adja-
cent cells swell, burst, and spill their contents into the interstitial
fluid, triggering an inflammatory response.
Negative feedback The principle governing most control systems; a
mechanism of response in which a stimulus initiates actions that
reverse or reduce the stimulus.
Neonatal (ne-o-NA-tal) Pertaining to the first four weeks after birth.
Neoplasm (NE-o-plazm) A new growth that may be benign or malig-
Nephron (NEF-ron) The functional unit of the kidney.
Nerve A cordlike bundle of neuronal axons and/or dendrites and
associated connective tissue coursing together outside the cen-
tral nervous system.
Nerve fiber General term for any process (axon or dendrite) project-
ing from the cell body of a neuron.
Nerve impulse A wave of depolarization and repolarization that self-
propagates along the plasma membrane of a neuron; also called
a nerve action potential.
Nervous tissue Tissue containing neurons that initiate and conduct
nerve impulses to coordinate homeostasis, and neuroglia that
provide support and nourishment to neurons.