Glossary
G-15
Muscle twitch
Te response of a muscle to a single brief threshold
stimulus.
Muscular dystrophy
A group of inherited muscle-destroying diseases.
Muscular system
Te organ system consisting of the skeletal muscles of
the body and their connective tissue attachments.
Myelencephalon (spinal brain)
A secondary brain vesicle; lower part of
the developing hindbrain, especially the medulla oblongata.
Myelin sheath
(mi
9
ĕ-lin) Fatty insulating sheath that surrounds all but
the smallest nerve fibers.
Myoblasts
Embryonic mesoderm cells from which all muscle fibers
develop.
Myocardial infarction (MI)
(mi
0
o-kar
9
de-al in-fark
9
shun) Condition
characterized by dead tissue areas in the myocardium; caused by inter-
ruption of blood supply to the area. Commonly called heart attack.
Myocardium
(mi
0
o-kar
9
de-um) Layer of the heart wall composed of
cardiac muscle.
Myofibril
(mi
0
o-fi
9
bril) Rodlike bundle of contractile filaments (myofila-
ments) found in muscle fibers (cells).
Myofilament
(mi
0
o-fil
9
ah-ment) Filament that constitutes myofibrils. Of
two types: actin and myosin.
Myoglobin
(mi
0
o-glo
9
bin) Oxygen-binding pigment in muscle.
Myogram
A graphic recording of mechanical contractile activity pro-
duced by an apparatus that measures muscle contraction.
Myometrium
(mi
0
o-me
9
tre-um) Tick uterine musculature.
Myopia
(mi-o
9
pe-ah) A condition in which visual images are focused in
front of rather than on the retina; nearsightedness.
Myosin
(mi
9
o-sin) One of the principal contractile proteins found in
muscle.
Myxedema
(mik
0
sĕ-de
9
mah) Condition resulting from underactive thy-
roid gland.
Nares
(na
9
rez) Nostrils.
Natural killer (NK) cell
Defensive cell (a type of lymphocyte) that can
kill cancer cells and virus-infected body cells before the adaptive immune
system is activated.
Necrosis
(nĕ-kro
9
sis) Death or disintegration of a cell or tissues caused
by disease or injury.
Negative feedback mechanisms
Te most common homeostatic con-
trol mechanism. Te net effect is that the output of the system shuts off
the original stimulus or reduces its intensity.
Neonatal period
Te four-week period immediately a±er birth.
Neoplasm
(ne
9
o-plazm) An abnormal mass of proliferating cells. Benign
neoplasms remain localized; malignant neoplasms are cancers, which can
spread to other organs.
Nephron
(nef
9
ron) Structural and functional unit of the kidney; consists
of the renal corpuscle and renal tubule.
Nerve
A bundle of axons in the peripheral nervous system.
Nerve fiber
Axon of a neuron.
Nerve growth factor (NGF)
Protein that promotes survival and develop-
ment of neurons; secreted by their target cells and many other cell types.
Nerve impulse
A self-propagating wave of depolarization; also called an
action potential.
Nerve plexuses
Interlacing nerve networks that occur in the cervical,
brachial, lumbar, and sacral regions and primarily serve the limbs.
Nervous system
Fast-acting control system that triggers muscle contrac-
tion or gland secretion.
Minerals
Inorganic chemical compounds found in nature; salts.
Mitochondria
(mi
0
to-kon
9
dre-ah) Cytoplasmic organelles responsible
for A²P generation for cellular activities.
Mitosis
Process during which the chromosomes are redistributed to two
daughter nuclei; nuclear division. Consists of prophase, metaphase, ana-
phase, and telophase.
Mitotic (M) phase
One of two major periods in the cell life cycle; in-
volves the division of the nucleus (mitosis) and the division of the cyto-
plasm (cytokinesis).
Mitral (bicuspid) valve
(mi
9
tral) Te le± atrioventricular valve.
Mixed nerves
Nerves containing the processes of motor and sensory
neurons; their impulses travel to and from the central nervous system.
Molar
(mo
9
lar) (1) A solution concentration determined by mass of
solute—1 liter of solution contains an amount of solute equal to its
molecular weight in grams. (2) Broad back teeth that grind and crush.
Molarity
(mo-lar
9
ĭ-te) A way to express the concentration of a solution;
moles per liter of solution.
Mole
(mōl) A mole of any element or compound is equal to its atomic
weight or its molecular weight (sum of atomic weights) measured in grams.
Molecule
Particle consisting of two or more atoms joined together by
chemical bonds.
Monoclonal antibodies
(mon
0
o-klo
9
nal) Pure preparations of identical
antibodies that exhibit specificity for a single antigen.
Monocyte
(mon
9
o-sīt) Large single-nucleus white blood cell; agranular
leukocyte.
Monosaccharide
(mon
0
o-sak
9
ah-rīd) Literally, one sugar; building block
of carbohydrates; e.g., glucose.
Morula
(mor
9
u-lah) Te mulberry-like solid mass of blastomeres result-
ing from cleavage in the early conceptus.
Motor areas
Functional areas in the cerebral cortex that control volun-
tary motor functions.
Motor (efferent) nerves
Nerves that carry impulses leaving the brain
and spinal cord, and destined for effectors.
Motor unit
A motor neuron and all the muscle cells it stimulates.
Mucous membranes (mucosae)
Membranes that form the linings of
body cavities open to the exterior (digestive, respiratory, urinary, and
reproductive tracts).
Mucus
(myoo
9
kus) A sticky, thick fluid secreted by mucous glands and
mucous membranes; keeps the free surface of membranes moist.
Multinucleate cell
(mul
0
tĭ-nu
9
kle-āt) Cell with more than one nucleus,
e.g., skeletal muscle cells, osteoclasts.
Multiple sclerosis (MS)
Demyelinating disorder of the CNS; causes
hardened patches (sclerosis) in the brain and spinal cord.
Multipolar neurons
Neurons with three or more processes; most com-
mon neuron type in the CNS.
Muscarinic receptors
(mus
0
kah-rin
9
ik) Acetylcholine-binding receptors
of the autonomic nervous system’s target organs; named for activation by
the mushroom poison muscarine.
Muscle fiber
A muscle cell.
Muscle spindle
Encapsulated receptor found in skeletal muscle that is
sensitive to stretch.
Muscle tension
Te force exerted by a contracting muscle on some
object.
Muscle tone
Low levels of contractile activity in relaxed muscle; keeps
the muscle healthy and ready to act.
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