G-12 Glossarynegative feedback loop A response mechanism
that serves to maintain a state of internal constancy,
or homeostasis. Effectors are activated by changes in
the internal environment, and the inhibitory actions
of the effectors serve to counteract these changes and
maintain a state of balance.
neoplasm (ne 9 ̆o-plazm) A new, abnormal growth
of tissue, as in a tumor.
nephron (nef 9 ron) The functional unit of the
kidneys, consisting of a system of renal tubules and
a vascular component that includes capillaries of the
glomerulus and the peritubular capillaries.
Nernst equation The equation used to calculate
the equilibrium membrane potential for given ions
when the concentrations of those ions on each side of
the membrane are known.
nerve A collection of motor axons and sensory
dendrites in the peripheral nervous system.
neurilemma (noor 0 ̆ı-lem 9 ̆a) The sheath of
Schwann and its surrounding basement membrane
that encircles nerve fibers in the peripheral nervous
system.
neuroglia (noo-rog 9 le- ̆a) The supporting cells of
the central nervous system that aid the functions
of neurons. In addition to providing support, they
participate in the metabolic and bioelectrical
processes of the nervous system, also called glial
cells.
neurohypophysis (noor 0 o-hi-pof 9 ̆ı-sis) The
posterior part of the pituitary gland that is derived
from the brain. It releases vasopression (ADH)
and oxytocin, both of which are produced in the
hypothalamus.
neuron (noor 9 on) A nerve cell, consisting of a
cell body that contains the nucleus; short branching
processes called dendrites that carry electrical
charges to the cell body; and a single fiber, or axon,
that conducts nerve impulses away from the cell
body.
neuropeptide (noor 0 o-pep 9 t ̄ıd) Any of various
polypeptides found in neural tissue that are
believed to function as neurotransmitters and
neuromodulators. Neuropeptide Y, for example, is
the most abundant polypeptide in the brain and has
been implicated in a variety of processes, including
the stimulation of appetite.
neurotransmitter (noor 0 o-trans 9 mit-er) A
chemical contained in synaptic vesicles in nerve
endings that is released into the synaptic cleft,
where it causes the production of either excitatory or
inhibitory postsynaptic potentials.
neurotrophin (noor 0 ̆o-trof 9 in) Any of a family
of autocrine regulators secreted by neurons and
neuroglial cells that promote axon growth and other
effects. Nerve growth factor is an example.
neutron (noo 9 tron) An electrically neutral particle
that exists together with positively charged protons
in the nucleus of atoms.
nexus (nek 9 sus) A bond between members of a
group; the type of intercellular connection found in
single-unit smooth muscles.
niacin (ni 9 - ̆a-sin) A water-soluble B vitamin
needed for the formation of NAD, which is a
coenzyme that participates in the transfer of
hydrogen atoms in many of the reactions of cell
respiration.
nicotinic receptors (nik 0 ̆o-tin 9 ik re-sep 9 torz)
Receptors for acetylcholine located in the
autonomic ganglia and in neuromuscular junctions.
Their name is derived from the fact that they can also
be stimulated by nicotine, derived from the tobacco
plant.
nidation (ni-da 9 shun) Implantation of the
blastocyst in the endometrium of the uterus.postganglionic parasympathetic neurons. Their
name is derived from the fact that they are also
stimulated by the chemical muscarine, derived from
a mushroom.
muscle spindle (mus 9 el spin 9 d 9 l) A sensory organ
within skeletal muscle that is composed of intrafusal
fibers. It is sensitive to muscle stretch and provides a
length detector within muscles.
myelin (mi 9 ̆e-lin) sheath A sheath surrounding
axons formed from the cell membrane of Schwann
cells in the peripheral nervous system and from
oligodendrocytes in the central nervous system.
myocardial infarction (mi 0 ̆o-kar 9 de-al
in-fark 9 shun) (MI) An area of necrotic tissue in the
myocardium that is filled in by scar (connective)
tissue.
myofibril (mi 0 ̆o-fi 9 bril) A subunit of striated
muscle fiber that consists of successive sarcomeres.
Myofibrils run parallel to the long axis of the muscle
fiber, and the pattern of their filaments provides the
striations characteristic of striated muscle cells.
myofilaments (mi-o-fil 9 a-ments) The thick and
thin filaments in a muscle fiber. The thick filaments
are composed of the protein myosin, and the thin
filaments are composed primarily of the protein actin.
myogenic (mi 0 ̆o-jen 9 ik) Originating within muscle
cells. This term is used to describe self-excitation by
cardiac and smooth muscle cells.
myoglobin (mi 0 ̆o-glo 9 bin) A molecule composed
of globin protein and heme pigment. It is related to
hemoglobin but contains only one subunit (instead
of the four in hemoglobin). Myoglobin is found in
striated muscles, wherein it serves to store oxygen.
myoneural (mi 0 ̆o-noor 9 al) junction A synapse
between a motor neuron and the muscle cell that it
innervates; also called the neuromuscular junction.
myopia (mi-o 9 pe- ̆a) A condition of the eyes in
which light is brought to a focus in front of the
retina because the eye is too long; also called
nearsightedness.
myosin (mi 9 ̆o-sin) The protein that forms the
A bands of striated muscle cells. Together with the
protein actin, myosin provides the basis for muscle
contraction.
myxedema (mik 0 s ̆ı-de 9 m ̆a) A type of edema
associated with hypothyroidism. It is characterized
by accumulation of mucoproteins in tissue fluid.N
NAD nicotinamide adenine dinucleotide
(nik 0 ̆o-tin ̆a-m ̄ıd ad 9 n ̄en di-noo 9 kle- ̆o-t ̄ıd) A coenzyme
derived from niacin that functions to transport
electrons in oxidation-reduction reactions. It helps
to transport electrons to the electron-transport chain
within mitochondria.
naloxone (na 0 l ok-s ̄on, n ̆a-lok 9 -s ̄on) A drug that
antagonizes the effects of morphine and endorphins.
natriuretic (na 0 tr ̆ı-yoo-ret 9 ik) hormone A
hormone that increases the urinary excretion of
sodium. This hormone has been identified as atrial
natriuretic peptide (ANP), produced by the atria of
the heart.
natural killer (NK) cells These are related to killer
T lymphocytes but are part of the innate immune
system. NK cells attack body cells infected with
viruses and tumor cells by direct contact, and also
secrete cytokines that activate B and T lymphocytes
as well as macrophages and neutrophils.
necrosis (n ̆e-kro 9 sis) Cellular death within tissues
and organs as a result of pathological conditions.
Necrosis differs histologically from the physiological
cell death of apoptosis.mineralocorticoid (min 0 er-al-o-kor 9 t ̆ı-koid) Any
of a class of steroid hormones of the adrenal cortex
(corticosteroids) that regulate electrolyte balance.
mitochondria (mi 0 to-kon 9 dre-ah), sing.
mitochondrion The organelles in cells that are
responsible for the production of most of the
ATP through aerobic respiration. The citric acid
cycle, the electron transport system, and oxidative
phosphorylation occur within the mitochondria.
mitosis (mi-to 9 sis) Cell division in which the
two daughter cells receive the same number of
chromosomes as the parent cell (both daughters and
parent are diploid).
molal (mo 9 lal) Pertaining to the number of moles
of solute per kilogram of solvent.
molar (mo 9 lar) Pertaining to the number of moles
of solute per liter of solution.
mole (m ̄ol) The number of grams of a chemical
that is equal to its formula weight (atomic weight for
an element or molecular weight for a compound).
monoamine (mon 0 o-am 9 ̄en) Any of a class of
neurotransmitter molecules containing one amino
group. Examples are serotonin, dopamine, and
norepinephrine.
monoamine oxidase (mon 9 o-am 9 ̄en ok 9 s ̆ı-d ̄as)
(MAO) An enzyme that degrades monoamine
neurotransmitters within presynaptic axon endings.
Drugs that inhibit the action of this enzyme thus
potentiate the pathways that use monoamines as
neurotransmitters.
monoclonal (mon-o-klo 9 nal) antibodies
Antibodies produced by a single clone of
lymphocytes. These are generated medically to bind
to a single antigenic determinant site of a molecule—
for example, to detect hCG in a pregnancy test.
monocyte (mon 9 o-s ̄ıt) A mononuclear, non-
granular, phagocytic leukocyte that can be
transformed into a macrophage.
monomer (mon 9 ̆o-mer) A single molecular unit
of a longer, more complex molecule. Monomers
are joined together to form dimers, trimers, and
polymers. The hydrolysis of polymers eventually
yields separate monomers.
mononuclear leukocyte (mon 0 o-noo 9 kle-ar loo 9 k ̆o-
s ̄ıt) Any of a category of white blood
cells that includes the lymphocytes and monocytes.
mononuclear phagocyte (fag 9 ̆o-s ̄ıt) system A term
used to describe monocytes and tissue macrophages.
monosaccharide (mon 0 ̆o-sak 9 ̆a-r ̄ıd) The monomer
of the more complex carbohydrates. Examples of
monomers are glucose, fructose, and galactose. Also
called a simple sugar.
motile (mo 9 til) Capable of self-propelled
movement.
motility (mo-til 9 i-te) The property of movement.
In the gastrointestinal (GI) tract, motility refers to
the ability to mix its contents and move them from
the oral to the anal end of the GI tract by muscular
contractions.
motor cortex (kor 9 teks) The precentral gyrus of
the frontal lobe of the cerebrum. Axons from this
area form the descending pyramidal motor tracts.
motor neuron (noor 9 on) An efferent neuron that
conducts action potentials away from the central
nervous system to effector organs (muscles and
glands). It forms the ventral roots of spinal nerves.
motor unit A lower motor neuron and all of the
skeletal muscle fibers stimulated by branches of
its axon. Larger motor units (more muscle fibers
per neuron) produce more force when the unit is
activated, but smaller motor units afford a finer
degree of neural control over muscle contraction.
muscarinic receptors (mus 0 k ̆a-rin 9 ik re-sep 9 torz)
Receptors for acetylcholine that are stimulated byfox36375_glo_G1-G19.indd 12fox36375_glo_G1-G19.indd 12 2/4/15 12:29 PM2/4/15 12:29 PM