G-16 glossary
nerve tract A bundle of myelinated axons of interneurons
inside the spinal cord and brain.
nervous system System of neurons oriented relative to one
another in precise message-conducting and information-
processing pathways.
nervous tissue Tissue composed of neurons and (in the cen-
tral nervous system) neuroglia.
neural tube Embryonic forerunner of the brain and spinal
cord.
neuromodulator A signaling molecule that influences the
effects of transmitter substances by enhancing or reducing
membrane responses in target neurons.
neuromuscular junction Chemical synapse between axon
terminals of a motor neuron and a muscle cell.
neuron A nerve cell; the basic unit of communication in the
nervous system. Neurons collectively sense environmental
change, integrate sensory inputs, then activate muscles or
glands that initiate or carry out responses.
neurotransmitter Any of the class of signaling molecules
that are secreted from neurons, act on adjacent cells, and are
then rapidly degraded or recycled.
neutrophil Phagocytic white blood cell that takes part in
inflammatory responses against bacteria.
niche (nitch) [L. nidas, nest] The full range of physical and
biological conditions under which members of a species can
live and reproduce.
nitrogen cycle Biogeochemical cycle in which gaseous
nitrogen is captured by nitrogen-fixing microorganisms and
then moves through organisms and ecosystems before being
returned to the atmosphere. The atmosphere is the largest res-
ervoir of nitrogen.
nitrogen fixation Process by which a few kinds of bacteria
convert gaseous nitrogen (N 2 ) to ammonia.
NK cell Natural killer cell; kills virus-infected cells and some
types of cancer cells.
nociceptor A receptor, such as a free nerve ending, that
detects stimuli causing tissue damage.
nondisjunction Failure of one or more chromosomes to sepa-
rate properly during mitosis or meiosis.
nonrenewable resource A natural resource that exists in a
finite amount and cannot be replenished.
nonshivering heat production Response to severe cold;
brown adipose tissue releases energy as heat rather than stor-
ing it as ATP.
nonsteroid hormone A type of water-soluble hormone,
such as a protein hormone, that cannot cross the lipid bilayer
of a target cell. These hormones enter the cell by receptor-
mediated endocytosis, or they bind to receptors that activate
membrane proteins or second messengers within the cell.
nosocomial infection An infection that is acquired in a
hospital, usually by direct contact with a microbe.multifactorial trait A trait that is shaped by more than one
gene as well as by environmental factors.
multiple allele system A gene that has three or more
different molecular forms (alleles).
muscle fatigue A decline in the ability of a muscle to
contract; occurs when a muscle has been kept in a state of
strong contraction as a result of continuous, high-frequency
stimulation.
muscle tension A mechanical force, exerted by a contracting
muscle, that resists opposing forces such as gravity and the
weight of objects being lifted.
muscle tissue Tissue having cells able to contract in
response to stimulation, then passively lengthen and so
return to their resting state.
muscle tone In muscles, a steady low-level contracted state
that helps stabilize joints and maintain general muscle health.
muscle twitch Muscle response in which the muscle
contracts briefly, then relaxes, when a brief stimulus activates
a motor unit.
muscular system Skeletal muscles, which attach to bones
and pull on them to move the body and its parts.
mutation, gene See gene mutation.
myelin sheath Of many sensory and motor neurons, an
axonal sheath that affects how fast action potentials travel;
formed from the plasma membranes of Schwann cells or oli-
godendrocytes that wrap repeatedly around the axon and are
separated from each other by a small node.
myocardium The cardiac muscle tissue.
myofibril (my-oh-fy-brill) One of many threadlike structures
inside a muscle cell; each is functionally divided into sarco-
meres, the basic units of contraction.
myosin (my-uh-sin) A contractile protein. In muscle cells, it
interacts with the protein actin to bring about contraction.
NADP Nicotinamide adenine dinucleotide phosphate;
a phosphorylated nucleotide coenzyme. When carrying
electrons and unbound protons (H+) between reaction sites, it
is abbreviated NADPH 2.
natural selection A difference in survival and reproduction
among members of a population that vary in one or more
traits.
negative feedback A homeostatic mechanism in which an
activity changes some condition in the internal environment
and so triggers a response that reverses the change.
nephron (neff-ron) [Gk. nephros, kidney] Of the kidney, a
slender tubule in which water and solutes filtered from blood
are selectively reabsorbed and in which urine forms.
nerve Cordlike communication line of the peripheral
nervous system, composed of axons of sensory neurons,
motor neurons, or both, encased in connective tissue. In the
brain and spinal cord, similar cordlike bundles are called
nerve tracts.
nerve impulse See action potential.
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