neurotransmitter Achemical made of amino acids
and peptides that switch on or off nerve impulses
across the synapse between neurons. Excitatory neuro-
transmitters stimulate the target cell, while inhibitory
ones inhibit the target cells. Examples of neurotrans-
mitters are acetylcholine, dopamine, noradrenaline,
and serotonin.
Acetylcholine is the most abundant neurotransmit-
ter in the body and the primary neurotransmitter
between neurons and muscles. It controls the stomach,
spleen, bladder, liver, sweat glands, blood vessels, heart,
and others. Dopamine is essential to the normal func-
tioning of the central nervous system. Noradrenaline,
or norepinephrine, act in the sympathetic nervous sys-
tem and produce powerful vasoconstriction. Serotonin
is associated with the sleep cycle.
neutral variation Genetic diversity that appears to
offer no selective advantage.
neutron An atomic particle found in the nuclei of
atoms that is similar to a proton but has no electric
charge.
See alsoELECTRON.
new chemical entity A compound not previously
described in the literature.
niche A habitat providing a particular set of environ-
mental conditions needed for the survival of a given
species. Species that occupy different niches may coex-
ist side by side in a stable manner with no competition.
However, if two species occupy the same niche, i.e., if
they require the same resources, there will be competi-
tion, and the weaker of the two will become extirpated.
Evolutionary effects can make a species adapt to a spe-
cialized niche that has a particular set of abiotic and
biotic factors within the habitat.
Nicolle, Charles-Jules-Henry (1866–1936) French
Biologist Charles-Jules-Henry Nicolle was born in
Rouen, France, Seine-Maritime, on September 21,
1866, to Eugène Nicolle, a local doctor. He entered a
local medical school, where he studied for three years
and then began working in Paris hospitals.
Hereceived his M.D. degree in 1893 and returned
to Rouen, becoming director of the bacteriological lab-
oratory in 1896. He left in 1903 to become director of
the Pasteur Institute in Tunis, a position he held until
his death in 1936.
Early in his career, Nicolle worked on cancer, and
at Rouen he investigated the preparation of diphtheria
antiserum. In North Africa, under his influence, the
Institute at Tunis quickly became a world-famous cen-
ter for bacteriological research and for the production
of vaccines and serums to combat most of the prevalent
infectious diseases.
He discovered in 1909 that the body louse trans-
mits typhus fever, and this discovery was vital to pre-
vention of the disease during the two world wars. He
also made several contributions on knowledge and pre-
vention of Malta fever, tick fever, scarlet fever, rinder-
pest, measles, influenza, tuberculosis, and trachoma. In
1928 he was awarded the Nobel Prize in physiology or
medicine for his work on typhus.
Nicolle wrote several important books, including
La Nature, conception et morale biologiques(1934);
Responsabilités de la Médecine(1936), and LaDes-
tinée humaine(1937).
He was also a philosopher and writer of stories,
such as Le Pâtissier de Bellone(1913), Les Feuilles de
la Sagittaire(1920), La Narquoise(1922), Les Menus
Plaisirs de l’Ennui (1924), Les deux Larrons(1929),
and Les Contes de Marmouse et ses hôtes(1930). He
died on February 28, 1936, in Tunis (Tunisia).nif A set of about 20 GENEs required for the assembly
of the NITROGENASE ENZYMEcomplex.nitrate reductase A METALLOENZYME, containing
molybdenum that reduces nitrate to nitrite.nitrite reductase A METALLOENZYME that reduces
nitrite. DISSIMILATORYnitrite reductases contain copper
and reduce nitrite to nitrogen monoxide. Assimilatory
nitrite reductases contain SIROHEMEand IRON-SULFUR
CLUSTERand reduce nitrite to ammonia.
See alsoASSIMILATION.244 neurotransmitter