Nerve growth factor- a cytokine that promotes the growth and repair of sensory nerves and maintenance of sympathetic nerves.     Neural crest cells - cells derived from the ectoderm layer in the embryo. This layer folds to form a neural tube which will later become the spinal cord of the animal. Cells from the crest of this fold, leave the ectoderm and migrate into the mesoderm layer. These neural crest cells are thus of ectomesenchymal origin. They migrate to form the dental mesenchyme, supportive cells of the nervous system, the adrenal cortex and melanocytes of the skin.     Neuromuscular junction the synapse between the axon terminal of a motoneurone and a skeletal muscle fibre. The release at this synapse of the neurotransmitter substance, acetyl choline causes the muscle to contract.     Neuropeptides - compounds which have extremely potent affect to excite or depress nerve cells, in very low concentration. In this regard they are distinctly different from neurotransmitters. They include substances such as prostaglandins, substance P, endorphins and enkephalins. Neuropeptides also have receptors on lymphocytes and thus influence the immune response.     Neurotransmitters - chemicals which are secreted into a synapse in order to transmit an electrical nerve impulse from a nerve axon of one cell to the cell body of another nerve cell. They bind to receptors on nerve cells and are produced rapidly in high concentrations at nerve synapses. In these respects they differ from neuropeptides. There are many different neurotransmitter substances. They include acetyl choline, epinephrine, histamine, serotonin GABA and glutamate.     Neutrophils - members of the family of white blood cells which are involved in non-specific phagocytosis of bacteria and foreign material. Neutrophils are also called polymorphonuclear leucocytes (PMN) because of their multi-lobed nucleus. Neutrophils also release toxic enzymes, stored safely in their cy lasm as lysosomes and released in the presence of foreign proteins. These toxic enzymes may do more harm to the host tissues than the bacteria they were released to kill.     Niche - an opportunity which can be exploited in order to make a living or survive in an ecosystem.     Noma - a highly destructive and usually fatal infection of the teeth and jaws which is a progression of ANUG Only found in malnourished children. - also called cancrum oris.     Nucleic acids - occur as chains of nucleotides, either asDNA (two chains) or RNA(one chain) and make up the genetic material of a cell.     Nucleotides - are made up of three components, a pentose sugar (ribose or deoxyribose), a phosphate group and an organic base which may either be a purine ( adenine or guanine) or a pyrimidine ( cytosine thymine or uracil).This basic structure is found in many important cell molecules such as in ADP, ATP and coenzymes. Nucleotides also form the subunits from which nucleic acids (DNA and RNA)are built.     Nucleus a cell organelle which contains the chromosomes whose genes control the structure of proteins within the cell. The nucleus is also a term used to refer to the mass of nerve cell bodies connected by tracts of nerve fibres, which occur in the brain.