NADP+/NADPH (nicotinamide adenine dinucleotide phosphate/reduced nicotinamide adenine dinucleotide phosphate) Electron carrier system closely related to NAD+/ NADH, but used almost exclusively in reductive biosynthetic, rather than catabolic, pathways. (Figure 2–36) naïve cell In immunology: a T or B lymphocyte that proliferates and differentiates into an effector cell or memory cell when it encounters its specific foreign antigen for the first time. (Figure 24–17) natural killer cell (NK cell) Cytotoxic cell of the innate immune system that can kill virus-infected cells and some cancer cells.
natural regulatory T cell   A regulatory T cell (Treg cell) that develops in the thymus and helps maintain self-tolerance.
negative selection   Process by which thymocytes expressing a T cell receptor with high affinity for a self peptide bound to a self-MHC protein are eliminated by undergoing apoptosis.
negative staining   A technique in electron microscopy enabling fine detail of isolated macromolecules to be seen. Samples are prepared such that a very thin film of heavy-metal salt covers everywhere except where excluded by the presence of macromolecules, which allow electrons to pass through, creating a reverse or negative image of the molecule.
Nernst equation   Equation that computes relates the electrical potential (voltage) generated by differences in ion concentrations across a membrane.
Netrin   Signal protein, secreted by cells of the neural tube floor plate, responsible for attracting growth cones of commissural axons toward and across the midline.
neural crest   Collection of cells located along the line where the neural tube pinches off from the surrounding epidermis in the vertebrate embryo. Neural crest cells migrate to give rise to a variety of tissues, including neurons and glia of the peripheral nervous system, pigment cells of the skin, and the bones of the face and jaws. (Figure 19–8) neural map   Regular mapping of neurons of a similar type from one territory to another, such that there are orderly projections of one array of neurons onto another.
neural tube   Tube of ectoderm that will form the brain and spinal cord in a vertebrate embryo. (Figure 21–56) neurofilament   Type of intermediate filament found in nerve cells. (Figure 16–72) neuromuscular junction Specialized chemical synapse between an axon terminal of a motor neuron and a skeletal muscle cell. (Figure 11–37) neuron (nerve cell) Impulse-conducting cell of the nervous system, with extensive processes specialized to receive, conduct, and transmit signals. (Figures 11–28 and 21–66) neuronal specificity   Nonequivalence among neurons; an intrinsic characteristic that guides axons to their appropriate target sites.
neurotransmitter   Small signal molecule secreted by the presynaptic nerve cell at a chemical synapse to relay the signal to the postsynaptic cell. Examples include acetylcholine, glutamate, GABA, glycine, and many neuropeptides.
neurotrophic factor   Factor released in limited amounts by a target tissue that the neurons innervating that tissue require to survive.
neurotrophin   Family of signal proteins that promote the survival and growth of specific classes of neurons.
neutrophil White blood cell that is specialized for the uptake of particulate material by phagocytosis. Enters tissues that become infected or inflamed. (Figure 24–5)