Acetylcholine -- A chemical neurotransmitter released from the ends of particular nerve fibers when stimulated. It transmits a signal to an adjacent nerve or muscle cell by binding to receptors on the target cell surface.

Adenyl cyclase, or adenylate cyclase -- An enzyme that catalyzes the formation of cylic AMP from ATP.

Amino acid -- The basic building block of proteins and polypeptides. It contains a basic amino group, an acidic carboxyl group, and a side chain attached to an alpha carbon atom. Amino acids link together to form proteins, function as chemical messengers, and act as intermediates in metabolism.

AMP, or adenosine monophosphate -- One of the four nucleotides in an RNA molecule. Two phosphates are added to AMP to form ATP.

Anaerobic glycolysis -- A sequence of biochemical reactions in which glycogen or glucose is rapidly metabolized to provide energy for intense muscular work in cases where not enough oxygen is being delivered to the muscle.

Analgesic -- Medication that reduces or eliminates pain.

Antibody -- Any of the protein molecules produced by specialized immune system cells (B cells) that can recognize and bind to a particular foreign antigen. If the antigen is on the surface of a cell, this binding leads to cell aggregation and subsequent destruction. Antibodies are also referred to as immunoglobulins.

Anticodon -- A sequence of three adjacent nucleotides in tRNA that binds to a complementary codon in mRNA and designates a specific amino acid during protein synthesis.

Antigen -- A substance that induces the formation of antibodies because the immune system recognizes it as a threat. It may be a foreign substance from the environment, such as chemicals, or formed within the body, such as bacterial or viral toxins.

Ascites -- An abnormal accumulation of watery fluid in the abdominal cavity.

ATP, or adenosine triphosphate -- Nucleoside triphosphate composed of adenine, ribose, and three phosphate groups that is the primary carrier of chemical energy in cells. The terminal phosphate groups are highly reactive in the sense that their hydrolysis, or transfer to another molecule, takes place with the release of a large amount of free energy.

Axon -- The thick branch of a neuron that sends messages.

Biochemistry -- The study of the chemical substances and vital processes occurring in living organisms; "the chemistry of life," or the study of the structure and properties of molecules in living organisms and how those molecules are made, changed, and broken down.

cAMP, or cyclic adenosine monophosphate -- A form of AMP, generated from ATP by adenylyl cyclase in response to the stimulation of many types of cell-surface receptors, used frequently as a second messenger in eukaryotics (cells containing one nucleus) and in catabolite repression in prokaryotes (cells containing no nucleus).

Cell-free system -- A mixture of cytoplasmic and/or nuclear components from cells used for in vitro protein synthesis, transcription, DNA replication, or other purposes.

Coding ratio -- The number of nucleotides in sequence required to code for amino acids. In a triplet code, the coding ratio is three.

Codon -- A sequence of three RNA or DNA nucleotides that specifies, i.e., codes for, either an amino acid or the termination of translation.

Complimentary DNA, or cDNA -- DNA synthesized by reverse transcriptase, the process of copying information from RNA into DNA.

Dependence -- A compulsive or chronic need, such as an addiction.

DNA, or deoxyribonucleic acid -- The primary genetic material of all cellular organisms and the DNA viruses. Located predominantly in cell nuclei, it is composed of two chains of nucleotides--deoxyribose and phosphate backbones with side chains of purine (adenine or guanine) or pyrimidine (cytosine and thymine) bases projecting inward. Hydrogen bonds link adenine to guanine, and cytosine to thymine. The two linked strands are twisted in a double helix.

Drosophila, or Drosophila melanogaster -- The common fruit-fly, often used as an animal model in scientific experiments.

Electrophysiology -- The branch of physiology that studies the relationship between electric phenomena and bodily processes.

Embryo -- An animal in the early stages of growth and differentiation that are characterized by cleavage, the laying down of fundamental tissues, and the formation of primitive organs and organ systems. In humans, the embryo is this developing organism from approximately the fourth day after fertilization to the end of the eighth week of gestation.

Epinephrine, or adrenalin -- A hormone secreted by the adrenal medulla and released into the bloodstream in response to physical or mental stress. It initiates many bodily responses, including the stimulation of heart action and an increase in blood pressure, metabolic rate, and blood glucose concentration.

Escherichia coli, or E. coli -- Common bacterium that has been studied intensively by geneticists because of its small genome size, normal lack of pathogenicity, and ease of culture in the laboratory.

Ganglia (plural of ganglion) -- Groups of nerve cell bodies in the central nervous system or peripheral nervous system.

Gene expression -- The process of producing a protein from its DNA-coding, or mRNA-coding, sequences.

Genetic code -- Linear sequences of three nucleotides, or triplets, that code for amino acids or termination codons during the process of translation at the ribosome.

Glia, also glial cells or neuroglia -- The major support cells of the brain. These cells are involved in the nutrition and maintenance of the nerve cells. Evidence indicates that certain types of glia may serve information processing roles as well.

Glioma -- A tumor originating in the neuroglia of the brain or spinal cord.

Glucagon -- A hormone produced by the pancreas that stimulates an increase in blood sugar levels, and therefore counteracts the action of insulin.

Glucose -- A simple sugar found in many fruits and plants, and in the normal blood of all animals. It is the end product of carbohydrate metabolism and is the chief source of energy for living organisms.

Homeobox -- A consensus sequence, a sequence of nucleotides in common between regions of homology in different but related DNA or RNA, of about 180 base pairs discovered in developmentally important genes in a range of life from yeast to human beings.

Homology -- Similarity of nucleotide or amino acid sequences in nucleic acids, peptides, or proteins. Also: structural similarity resulting from descent from a common form.

Hybrid cell -- A cell derived from the fusion of cells from different cultured cell lines, pedigrees of cells related through asexual reproduction.

Inducer -- An environmental agent that triggers transcription, the synthesis of RNA from a DNA template, from a set of adjacent structural genes, or operon.

Macrophage -- An immune cell found everywhere in the body. Macrophages act as scavengers that engulf dead cells, foreign substances, and other debris.

Medulla oblongata -- The lowermost portion of the vertebrate brain, continuous with the spinal cord. It is responsible for the control of respiration, circulation, and certain other bodily functions.

Messenger RNA, or mRNA -- An RNA molecule transcribed from the DNA of a gene, and from which a protein is translated by the action of ribosomes. The basic function of the nucleotide sequence of mRNA is to determine the amino acid sequence in proteins.

Metabolic pathway -- A series of individual chemical reactions that combine to either break down a large compound into smaller units or that synthesize more complex molecules from smaller ones. The product of one reaction in a pathway serves as the substrate, or the substance acted upon, for the following reaction.

Monocyte -- A large, circulating white blood cell that engulfs and absorbs waste material. A monocyte has a single well-defined nucleus and very fine granulation in the cytoplasm.

Morphine, or morphia -- A bitter crystalline alkaloid extracted from opium. The soluble salts of morphine are used in medicine as an analgesic, a light anesthetic, or a sedative.

Narcotic -- A frequently addictive depressant drug (e.g., opium and derivatives such as morphine) that in moderate doses dulls the senses, relieves pain, and induces profound sleep, but in excessive doses causes stupor, convulsions, or coma. Also used in reference to other drugs, such as LSD or marijuana, subject to similar legal restrictions whether physiological addictive or not.

Nematode, or roundworm -- Any of several worms of the phylum Nematoda, having unsegmented, cylindrical bodies, often narrowing at each end, and including parasitic forms such as the hookworm and pinworm.

Neurobiology -- The biological study of the nervous system or its components.

Neuroblastoma -- A malignant tumor composed of neuroblasts, dividing neuronal precursor cells, originating in the autonomic nervous system or the adrenal medulla and occurring primarily in infants and young children.

Neuron, or nerve cell -- The fundamental unit of the nervous system. Its structure and properties allow it to conduct signals via an electrical charge across its cell membrane.

Neuroreceptor -- Specific sites on the surface of a nerve cell to which neurotransmitters bind, initiating the conduction of impulses, or signals, to other nerve cells.

Neurotransmitter -- A chemical substance, such as acetylcholine or dopamine, which transmits nerve impulses across a synapse.

Nucleotide -- A unit that polymerizes into nucleic acids (DNA or RNA). Each nucleotide consists of a purine (adenine or guanine) or pyrimidine (cytosine, thymine, or uracil) base, a sugar (ribose in the case of RNA, deoxyribose in the case of DNA), and a phosphate molecule.

Oligonucleotide -- A molecule usually composed of 25 or fewer nucleotides; used as a DNA synthesis primer.

Organelle -- A specialized structure within a cell, such as a mitochondrion or chloroplast, which performs a particular function.

Phenylalanine -- A nonpolar amino acid.

Phosphorylase -- An enzyme that catalyzes the production of glucose phosphate from glycogen and inorganic phosphate.

Plasticity -- The capability of building tissue.

Polymer -- A molecule composed of repeated subunits, or individual polypeptide chains in a protein containing more than one polypeptide chain.

Polymerase -- An enzyme that catalyzes the synthesis of nucleic acids on preexisting nucleic acid templates, such as assembling RNA from ribonucleotides or DNA from deoxyribonucleotides.

Polynucleotide -- A DNA polymer composed of multiple nucleotides.

Polyribonucleotide -- An oligonucleotide, a short polymer of two to twenty nucleotides, consisting of a number of ribonucleotides, which are the nucleotides that contain ribose as their sugar, and which are components of RNA.

Polyuridylic acid, or poly-U -- RNA or a segment of RNA that is composed of a polynucleotide chain consisting entirely of uracil.

Protein -- A large molecule composed of one or more chains of amino acids in a specific order. The order is determined by the base sequence of nucleotides in the gene that codes for the protein. Proteins are required for the structure, function, and regulation of the body's cells, tissues, and organs. Each protein has unique functions.

Protein synthesis -- The process in which the genetic code carried by messenger RNA directs cellular organelles called ribosomes to produce proteins from amino acids.

Receptor -- A specialized protein on a cell's surface that binds to substances that affect the activities of the cell.

Ribosome -- A complex organelle composed of proteins and rRNA that catalyzes translation of messenger RNA into an amino acid sequence. Ribosomes consist of two non-identical subunits each consisting of a different rRNA and a different set of proteins.

RNA, or ribonucleic acid -- A single-stranded nucleic acid that performs an important role in the flow of genetic information. It is similar to DNA, but has ribose sugar, rather than deoxyribose sugar, and uracil, rather than thymine, as one of the pyrimidine bases.

sRNA, or soluble ribonucleic acid -- Also called transfer RNA, or tRNA. Small RNA molecules carry amino acids to the ribosome for polymerization into a polypeptide. During translation, the amino acid is inserted into the growing polypeptide chain when the anticodon of the tRNA pairs with a codon on the mRNA being translated.

Sympathetic neuroblast -- Also called sympathetoblast or sympathicoblast. A pluripotential cell, a cell that has the potential, depending on its environment, to develop into more than one type of mature cell, in the embryo that will develop into a sympathetic nerve cell or a chromaffin cell.

Synapse -- The junction across which a nerve impulse passes from an axon terminal to a neuron, muscle cell, or gland cell.

Synaptogenesis -- The formation of a synapse.

Tobacco mosaic virus, tobamovirus, or TMV -- Member of a group of plant viruses with rigid, rod-shaped particles containing one molecule of linear RNA, easily sap-transmitted and transmitted in nature by contact and in seed. Frequently used in molecular biology and virology studies for its nonpathogenicity to animals and ease of growth.

Tolerance -- Physiological resistance to a poison or the capacity to absorb a drug in large doses without adverse effect. This diminution in the response is often due to prolonged use or exposure.

Transcription -- The process whereby RNA is synthesized from a DNA template.

Trinucleotide -- A triplet of nucleotides, or a codon.

Triplet -- The three nucleotide pairs that compose a codon.

Uptake -- The absorption and incorporation of a substance by living tissue.

Uracil -- A pyrimidine (the parent compound of many drugs, including the barbiturates) base that is an essential constituent of RNA, but not DNA.