Acetylcholine A chemical neurotransmitter that functions in the transmission at all neuromuscular and glandular junctions of the peripheral nervous system, as well as many synapses within the CNS. Acetylcholinesterase An enzyme found in the synaptic cleft that breaks down and inactivates acetylcholine molecules, thereby controlling neurotransmission. Acetylcholinesterase Inhibitors Chemicals that inhibit the enzyme acetylcholinesterase at neural synapses.  This prevents the acetylcholinesterase from stopping the action of acetylcholine and allows for continued stimulation of the effector.  The result is spasms and paralysis, which can cause paralysis and death.  Some important acetylcholinesterase inhibitors are organophosphate pesticides, carbamates, and some wargases. Action potential A conducted change in the membrane potential of cells, initiated by an alteration of the membrane permeability to sodium ions, and subsequent propagation of an electrical impulse down an axon.  Same as nerve impulse. Adenosine triphosphate (ATP) An important high-energy compound located in the cytoplasm of cells, which serves as a source of cellular energy. Afferent nerve A nerve that relays sensory information to the CNS. Amyotrophic Lateral Sclerosis A disease in which the myelin around nerves is lost causing paralysis and loss of sensory and motor function.  Same as Lou Gehrig's Disease. Anaplasia An alteration of cells from normal appearance to poorly-differentiated or undifferentiated morphology.  They have irregular nuclei and cell structure with numerous mitotic figures.  Anaplasia is frequently associated with malignancies and serves as one criterion for grading the aggressiveness of a cancer. Anoxia The lack of oxygen supply to tissues. Anxiety A feeling of apprehension, uncertainty, and fear without apparent stimulus, and associated with tachycardia, sweating and tremors. Apoptosis Individual or single cell death by a process of self-destruction of the cell nucleus.  In apoptosis, dying cells are not contiguous but are scattered throughout a tissue.  Often referred to as "programmed cell death". Astrocyte A type of glial cell in the CNS.  They are big cells that maintain the blood-brain barrier and provide rigidity to the brain structure. Atrophy A decrease in the size of cells.  If a sufficient number of cells are involved, the tissue or organ may also decrease in size. Atropine An anticholinergic drug that blocks acetylcholine receptors. Autonomic Nervous System The part of the nervous system involved in the unconscious regulation of visceral functions by transmitting motor information to smooth muscles, cardiac muscle, and various glands. Axon The elongation of a neuron that conducts an action potential.  It may extend long distances from one part of the body to another.