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Glossary 481GABA A receptorCNS.A type of GABA receptor that is found ubiquitously in theGlial cells Commonly referred to as neuroglia, or just glia, the nonneouronalcells in the nervous system that maintain homeostasis, form the myelin sheatharound neurons, provide support and nutrition, and have important developmentalroles. Recent findings indicate that they are active participants in synaptic transmission,and may be more crucial then previously thought.Glucocorticoid A hormone that affects the metabolism of carbohydrates, fats,and proteins, regulates cardiovascular and homeostatic functions, acts as an antiinflammatory,immunosuppressive agent that is crucial to the stress response.Glucocorticoids are made in the adrenal gland and chemically classified as steroids;cortisol is the major natural glucocorticoid in primates.GlutamateThe major excitatory neurotransmitter and a common amino acid.Gray matter One of two main solid components of the CNS, consisting of nervecell bodies and the nonmylelinated sections of axons and dendrites, crucial forinformation processing.Habituation A form of nonassociative learning in which there is a gradual attenuationof the behavioral response with repetition of an innocuous stimulus.Hippocampus A bilateral brain structure located in the temporal lobe and anintegral part of the limbic system that plays an important role in aspects ofdeclarative memory. It is also implicated in disorders such as amnesia, Alzheimer’sdisease, schizophrenia, and various mood disorders. For localization, seeFigure B1.Histone Small, basic proteins found in the nuclei of the cells of most organisms,other than viruses and bacteria (eukaryotic cells); the chief proteins of chromatin,playing a role in gene regulation and acting as spools around which DNAwinds.HomologousA likeness because of shared ancestry.HPA (hypothalamic pituitary adrenal) axis A neural system activated by stressthat governs a neurohormonal cascade that has pervasive effects on brain function.Activation of the HPA axis leads to subsequent release of corticotrophinreleasing hormone (CRH) from the hypothalamus, adenocorticotropichormone (ACTH) from the pituitary, and glucocorticoides from the adrenals.