Action Potential: This occurs when a neuron is activated and temporarily reverses the electrical state of its interior membrane from negative to positive. This electrical charge travels along the axon to the neuron's terminal where it triggers or inhibits the release of a neurotransmitter and then disappears.

Adrenal Cortex: An endocrine organ that secretes corticosteroids for metabolic functions: aldosterone for sodium retention in the kidneys, androgens for male sexual development, and estrogens for female sexual development.

Adrenal Medulla: An endocrine organ that secretes epinephrine and norepinephrine for the activation of the sympathetic nervous system.

Affective Psychosis: A psychiatric disease relating to mood states. It is generally characterized by depression unrelated to events in the life of the patient, which alternates with periods of normal mood or with periods of excessive, inappropriate euphoria and mania.

Agonist: A neurotransmitter, a drug or other molecule that stimulates receptors to produce a desired reaction.

Amino Acid Transmitters: The most prevalent neurotransmitters in the brain, these include glutamate and aspartate, which have excitatory actions, and glycine and gamma-amino butyric acid (GABA) which have inhibitory actions.

Amygdala: A structure in the forebrain that is an important component of the limbic system.

Androgens: Sex steroid hormones, including testosterone, found in higher levels in males than females. They are responsible for male sexual maturation.

Antagonist: A drug or other molecule that blocks receptors. Antagonists inhibit the effects of agonists.

Aphasia: Disturbance in language comprehension or production, often as a result of a stroke.

Auditory Nerve: A bundle of nerve fibers extending from the cochlea of the ear to the brain, which contains two branches: the cochlear nerve that transmits sound information and the vestibular nerve that relays information related to balance.

Autonomic Nervous System: A part of the peripheral nervous system responsible for regulating the activity of internal organs. It includes the sympathetic and parasympathetic nervous systems.

Axon: The fiberlike extension of a neuron by which the cell sends information to target cells.

Basal Ganglia: Clusters of neurons, which include the caudate nucleus, putamen, globus pallidus and substantia nigra, that are located deep in the brain and play an important role in movement. Cell death in the substantia nigra contributes to Parkinsonian signs.

Brainstem: The major route by which the forebrain sends information to and receives information from the spinal cord and peripheral nerves. It controls, among other things, respiration and regulation of heart rhythms.

Broca's Area: The brain region located in the frontal lobe of the left hemisphere that is important for the production of speech.

Catecholamines: The neurotransmitters dopamine, epinephrine and norepinephrine that are active both in the brain and the peripheral sympathetic nervous system. These three molecules have certain structural similarities and are part of a larger class of neurotransmitters known as monoamines.

Cerebral Cortex: The outermost layer of the cerebral hemispheres of the brain. It is responsible for all forms of conscious experience, including perception, emotion, thought and planning.

Cerebral Hemispheres: The two specialized halves of the brain. The left hemisphere is specialized for speech, writing, language and calculation; the right hemisphere is specialized for spatial abilities, face recognition in vision and some aspects of music perception and production.

Cerebrospinal Fluid: A liquid found within the ventricles of the brain and the central canal of the spinal cord.

Cholecystokinin: A hormone released from the lining of the stomach during the early stages of digestion which acts as a powerful suppressant of normal eating. It also is found in the brain.

Circadian Rhythm: A cycle of behavior or physiological change lasting approximately 24 hours.

Classical Conditioning: Learning in which a stimulus that naturally produces a specific response (unconditioned stimulus) is repeatedly paired with a neutral stimulus (conditioned stimulus). As a result, the conditioned stimulus can become able to evoke a response similar to that of the unconditioned stimulus.

Cochlea: A snail-shaped, fluid-filled organ of the inner ear responsible for transducing motion into neurotransmission to produce an auditory sensation.

Cognition: The process or processes by which an organism gains knowledge of or becomes aware of events or objects in its environment and uses that knowledge for comprehension and problem-solving.

Cone: A primary receptor cell for vision located in the retina. It is sensitive to color and used primarily for daytime vision.

Cornea: A thin, curved transparent membrane on the surface of the front of the eye. It begins the focusing process for vision.

Corpus Callosum: The large bundle of nerve fibers linking the left and right cerebral hemispheres.

Cortisol: A hormone manufactured by the adrenal cortex. In humans, it is secreted in greatest quantities before dawn, readying the body for the activities of the coming day.

Dendrite: A tree-like extension of the neuron cell body. Along with the cell body, it receives information from other neurons.

Dopamine: A catecholamine neurotransmitter known to have multiple functions depending on where it acts. Dopamine-containing neurons in the substantia nigra of the brainstem project to the caudate nucleus and are destroyed in Parkinson's victims. Dopamine is thought to regulate emotional responses, and play a role in schizophrenia and cocaine abuse.

Dorsal Horn: An area of the spinal cord where many nerve fibers from peripheral pain receptors meet other ascending nerve fibers.

Endocrine Organ: An organ that secretes a hormone directly into the bloodstream to regulate cellular activity of certain other organs.

Endorphins: Neurotransmitters produced in the brain that generate cellular and behavioral effects like those of morphine.

Epinephrine: A hormone, released by the adrenal medulla and the brain, that acts with norepinephrine to activate the sympathetic division of the autonomic nervous system. Sometimes called adrenaline.

Estrogens: A group of sex hormones found more abundantly in females than males. They are responsible for female sexual maturation and other functions.

Evoked Potentials: A measure of the brain's electrical activity in response to sensory stimuli. This is obtained by placing electrodes on the surface of the scalp (or more rarely, inside the head), repeatedly administering a stimulus, and then using a computer to average the results.

Excitation: A change in the electrical state of a neuron that is associated with an enhanced probability of action potentials.

Follicle-Stimulating Hormone: A hormone released by the pituitary gland. It stimulates the production of sperm in the male and growth of the follicle (which produces the egg) in the female.

Forebrain: The largest division of the brain, which includes the cerebral cortex and basal ganglia. It is credited with the highest intellectual functions.

Frontal Lobe: One of the four divisions (parietal, temporal, occipital) of each hemisphere of the cerebral cortex. It has a role in controlling movement and associating the functions of other cortical areas.

Gamma-Amino Butyric Acid (GABA): An amino acid transmitter in the brain whose primary function is to inhibit the firing of neurons.

Glia : Specialized cells that nourish and support neurons.

Glutamate: An amino acid neurotransmitter that acts to excite neurons. Glutamate probably stimulates N-methyl-D-aspartate (NMDA) receptors that have been implicated in activities ranging from learning and memory to development and specification of nerve contacts in a developing animal. Stimulation of NMDA receptors may promote beneficial changes, while overstimulation may be the cause of nerve cell damage or death in neurological trauma and stroke.

Gonad: Primary sex gland: testis in the male and ovary in the female.

Growth Cone: A distinctive structure at the growing end of most axons. It is the site where new material is added to the axon.

Hippocampus: A seahorse-shaped structure located within the brain and considered an important part of the limbic system. It functions in learning, memory and emotion.

Hormones: Chemical messengers secreted by endocrine glands to regulate the activity of target cells. They play a role in sexual development, calcium and bone metabolism, growth and many other activities.

Hypothalamus: A complex brain structure composed of many nuclei with various functions. These include regulating the activities of internal organs, monitoring information from the autonomic nervous system and controlling the pituitary gland.

Immediate Memory: A phase of memory that is extremely short-lived, with information stored only for a few seconds. It also is known as short-term and working memory.

Inhibition: In reference to neurons, it is a synaptic message that prevents the recipient cell from firing.

Ions: Electrically charged atoms or molecules.

Iris: A circular diaphragm that contains the muscles which alter the amount of light that enters the eye by dilating or constricting the pupil. It has an opening in its center.

Korsakoff's Syndrome: A disease associated with chronic alcoholism, resulting from a deficiency of vitamin B-1. Patients sustain damage to part of the thalamus and cerebellum. Symptoms include inflammation of nerves, muttering delirium, insomnia, illusions and hallucinations and a lasting amnesia.

Limbic System: A group of brain structures - including the amygdala, hippocampus, septum and basal ganglia - that work to help regulate emotion, memory and certain aspects of movement.

Long-Term Memory: The final phase of memory in which information storage may last from hours to a lifetime.

Mania: A mental disorder characterized by excessive excitement. A form of psychosis with exalted feelings, delusions of grandeur, elevated mood, psychomotor overactivity and overproduction of ideas.

Melatonin: Produced from serotonin, melatonin is released by the pineal gland into the bloodstream. It affects physiological changes related to time and lighting cycles.

Memory Consolidation: The physical and psychological changes that take place as the brain organizes and restructures information in order to make it a permanent part of memory.

Metabolism: The sum of all physical and chemical changes that take place within an organism and all energy transformations that occur within living cells.

Mitochondria: Small cylindrical particles inside cells that provide energy for the cell by converting sugar and oxygen into special energy molecules.

Monoamine Oxidase (MAO): The brain and liver enzyme that normally breaks down the catecholamines norepinephrine, serotonin and dopamine.

Motor Neuron: A neuron that carries information from the central nervous system to the muscle.

Myasthenia Gravis: A disease in which acetylcholine receptors on the muscle cells are destroyed, so that muscles can no longer respond to the acetylcholine signal in order to contract. Symptoms include muscular weakness and progressively more common bouts of fatigue. Its cause is unknown but is more common in females than in males and usually strikes between the ages of 20 and 50.

Myelin: Compact fatty material that surrounds and insulates axons of some neurons.

Nerve Growth Factor: A substance whose role is to guide neuronal growth during embryonic development, especially in the peripheral nervous system.

Neuron: Nerve cell. It is specialized for the transmission of information and characterized by long fibrous projections called axons, and shorter, branch-like projections called dendrites.

Neurotransmitter: A chemical released by neurons at a synapse for the purpose of relaying information via receptors.

Nociceptors: In animals, nerve endings that signal the sensation of pain. In humans, they are called pain receptors.

Norepinephrine: A catecholamine neurotransmitter, produced both in the brain and in the peripheral nervous system. It seems to be involved in arousal, reward and regulation of sleep and mood, and the regulation of blood pressure.

Organelles: Small structures within a cell that maintain the cells and do the cells' work.

Parasympathetic Nervous System: A branch of the autonomic nervous system concerned with the conservation of the body's energy and resources during relaxed states.

Parietal Lobe: One of the four subdivisions of the cerebral cortex. It plays a role in sensory processes, attention and language.

Peptides: Chains of amino acids that can function as neurotransmitters or hormones.

Periaqueductal Gray Area: A cluster of neurons lying in the thalamus and pons. It contains endorphin-producing neurons and opiate receptor sites and thus can affect the sensation of pain.

Peripheral Nervous System: A division of the nervous system consisting of all nerves not part of the brain or spinal cord.

Phosphorylation: A process that modifies the properties of neurons by acting on an ion channel, neurotransmitter receptor or other regulatory molecule. During phosphorylation, a phosphate molecule is placed on another molecule resulting in the activation or inactivation of the receiving molecule. It may lead to a change in the functional activity of the receiving molecule. Phosphorylation is believed to be a necessary step in allowing some neurotransmitters to act and is often the result of second messenger activity.

Pineal Gland: An endocrine organ found in the brain. In some animals, it seems to serve as a light-influenced biological clock.

Pituitary Gland: An endocrine organ closely linked with the hypothalamus. In humans, it is composed of two lobes and secretes a number of hormones that regulate the activity of other endocrine organs in the body.

Pons: A part of the hindbrain that, with other brain structures, controls respiration and regulates heart rhythms. The pons is a major route by which the forebrain sends information to and receives information from the spinal cord and peripheral nervous system.

Receptor Cell: Specialized sensory cells designed to pick up and transmit sensory information.

Receptor Molecule: A specific molecule on the surface or inside of a cell with a characteristic chemical and physical structure. Many neurotransmitters and hormones exert their effects by binding to receptors on cells.

Reuptake: A process by which released neurotransmitters are absorbed for subsequent re-use.

Rod: A sensory neuron located in the periphery of the retina. It is sensitive to light of low intensity and specialized for nighttime vision.

Second Messengers: Recently recognized substances that trigger communications between different parts of a neuron. These chemicals are thought to play a role in the manufacture and release of neurotransmitters, intracellular movements, carbohydrate metabolism and, possibly, even processes of growth and development. Their direct effects on the genetic material of cells may lead to long-term alterations of behavior, such as memory.

Sensitization: A change in behavior or biological response by an organism that is produced by delivering a strong, generally noxious, stimulus.

Serotonin: A monoamine neurotransmitter believed to play many roles including, but not limited to, temperature regulation, sensory perception and the onset of sleep. Neurons using serotonin as a transmitter are found in the brain and in the gut. A number of antidepressant drugs are targeted to brain serotonin systems.

Short-Term Memory: A phase of memory in which a limited amount of information may be held for several seconds to minutes.

Stimulus: An environmental event capable of being detected by sensory receptors.

Stroke: The third largest cause of death in America, stroke is an impeded blood supply to the brain. It can be caused by a blood clot forming in a blood vessel, a rupture of the blood vessel wall, an obstruction of flow caused by a clot or other material, or by pressure on a blood vessel (as by a tumor). Deprived of oxygen, which is carried by blood, nerve cells in the affected area cannot function and die. Thus, the part of the body controlled by those cells, cannot function either. Stroke can result in loss of consciousness and brain function, and death.

Sympathetic Nervous System: A branch of the autonomic nervous system responsible for mobilizing the body's energy and resources during times of stress and arousal.

Synapse: A gap between two neurons that functions as the site of information transfer from one neuron to another.

Temporal Lobe: One of the four major subdivisions of each hemisphere of the cerebral cortex. It functions in auditory perception, speech and complex visual perceptions.

Thalamus: A structure consisting of two egg-shaped masses of nerve tissue, each about the size of a walnut, deep within the brain. It is the key relay station for sensory information flowing into the brain, filtering out only information of particular importance from the mass of signals entering the brain.

Ventricles: Of the four ventricles, comparatively large spaces filled with cerebrospinal fluid, three are located in the brain and one in the brainstem. The lateral ventricles, the two largest, are symmetrically placed above the brainstem, one in each hemisphere.

Wernicke's Area: A brain region responsible for the comprehension of language and the production of meaningful speech.