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parallel processing: The division of an information-processing job into smaller parts that are each handled simultaneously by various cortical fields and brain nuclei.

parietal lobe: One of the four subdivisions of the cerebral cortex; it is involved in sensory processes, attention, and language.

phencyclidine (PCP): Originally developed as an anesthetic, PCP may act as an hallucinogen, stimulant, or sedative.

pituitary gland: An endocrine organ closely linked with the hypothalamus. The pituitary secretes a number of hormones that regulate the activity of other endocrine organs in the human body.

plasticity: The capacity of the brain to change its structure and function within certain limits. Plasticity underlies brain functions such as learning and allows the brain to generate normal, healthy responses to long-lasting environmental changes.

positron: A positively charged particle having the same mass and spin as, but opposite charge of, an electron.

positron emission tomography (PET): An imaging technique for measuring brain function in living subjects by detecting the location and concentration of small amounts of radioactive chemicals.

postsynaptic neuron: A neuron that receives messages from other neurons.

presynaptic neuron: A neuron that releases neurotransmitters into synapses to send messages to other neurons.

psychedelic drug: A drug that distorts perception, thought, and feeling. This term is typically used to refer to drugs with actions like those of LSD.

psychoactive drug: A drug that changes the way the brain works.

psychosocial therapy: Therapy that uses a combination of individual psychotherapy and group (social) therapy approaches to rehabilitate or provide the interpersonal and intrapersonal skills an addict needs to live without drugs.

receptor: A large molecule that recognizes specific chemicals (normally neurotransmitters, hormones, and similar endogenous substances) and transmits the message carried by the chemical into the cell on which the receptor resides.

relapse: In drug abuse, relapse is the resumption of drug use after trying to stop taking drugs. Relapse is a common occurrence in many chronic disorders, including addiction, that require behavioral adjustments to treat effectively.

resting membrane potential: The difference in electrical charge between the inside and the outside of a nerve cell when the cell is not firing. The inside of a resting neuron has a greater negative charge than the outside of the neuron.

reuptake: The process by which neurotransmitters are removed from the synapse by being "pumped" through transporters back into the axon terminals that first released them.

reuptake pump (transporter): The large molecule that actually transports neurotransmitter molecules back into the axon terminals that released them.

reward: The process that reinforces behavior. It is mediated at least in part by the release of dopamine into the nucleus accumbens. Human subjects report that reward is associated with feelings of pleasure.

reward system (or brain reward system): A brain circuit that, when activated, reinforces behaviors. The circuit includes the dopamine-containing neurons of the ventral tegmental area, the nucleus accumbens, and part of the prefrontal cortex. The activation of this circuit causes feelings of pleasure.

route of administration: The way a drug is put into the body. Drugs can enter the body by eating, drinking, inhaling, injecting, snorting, smoking, or absorbing a drug through mucous membranes.

rush: Intense feelings of euphoria a drug produces when it is first consumed.

second messenger: A molecule produced inside neurons as a step in the process of communication between cells. The second messenger lets other parts of the cell know that a specific receptor has been activated, thereby completing the message carried by the neurotransmitter that bound to the receptor. Some receptors (dopamine and opiate receptors, for example) use second messengers. Others (nicotine and GABA receptors, for example) do not.

sensitization: An increased response to a drug caused by repeated administration. Sensitization is most commonly seen in some responses to stimulants.

serotonin: A neurotransmitter that regulates many functions, including mood, appetite, and sensory perception.

single photon emission computed tomography (SPECT): An imaging process that measures the emission of single photons of a given energy from radioactive tracers in the human body.

stimulants: A class of drugs that elevates mood, increases feelings of well-being, and increases energy and alertness. These drugs produce euphoria and are powerfully rewarding. Stimulants include cocaine, methamphetamine, and methylphenidate (Ritalin).

synapse: The site where presynaptic and postsynaptic neurons communicate with each other.

synaptic space (or synaptic cleft): The intercellular space between the presynaptic and postsynaptic neurons.

temporal lobe: The lobe of the cerebral cortex at the side of the head that hears and interprets music and language.

thalamus: Located deep within the brain, the thalamus is the key relay station for sensory information flowing into the brain, filtering out important messages from the mass of signals entering the brain.

tetrahydrocannabinol (THC): The active ingredient in marijuana that is primarily responsible for producing the drug's psychoactive effects.

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

tolerance: A physiological change resulting from repeated drug use that requires the user to take larger amounts of the drug to get the same effect initially felt from a smaller dose.

transporter: A large protein on the cell membrane of the axon terminals. It removes neurotransmitter molecules from the synapse by carrying them back into the axon terminal that released them.

ventral tegmental area (VTA): The group of dopamine-containing neurons that make up a key part of the brain reward system. These neurons extend axons to the nucleus accumbens and the prefrontal cortex.

vesicle: A membranous sac within an axon terminal that stores and releases neurotransmitter.

withdrawal: Physical symptoms in the body and brain that occur when a person who is physically dependent on a drug stops using the drug.

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