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Pathways of Addiction: Opportunities in Drug Abuse Research
tiple dendrites that arise from the cell body to receive incoming signals; and usually a single axon that also arises from the cell body. Axons can be very long and give rise to outgoing signals through their branched ends (terminals). A single neuron can possess thousands of axon terminals and thereby form connections (called synapses) with up to thousands of other neurons. The brain utilizes a chemical process of neurotransmission to transfer information across synapses. Briefly, an electrochemical impulse produced by changes in concentrations of ions across the axon membrane travels down the axon of one neuron, invades the axon's nerve terminals, and triggers the release of a chemical substance, called a neurotransmitter, from the terminals. The neurotransmitter diffuses across the synaptic cleft (the space between the two neurons) and binds to specific receptor proteins located on the surface of the cell, or plasma membrane, of the next neuron. The binding of a neurotransmitter to its receptor activates the receptor and causes a change in the flow of ions across the cell membrane, which can either lead to or inhibit the generation of electrical impulses in that next neuron. The neurotransmitter stimulus is then "turned off" either by enzymatic degradation in the synaptic cleft or by proteinmediated reuptake of neurotransmitter into the nerve terminal. Neurons receive incoming signals from hundreds or thousands of nerve terminals. Whether a neuron fires an impulse is determined by the summation of those numerous inputs.
Neuronal membranes contain classes of proteins, termed ion pumps, that maintain unequal concentrations of ions (e.g., Na+, K+, Ca2+, C1-) between the outside and inside of the cell. The most important pump is termed the Na+-K+ ATPase (adenosine triphosphatase). Neurons are polarized, meaning that the inside of the cell is negatively charged with respect to the outside. Neurons also possess other proteins in their plasma membrane, termed ion channels, that allow passage of specific ions across the cell membrane. Neurotransmitters regulate the electrical properties of neurons by activating or inhibiting the activity of specific types of ion channels.
Neurotransmitters and Their Receptors
The majority of neurotransmission in the brain is performed by amino acid neurotransmitters, which are contained in two-thirds of all synapses in the brain. Glutamate is the major excitatory neurotransmitter in the brain because its receptor channel permits Na+ (and in some cases Ca2+) to flow into the cell; the major inhibitory neurotransmitter in the brain is gamma-aminobutyric acid (GABA) (GABA's receptor channel carries C1 into the cell).
Most other neurotransmitters in the brain bind to receptor proteins