The brain accumulates information from the surroundings, compares it with previous experiences, subjects it to cognitive-emotional evaluation, and initiates adequate behaviour via the motor system. All these actions are mediated by nerve cells (neurons) that have to communicate with each other. Within the nerve cell, that in some cases extend for more than one meter, a signal (excitation) is propagated by opening of voltage-sensitive ion channels. At the contact site between two neurons (synapse) a chemical compound (neurotransmitter) is released that diffuses over a narrow distance (synaptic cleft) to the next neuron and interacts with so-called receptors for this compound; this causes excitation or inhibition of the next neuron. The receiving neuron sums up all excitatory and inhibitory input and, if excitation overcomes a certain treshold, transmits the information further. After interaction with the receptor, the neurotransmitter has to be eliminated from the synaptic cleft, either by enzymatic cleavage or by specific transporters. Any of these processes may be disturbed in various diseases. Most of the drugs available for the treatment of psychiatric and neurologic diseases interfere with specific neurotransmitter receptors, signal propagation, or elimination of the neurotransmitter.
