<P> The functions of the brain depend on the ability of neurons to transmit electrochemical signals to other cells, and their ability to respond appropriately to electrochemical signals received from other cells . The electrical properties of neurons are controlled by a wide variety of biochemical and metabolic processes, most notably the interactions between neurotransmitters and receptors that take place at synapses . </P> <P> Neurotransmitters are chemicals that are released at synapses when an action potential activates them--neurotransmitters attach themselves to receptor molecules on the membrane of the synapse's target cell, and thereby alter the electrical or chemical properties of the receptor molecules . With few exceptions, each neuron in the brain releases the same chemical neurotransmitter, or combination of neurotransmitters, at all the synaptic connections it makes with other neurons; this rule is known as Dale's principle . Thus, a neuron can be characterized by the neurotransmitters that it releases . The great majority of psychoactive drugs exert their effects by altering specific neurotransmitter systems . This applies to drugs such as cannabinoids, nicotine, heroin, cocaine, alcohol, fluoxetine, chlorpromazine, and many others . </P> <P> The two neurotransmitters that are used most widely in the vertebrate brain are glutamate, which almost always exerts excitatory effects on target neurons, and gamma - aminobutyric acid (GABA), which is almost always inhibitory . Neurons using these transmitters can be found in nearly every part of the brain . Because of their ubiquity, drugs that act on glutamate or GABA tend to have broad and powerful effects . Some general anesthetics act by reducing the effects of glutamate; most tranquilizers exert their sedative effects by enhancing the effects of GABA . </P> <P> There are dozens of other chemical neurotransmitters that are used in more limited areas of the brain, often areas dedicated to a particular function . Serotonin, for example--the primary target of antidepressant drugs and many dietary aids--comes exclusively from a small brainstem area called the raphe nuclei . Norepinephrine, which is involved in arousal, comes exclusively from a nearby small area called the locus coeruleus . Other neurotransmitters such as acetylcholine and dopamine have multiple sources in the brain, but are not as ubiquitously distributed as glutamate and GABA . </P>

The muscle that is mostly involved in any specific action is called