Many of the parts of the brain are big enough to see with the naked eye, but the individual sites in the brain that are affected by psychoactive drugs can't be seen without the aid of powerful microscopes.

These are the key points of contact between two neurons, a tiny gap called the synaptic cleft. An electrical impulse in one brain cell, the presynaptic neuron, causes the release of a chemical messenger, a neurotransmitter, into this space. The neurotransmitter released by the first neuron drifts across the gap and makes contact with a second, target brain cell, which is called the postsynaptic neuron.

Into the Gap

Once across the gap, the neurotransmitter contacts a specific receptor molecule that sticks out from the surface of the postsyn-aptic neuron. The interaction between the neurotransmitter and its receptor starts a sequence of biochemical changes in the neuron on the receiving end. This contributes to its becoming more or less electrically excitable.

If the sum total of the thousands of neuronal messages that come into a postsynaptic neuron tells it to fire, then the postsynaptic cell initiates its own electrical signal and becomes the presynaptic cell with respect to the next neuron down the line.

This process is conducted continuously, even while you sleep, on an unimaginably vast scale involving billions of brain cells. In ways we don't understand, it helps the brain produce consciousness, dreams, thoughts, feelings, and emotions. When the mechanism malfunctions, the result can be hallucinations, paranoia, depression, and other symptoms of mental illness.