The synapse is where individual neurons transmit information to other neurons. Neurons come very close to each other at the synapse, but do not actually touch. Communication between neurons happens when chemical signals are transmitted across a gap known as the synaptic cleft.
When an action potential reaches the end of the first neuron (1) it causes little packets of chemicals inside the cell called vesicles (2) to move to the edge of the cell and to release their contents into the synapse(3). These chemicals are called neurotransmitters, and are the key to information transfer between neurons. In fact, neurotransmitters are similar to keys – on the other side of the synapse, there are specially shaped receptors that are shaped to accept the neurotransmitter, much like a key fitting into a lock. When enough neurotransmitter binds into the receptors (4), the neuron will generate an action potential of its own (5). This communication between neurons is brief, though, there is a clean-up mechanism in the synapse that clears away the neurotransmitters, either by breaking them down or by re-absorbing them.
The brain uses many different neurotransmitters, and the following are just a few:
Serotonin is responsible for mood and feelings of satiety, as well as many other functions. Many popular anti-depressants available now work on serotonergic systems.
Dopamine is used in brain areas responsible for generating feelings of reward and pleasure. Cocaine and heroine activate the dopaminergic system.
Acetylcholine is used throughout the brain. Alzheimer's disease causes specific wasting of neurons that use acetylcholine.