Because neurons do not touch one another, they communicate chemically rather than electronically. After a neuron fires, the axon terminal at the end of the neuron transmits the signal it has received by releasing neurotransmitters. A neurotransmitter is a hormone released into the gap between the axon of a neuron and another neuron's dendrites. This gap between two nerve cells, across which neurotransmitters travel, is called the synapse. Neurotransmitters can be divided into two broad groups based on their effects. An agonist is a neurotransmitter that excites or stimulates neurons, and an antagonist is a neurotransmitter that inhibits or suppresses neurons. There are over 100 kinds of neurotransmitters. Major ones include serotonin (which influences mood), dopamine (which induces feelings of reward and pleasure), histamine (which regulates immune response), and norepinephrine (which is released when a stressful event occurs). The level of neurotransmitters in the brain differs among individuals due to their genetic makeup and life events. For example, people prone to thrill-seeking tend to have high baseline levels of dopamine. Studies suggest that this sensation-seeking trait is 60% genetic and linked to a specific dopamine-producing gene.
Psychoactive drugs work by influencing or mimicking neurotransmitter activity. Depressants (such as alcohol and morphine) reduce neural activity and slow bodily functions, while stimulants (such as cocaine and amphetamines) have the opposite effect. Opiates (such as oxycodone) reduce pain. Hallucinogens (psychedelics, such as LSD) distort perceptions and trigger hallucinations.
Long-term potentiation (LTP) is an increase in the strength of a synapse that lasts from minutes to several days. Synaptic strength is a measure of how easy it is for one neuron to be affected by inputs from another neuron. LTP means that after a given neuron (Neuron A) stimulates another neuron (Neuron B), Neuron A will need less signal strength to stimulate Neuron B in the future. Repeated stimulation leads to a neural circuit as the receiving neuron adds more neurotransmitter receptors. Neuroscientists have the saying, "Neurons that fire together, wire together."
Neurotransmitters that suppress activity in the receiving neuron can produce long-term depression rather than LTP of the synapse. This makes the receiving neuron less likely to fire when it receives inputs. Any kind of learning consists of modifying synapses—making them more or less likely to receive and send signals in the presence of particular inputs. Synaptic changes can result from external inputs (sensation and perception) or internal activity (thinking).