Synapses
The human brain consists of different parts with different functions. Also in the largest part of the brain, cerebrum, different areas have different functions. The neurons and chemicals around them perform the thinking processes.
Neurons work in the same way as all the other cells in the body. They e.g. have ion channels through which ions can pass the cell membrane in a controlled way, and receptors where different proteins can connect. The special thing about neurons is that they have a special form adapted to receive chemical signals from other neurons and an electrical method to send a signal a long distance. Those parts of the neuron that receives signals from other neurons are called dendrites. That part of a neuron that sends away a signal to the next neuron is called an axon. The contact points between neurons are called synapses. The synapse is usually the gap between the end of the sending neuron’s axon and the receiving neuron’s dendrite. A neuron can have up to 100000 synapses.
The incoming signals are summed, and if the sum exceeds a certain threshold voltage the neuron fires and the electrical signal is sent along the axon to the synapse. This electrical signal is not sent in the same way as in a copper wire but in a much slower way by the opening of ion channels in the axon membrane. The speed in copper wire is 200000 km/s, in axon 200 m/s. The axons are surrounded by myelin sheaths that consist of fat. This diminishes the cross talk between neurons and increases the speed of the signal. If your myelin sheaths deteriorate you get multiple sclerosis.
The signal voltage has a fixed value. A stronger in signal leads to that the neuron sends signal pulses with a higher frequency - up to 500 Hz. When the electric signal arrives at the synapse at the end of the axon, transmitters are emitted from small bubbles where they are stored: vesicles. These transmitters then float across to the dendrite of the next cell where they dock with receptors and induce a voltage. If all the voltages at this cell are summed to a value above the threshold voltage, the neural signal is sent along to the next cell etc. A neuron has receptors for several different transmitters.
Neurons in different parts of the brain use different transmitters. This means that you through drugs, i.e. molecules in the blood, can increase or lower the activity in a certain part of the brain.
Some transmitters are produced in one place and are led through axons to a different place where they are used. The most common is that transmitters that regulate wakefulness and arousal levels are produced by cells near the brain stem and are the led to the cortex.
Some different effects a drug can have:
1.	It can be or be similar to a transmitter (e.g. nicotine»acetylcholine, morphine»endorphin).
2.	It can release the transmitters of the synapse (e.g. amphetamine releases noradrenaline and dopamine, ecstasy releases serotonin from cells in the brain stem that leads to the cortex - these cells die from repeated drug use.)
3.	It can inhibit the absorption of transmitters (e.g. Prozac blocks the absorption of serotonin, cocaine blocks the absorption of noradrenaline, amphetamine blocks the absorption of noradrenaline and dopamine).
4.	It can block the receptors (e.g. curare blocks the transmitter between neurons and muscle cells: acetylcholine).
5.	It can break down transmitters (enzymes).
6.	It can prevent the action of the enzymes that break down transmitters (e.g. tea).
7.	It can regulate how the cell will respond to transmitters in the future.

Some neuron-to-neuron connections are fused without a chemical synapse. These are faster and are not affected by transmitters, but they cannot change their sensitivity due to learning.
Nicotine is a caricature of acetylcholine and binds only to one receptor compared to acetylcholine that binds to many. Nicotine gives a stressed condition, as do amphetamine and cocaine that increases the availability of noradrenaline. Amphetamine also increases the availability of dopamine, which leads to increased ability to focus. Morphine makes pain less unpleasant as do its natural correspondent endorphin. Ecstasy and Prozac gives a sense of happiness because they increase the availability of serotonin. Curare leads to paralyzation and death because the heart stops.
The reason why one can be addicted to drugs is that the natural sensitivity to a transmitter diminishes when one uses artificial transmitters or prevents natural transmitters to be absorbed. Drug use can also lead to that the production of the transmitter diminishes because the neurons that are forced to overproduce the transmitter die. One enters a spiral where one must increase the use of the drug in order to achieve the same effect.
One can influence the access to transmitters not only with drugs but also using common food. When there is produced a lot of dopamine, there is produced less serotonin, and vice versa. The production of dopamine is promoted if there are big amino acid molecules in the blood, which you get by eating foods rich in protein e.g. fish. The production of serotonin is promoted if there is the amino acid l-tryptophan and glucose in the blood. L-tryptophan is one of the building blocks in serotonin and can be found in pasta, popcorn and chicken. To get a higher and stable level of glucose you can eat food rich in starch. You sleep better if you have a lower level of dopamine and thus it is better to eat food more rich in starch in the evening.
Substances in green and to some extent black tea seems to inhibit the enzyme that breaks down dopamine and can also prevent the manufacture of faulty transmitters, plaques, which is the problem in Alzheimer's disease.

References:
“The Usborne Illustrated Dictionary of Biology”
“The Human Brain : A Guided Tour”, Susan A. Greenfield
“Rediscovering Biology”, ch. “Neurobiology”, http://www.learner.org/resources/series187.html#jump10
“Change Your Brain, Change Your Life”, Daniel G. Amen
“Black, Green Tea May Slow Alzheimer's Disease”, Miranda Hitti, http://my.webmd.com/content/article/96/103548?src=rss_cbsnews