Once a drug has entered the human body and made its way to the brain, what happens next? This is a very interesting question - the answers to which we are only now beginning to uncover. The study of the workings of the brain, the complex relationships between chemical and electrical activity and their effects on personality and mood, is still in its infancy.

It is mainly through research on the actions of many of the prohibited drugs such as cocaine, heroin, LSD and, more recently, ecstasy, that we are beginning to be able to put some pieces of the jigsaw together.

The central nervous system

What we do know is that the brain is made up of billions of nerve cells called neurons. These receive and transmit messages to and from all parts of the body. Together they are a sort of massive switchboard.

Extending down from the brain is a long bundle of nerve cells called the spinal cord, which together with the brain makes up the central nervous system (CNS).

Messages cross from one nerve cell to another through the medium of chemicals known as neurotransmitters. These carry signals across the gap or synapse, between neurons. There are two types of neurotransmitters - excitatory, which stimulate action and inhibitory, which reduce it.

One neuron on one side of a synapse will release (or 'fire') neurotransmitters which bind to a receptor site on a neighbouring neuron to make the connection (or 'uptake'). The rate of release and uptake of neurotransmitters may affect the long-term personality or short-term mood of an individual but not too much is yet known about the detail of how these mechanisms work.

Mental awareness will vary throughout the day as it is controlled by the levels of naturally ocurring chemicals in the brain, which can have either a depressant or stimulant action on central nervous system activity. If the person is drowsy, a cup of coffee might stimulate more activity. Coffee contains the stimulant drug caffeine which would increase the release of excitatory neurotransmitters thereby giving the user a lift. As a single cup of coffee contains only a relatively low dose of caffeine the effect will be only modest and short lived.

Stimulant effects.

Other stimulant drugs, such as cocaine and amphetamines have a greater impact on the release of excitatory neurotransmitters and thus produce a higher level of wakefulness and a more radically altered mood. That is why these stimulant drugs are sometimes known as "speed".

Depressant effects.

Depressant drugs, like alcohol and heroin, work in much the same way on mood and personality but activate inhibitory chemical messengers. However, the repeated use of such drugs over an extended period of time can cause the body to adjust the amount of naturally occurring inhibitory chemicals it produces. This leads to the phenomena of tolerance. More and more of the drug has to be taken in order to get the desired effect. In building tolerance to the effects of a drug, the user may be taking the first steps on the road to physical drug dependence.

Psychedelic effects.

Psychedelic drugs, like LSD and certain 'magic' mushrooms, affect those areas of the brain which control sensory perception and thought patterns. They do this by altering the way in which the messages are received and interpreted. The change in mood or personality brought about by psychedelic drugs is more likely to be influenced by the set and setting of the drug use than the purely pharmacological action of the drugs themselves within the central nervous system.

Dual action drugs.

The arrival of a new range of drugs which seem to have a dual action has further complicated the picture. These are the stimulant psychedelics, of which ecstasy is the most well known.

Ecstasy, or methylenedioxymethylamphetamine (MDMA) to give it its scientific name, belongs to a family of synthetic compounds related to the amphetamines. Because of this family link, ecstasy has stimulant properties like amphetamine, but it also has certain effects in common with LSD. It works on the brain in much the same way as LSD by the release of the neurotransmitter serotonin which has been reported by users as making them feel happier and increasing their feelings of empathy for others.

Drug, set and setting.

Before finishing this brief overview section on the effects within the brain of drugs it is important to stress the roles of drug experience and user expectation as well as the pharmacology. The effects actually experienced by users from all drugs are influenced greatly by their expectation of the drug's likely effect and this is influenced by their previous experience of it as well as by the setting in which the drug is taken. Drug effects are learned as much as experienced anew each time of taking. This is explored more fully in the next section - "How drugs work".