The human brain is the most complex organ in the body. In brief, the brain regulates your basic body functions; enables you to interpret and respond to everything you experience; and shapes your thoughts, emotions, and behavior.

The brain is made up of many parts that all work together as a team. Different parts of the brain are responsible for coordinating and performing specific functions.

Brain areas affected by drug use
The brain stem controls basic functions critical to life, such as heart rate, breathing, and sleeping.

The limbic system contains the brain’s reward circuit—it links together a number of brain structures that control and regulate our ability to feel pleasure. Feeling pleasure motivates us to repeat behaviors such as eating, actions that are critical to existence. The limbic system is activated when we perform these activities and also by drugs. In addition, the limbic system is responsible for our perception of other emotions, both positive and negative, which explains the mood-altering properties of many drugs.

The cerebral cortex is divided into areas that control specific functions. Different areas process information from our senses, enabling us to see, feel, hear, and taste. The front part of the cortex, the frontal cortex, is the thinking center of the brain; it powers our ability to think, plan, solve problems, and make decisions.

How the brain communicates
The brain is a communications center consisting of billions of neurons, or nerve cells. Networks of neurons pass messages back and forth to different structures within the brain, the spinal column, and the peripheral nervous system. These nerve networks coordinate and regulate everything we feel, think, and do.

Neuron to Neuron: Each nerve cell in the brain sends and receives messages in the form of electrical impulses. Once a cell receives and processes a message, it sends it on to other neurons.

Neurotransmitters - The Brain’s Chemical Messengers: The messages are carried between neurons by chemicals called neurotransmitters. (They transmit messages between neurons.)

Receptors - The Brain’s Chemical Receivers: The neurotransmitter attaches to a specialised site on the receiving cell called a receptor. A neurotransmitter and its receptor operate like a “key and lock,” an exquisitely specific mechanism that ensures that each receptor will forward the appropriate message only after interacting with the right kind of neurotransmitter.

Transporters - The Brain’s Chemical Recyclers: Located on the cell that releases the neurotransmitter, transporters recycle these neurotransmitters (i.e., bring them back into the cell that released them), thereby shutting off the signal between neurons.

How brain cells communicate

  • To send a message a brain cell releases a chemical (neurotransmitter) into the space separating two cells called the synapse.
  • The neurotransmitter crosses the synapse and attaches to proteins (receptors) on the receiving brain cell.
  • This causes changes in the receiving brain cell and the message is delivered.

How drugs work in the brain
Just as neurotransmitters are chemicals, so are drugs. They work in the brain by tapping into the brain’s communication system and interfering with the way nerve cells normally send, receive, and process information.

Some drugs, such as marijuana and heroin, can activate neurons because their chemical structure mimics that of a natural neurotransmitter. This similarity in structure “fools” receptors and allows the drugs to lock onto and activate the nerve cells. Although these drugs mimic brain chemicals, they don’t activate nerve cells in the same way as a natural neurotransmitter, and they lead to abnormal messages being transmitted through the network.

Other drugs, such as amphetamine or cocaine, can cause the nerve cells to release abnormally large amounts of natural neurotransmitters or prevent the normal recycling of these brain chemicals. This disruption produces a greatly amplified message, ultimately disrupting communication channels. The difference in effect can be described as the difference between someone whispering into your ear and someone shouting into a microphone.

How drugs work in the brain to produce pleasure (or 'reward')
Most psychoactive drugs directly or indirectly target the brain’s reward system by flooding the circuit with dopamine. Dopamine is a neurotransmitter present in regions of the brain that regulate movement, emotion, cognition, motivation, and feelings of pleasure. The overstimulation of this system, with the release of dopamine, produces euphoric effects.

How the stimulation of the brain’s reward centres can influence repeated drug using behaviour
Brains are wired to ensure that we will repeat life-sustaining activities by associating those activities with pleasure or reward. Whenever this reward circuit is activated, the brain notes that something important is happening that needs to be remembered, and teaches us to do it again and again, without thinking about it. Because psychoactive drugs stimulate the same circuit, that behaviour is reinforced at the neurological level.

Why are drugs more 'rewarding' than natural causes of dopamine release?
When some psychoactive drugs are taken, they can release 2 to 10 times the amount of dopamine that natural rewards do. In some cases, this occurs almost immediately (i.e. when drugs are smoked or injected), and the effects can last much longer than those produced by natural rewards. The resulting effects on the brain’s pleasure circuit dwarfs those produced by naturally rewarding behaviors such as eating and sex. The effect of such a powerful reward motivates people to take drugs again.

The text content of this page is reproduced with permission from: Drugs, Brains & Behaviour (2010), NIDA, National Institutes of Health, U.S.A.