Neurons and Neurotransmitters
The brain is made up of a series of nerve cells called neurons. Neurons are responsible for controlling cognition, muscle movement, sensory information and emotions. Neurons are able to communicate with each other using small junctions called synapses. Sometimes thousands of neurons will connect at one synapse. When one neuron is activated, it releases special chemicals into the synapse called neurotransmitters. These neurotransmitters travel through the synapse and bind to special proteins on the other neurons called receptors. When a neurotransmitter binds to its receptor, it can either activate or block the other neurons from sending their own signals. The effect of the neurotransmitter depends on its identity and the type of receptor to which it binds; the same neurotransmitter can have different effects on different neurons depending on the receptors present.
Immediate Drug Effects
Drugs that affect the brain work by changing the brain's chemistry. As DrugAbuse.gov explains, different drugs can affect neuron signaling in different ways. Some drugs are similar in structure to neurotransmitters and can bind to neurotransmitter receptors on neurons. Marijuana and heroin, for example, are able to affect the brain in this way. Other drugs, such as cocaine and amphetamines, stimulate neurons to release abnormally high amounts of certain neurotransmitters. Drugs that can be abused also activate the brain's pleasure system by releasing dopamine, which is a neurotransmitter that gets released when the body experiences pleasurable sensations.
Tolerance and Dependence
Over time, the neurons in the brain will adjust to these new levels of neurotransmitters. According to the Genetic Science Learning Center, the body will reduce the number of receptors for the neurotransmitters. This reduces the signaling caused by those neurotransmitters. If this occurs, it is called tolerance because more of the drug is needed to generate its effects. Because the brain has adjusted its internal chemistry to compensate for the drugs effects, stopping the drug will cause the brain's chemistry to become unbalanced, but this time in an uncomfortable way. This is called dependence and it leads to symptoms of drug withdrawal.