David H. answered • 03/15/19
Columbia Researcher Looking to Tutor in Science
In terms of psychoactive drugs (i.e. drugs that act on your brain), tolerance refers to the phenomenon whereby your brain makes chemical changes in order to adapt to constant pharmacological stimulation applied by the drug. Your brain is plastic - it is constantly making changes in response to the stimulus you receive.
The neurons in your brain communicate with one another via neurotransmitters - the end of one neuron releases neurotransmitters that bind and interact with receptors on an adjacent neuron (the place where this happens is called a synapse). The result of this interaction is a signal that either tells the next neuron to “fire” or to “not fire”. Most psychoactive drugs impact this transmission on some level, either by changing the amount of neurotransmitter released, changing the amount of receptors on the second neuron or changing the clearance of neurotransmitters from the synapse. For example, alcohol is a depressant and typically decreases the ability for neurons to signal to one another.
In response to continual drug abuse your brain will try to compensate to return to its normally functional level (homeostasis). Taking the alcohol example from above, abuse with depressants can lead to your brain compensating by increasing the amount of receptors or neurotransmitters released in the synapse, therefore the more someone abuses alcohol the more alcohol it will take to make them drunk. In certain circumstances, severe alcoholics can even undergo seizures when sober because their brain has increased the amount of activity occurring within the synapses (although this is rare because alcoholism leads to other severe conditions such as liver failure before this occurs). This is an oversimplification (as alcohol can act as a depressant by increasing inhibitory neurotransmitters), but the homeostatic concepts involved hold true for all psychoactive drugs.
