Synapse is the junction between axon of one neuron and dendrite of another neuron. Synapses often change their responses by becoming strengthened or weaken, this is called synaptic plasticity.
With chronic use of drugs, the tolerance develops and the expression of liking decreases. In contrast, the other system associated with wanting become more sensitized and craving increases. This includes plastic changes in brain.
Post-traumatic stress disorder (PTSD) is an anxiety disorder developed due to the occurrence of traumatic events that include accidents by vehicle, events of terrorism, sexual assault, etc.
Symptoms of PTSD include hyper-arousal, recurring flashbacks of traumatic events, avoiding the memories of the event etc. that continue for months or years after the traumatic event. This indicates the enhanced sensitization of an individual towards the traumatic event.