Neurotransmitters are responsible for relaying information from one neuron to another - if a person has low levels of serotonin then normal transmission of mood-relevant information does not take place and a person may experience low moods
At least some cases of OCD may be explained by a reduction in the functioning of the serotonin system in the brain
Decision-making systems

Some cases of OCD, in particular hoarding disorder, seem to be associated with impaired decision making - this in turn may be associated with abnormal functioning of the lateral frontal lobes of the brain
The frontal lobes are the part of the brain responsible for logical thinking and decision-making
There is also evidence to suggest that the parahippocampal gyrus, associated with processing unpleasant emotions, functions abnormally in OCD
