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OCD .2 - Coggle Diagram
OCD .2
Genetic Explanation
Genetic explanations have focused on identifying specific candidate genes which are implicated in OCD. It is believed that OCD is a polygenic condition, which means that several genes are involved.
Taylor (2003) suggests that as many as 230 genes may be involved in the condition and perhaps different genetic variations contribute to the different types of OCD.
The COMT gene is associated with the product of catechol-O-methyltransferase (COMT), which regulates the neurotransmitter dopamine.
Although all genes come in different forms, one variation of the COMT gene results in higher levels of dopamine and this variation is more common in patients with OCD, compared to people without OCD.
Secondly, the SERT gene (5-HTT gene) is linked to serotonin and affects the transport of this neurotransmitter (SERotonin Transporter). Transportation issues cause lower levels of serotonin to be active within the brain and are associated with OCD (and depression)
Ozaki et al (2003) published results from a study of two unrelated families who both had mutation of the SERT gene. It concided with six out of seven of the family members having OCD.
Neurotransmitters.
The neurotransmitter serotonin is believed to play a role in OCD. Serotonin regulates mood and lower levels of serotonin are associated with mood disorders, such as depression. Some cases of OCD are also associated with reduced levels of serotonin, which may be caused by the SERT gene.
Evidence for the role of serotonin in OCD comes from research examining antidepressants (SSRIs) such as that conducted by Piggott et al (1990) who found that drugs which increase the level of serotonin in the synaptic gap are effective in treating patients with OCD.
In addition, the neurotransmitter dopamine has also been implicated in OCD. In contrast to serotonin, higher levels of dopamine have been associated with some of the symptoms of OCD, in particular, compulsive disorders.
Evaluation.
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Further support for the biological explanation of OCD comes from twin studies which have provided strong evidence for a genetic link.
Billett et al (1998) conducted a meta-analysis of 14 twin studies investigating the genetic inheritance rate of OCD. It was concluded that monozygotic (MZ) twins had double the risk of developing OCD compared to dizygotic (DZ) twins, if one pair had the disorder
Since concordance rates in twin studies are never 100%, it suggests that the diathesis-stress model may be a better explanation whereby a genetic vulnerability is inherited and triggered by a stressor in the environment
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Brain Structure.
It is believed that several regions in the frontal lobes of the brain have abnormal brain circuits in patients with OCD. Two brain regions implicated specifically in OCD are: the basal ganglia and orbitofrontal cortex
The orbitofrontal cortex is a region which converts sensory information into thoughts and actions. PET scans have found higher activity in the orbitofrontal cortex in patients with OCD when, for example, a patient is asked to hold a dirty item with a potential germ hazard. One suggestion is that the heightened activity in the orbitofrontal cortex increases the conversion of sensory information to actions (behaviours) which results in compulsions.