Neurobiology Essay – OCD

My project will centre around obsessive compulsive disorder (OCD), a disorder for which I am somewhat familiar. I am not sure I agree with the ‘disorder’ part, however that is an aside and thus is one I will not touch upon any further here.

OCD causes anxiety which manifests through repetitive compulsions or behaviours. OCD is not fully understood at present but there is evidence that its origin can be found in neurobiology, as show in figure 1 (sourced from www.ocd.commons.yale.edu/ocdbrain/), so let us explore that avenue further.

Figure 1: PET scan – The top row of brains here are from an individual without any diagnosis; the bottom row is from a patient with OCD. The individuals are at rest. The images are reconstructed horizontal slices through the brain; the front of the brain is at the top, as if the person was lying on his or her back. The colours correspond to brain activity; reds and yellows correspond to brain regions with higher activity, blues and greens are brain areas with less activity. By comparing the images in the top row with those in the bottom, you can see at a glance that, while the overall patterns are quite similar, a few brain regions are markedly more active in the OCD brain. The two most prominent are the orbitofrontal cortex (OFC), at the front of the brain just above the eyeballs, and the caudate nucleus, a component of the basal ganglion deep within the brain.

It is thought that there are three anatomical brain regions involved in OCD. These form a looped pathway and are as follows:

  • The orbitofrontal cortex (OFC)
  • The anterior cingulated cortex (ACC)
  • The basal ganglion

The basal ganglion can be subdivided into:

  • The subthalamic nucleus
  • The nucleus accumbens
  • The substantia nigra
  • The globus pallidus
  • The striatum

The striatum can also be subdivided into:

  • The putamen
  • The caudate nucleus

One of the functions of the OFC is to detect when there is something wrong and to send a ‘worry’ signal to the brain’s interpreter, the thalamus. The thalamus directs signals from many parts of the brain to regions that can interpret them, in this case back to the OFC. These nerve cell connections therefore form a loop. The caudate nucleus lies between the OFC and the thalamus and regulates signals sent between them. When the thalamus receives a ‘worry’ signal, it becomes excited and sends strong signals back through the loop to the OFC, which interprets them. Normally, the caudate nucleus acts as a brake, suppressing the original ‘worry’ signals sent by the OFC to the thalamus. This prevents the thalamus from becoming overexcited. It is possible that in OCD, the caudate nucleus does not suppress these signals to the same intensity, if at all, and thus the thalamus sends a strong signal back to the OFC, which responds by increasing compulsive behaviour and anxiety.

As discussed by Professor Mason in her lectures, the basal ganglion acts as a selector. It receives a multitude of information and from these inputs it ‘decides’ what the output should be. One possible explanation for the compulsions of OCD is that the basal ganglion repeatedly selects the same output over and over again instead of just selecting it once, resulting in a repetitive compulsive behaviour.

The ACC is highly connected to the OFC and the basal ganglion and is believed to contribute to the emotional response to obsessive thoughts. This area of the brain tells the person with OCD to perform compulsions in order to relieve their anxiety.

All of these regions have cells which are affected by serotonin, a neurotransmitter thought to be involved in regulating; anxiety, memory, sleep, mood, aggression, impulse control, appetite, body temperature, and pain. In OCD, some receptors are thought to block serotonin from entering the cell. This leads to a deficiency of the neurotransmitter in key areas of the brain and thus, someone with OCD can have issues with some or all of the functions listed above. It is also the reason many people with OCD take selective serotonin reuptake inhibitors (SSRIs) which stop nerve cells that have just released serotonin from absorbing it back into the cell. This makes it readily available at synapses for other neurons and hence can reduce symptoms in some people.

Unfortunately OCD and similar conditions are not yet fully understood and tend to therefore have a certain level of stigma attached to them. However, by learning what we can about them, through courses such as this one given by Professor Mason, we can learn to accept them and to empathise with those who experience the world in ways different to the ‘norm’. This was what brought me to this course and has indeed now been amplified further by my undertaking of the course. I will continue to strive for more understanding on all things neurobiology, to quench my own curiosity, but also to increase my ability to interact with those around me. I cordially encourage you to do the same.

REFERENCES

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