One of the biggest memes last year was Arthur's Fist. We all know the feeling, stressful situations have you fighting the urge to deck someone. Now, researchers have found neuronal connections between the prefrontal cortex and an area of the brainstem that is directly responsible for controlling our instinctive responses.
Scientists have uncovered exactly which neuronal projections prevent social animals like human beings from acting out our base impulses like the urge to lash out physically in stressful situations. The study, published in Nature Neuroscience, could have implications for schizophrenia and mood disorders like depression.
"We need to be able to dynamically control our instinctive behaviours, depending on the situation."“Instincts like fear and sex are important, but you don’t want to be acting on them all the time,” says Cornelius Gross, who led the work at the European Molecular Biology Laboratory (EMBL). “We need to be able to dynamically control our instinctive behaviours, depending on the situation.”
The region at the base of the brain—the brainstem, just above the spinal chord drives our instincts. Scientists have known for some time that another brain region, the prefrontal cortex, plays a role in keeping those instincts in check (see background information down below). But exactly how the prefrontal cortex puts a break on the brainstem has remained unclear.
Now, Gross and colleagues have actually found the connection between prefrontal cortex and brainstem. The EMBL scientists teamed up with Tiago Branco from the Medical Research Council Laboratory of Molecular Biology (MRC LMB) at Cambridge University, and traced connections between neurons in a mouse brain.
The researchers have discovered that the prefrontal cortex makes prominent connections directly to the brainstem. The teams also found that this physical connection was the mechanism that inhibits instinctive behaviour.
They found that in mice that have been repeatedly defeated by another mouse – the mouse equivalent to being bullied – this connection weakens, and the mice act more scared. The scientists found that they could elicit those same fearful behaviours in mice that had never been bullied, simply by using drugs to block the connection between prefrontal cortex and brainstem.
These findings provide an explanation, based on the anatomy, for why it’s much easier to stop yourself from hitting someone than it is to stop yourself from feeling the urge to do so. The scientists found that the connection from the prefrontal cortex is to a very specific region of the brainstem, the PAG, which is responsible for the acting out of our instincts. However, it doesn’t affect the hypothalamus, the region that controls feelings and emotions. So the prefrontal cortex keeps behaviour in check, but doesn’t affect the underlying instinctive feeling: it stops you from running off-stage, but doesn’t stop the butterflies in your stomach.
The work has implications for schizophrenia and mood disorders such as depression, which have been linked to problems with prefrontal cortex function and maturation.
“One fascinating implication we’re looking at now is that we know the prefrontal cortex matures during adolescence. Kids are really bad at inhibiting their instincts; they don’t have this control,” says Gross, “so we’re trying to figure out how this inhibition comes about, especially as many mental illnesses like mood disorders are typically adult-onset.”