The human brain is anatomically organized into four lobes. These four lobes include the frontal, parietal, occipital, and temporal lobes. The margins of these lobes are defined by specific gyri and sulci. Gyri and sulci are the respective hills and valleys in the brain that allow for a large surface area of brain volume. Think of these gyri and sulci as the indentations and protrusions that are essential to proper brain functioning. The large surface area of the brain houses the brain’s neurons, glia as supporting cells, and blood vasculature. One specific sulcus that separates the frontal lobe from the temporal lobe and the parietal lobe from the temporal lobe is the lateral sulcus, or what is commonly referred to as the Sylvian fissure. Located deep within the Sylvian fissure, with a size similar to that of a walnut, and a function integral to life is the insular cortex or the insula.
The insula mainly functions in interoception. Interoception is the sense of awareness of the internal processes in the body. These internal processes include physiological functions of the autonomic nervous system like regulating heartrate, hunger, thirst, bladder control, pain, temperature, and consciousness. These functions are essential to human life and survival. Another function of the insula is the control and regulation of cognitive and emotional states of mind. Cognition is the brain’s ability to process and integrate thoughts, whereas emotion regulation is the brain’s ability to process emotions and react accordingly. So, the neurons of the insula fire when one is thinking about the type of food they would like to eat because they are hungry, or the pain they feel when they are injured. For example, you are playing basketball and unintentionally sprain your ankle as you are sprinting down the court. Your brain is firing neurons within the insula, causing you to experience both the physical and mental components of pain.
With proper functioning of the insula, interoception is balanced; however, improper functioning leads to imbalance and disorder. Disorders related to damage in the structure and function of the insula result in autonomic nervous system disruptions as well as psychiatric disorders due to dysregulation of cognitive and emotional processes. Disorders known to result from dysregulation of neurons in the insula are bipolar disorder (BD) and major depressive disorder (MDD). Scientists used fMRI neuroimaging, a technique that measures brain activity through analyzing blood oxygenation in the brain, to understand that MDD results in increased neuronal activity in the insula, whereas neuronal activity in BD is less than that in MDD, but still greater than baseline. This means that more neurons are firing and relaying information with each other in the insular cortex, causing individuals to undergo states of intense sadness and a lack of desire to complete once favorable activities. More neuronal activity in the insula resulting in MDD has led scientists to hypothesize that a greater focus on one’s internal states leads to rumination and intense negative emotions.
Another psychiatric disorder, schizophrenia, has been linked to dysregulation of neurons in the insular cortex. Studies completed postmortem have shown an underdeveloped insular cortex by way of a smaller number of neurons in each insular layer as well as a decreased size in the neuronal and glial soma. It was discovered that several proteins were abnormally expressed in the neurons in the insula of patients diagnosed with schizophrenia. Additionally, the present insular pathology of patients with schizophrenia was shown to lead individuals to struggle with differentiating facial expressions like disgust, happiness, and anger. Further analysis of the role of the insular cortex in schizophrenia is being researched to get a more holistic understanding.
Altogether, the insular cortex is vital to proper human autonomic, cognitive, and emotional functioning. Even with its incredibly small size, its function is not overshadowed. Continual research on the insula’s role in psychiatric mood disorders as well as its role in interoception are essential to a comprehensive understanding of the insula. The brain is composed of an intense network of neurons, and with one small disruption, normalcy can be reversed.
King Lab at Creighton University 