Table of Contents
Overview:
All brain functions are magnificent and so amazing, that one finds it hard to believe that all our thoughts, emotions, ideas and feelings, and more are processed within this organ. Different parts of the brain perform a specific task, and one of them is the insular cortex.
The insular cortex (also insula and insular lobe) is a part of the cerebral cortex wrapped deep inside the lateral sulcus (the fissure dividing the temporal lobe from the parietal and frontal lobes) of every mammal brain hemisphere.
In this article, the structural arrangement, the location, and the functions of the insular cortex will be explained. Certain complications involve the insular cortex, and they are also described.
Summary:
The insulae are thought to be engaged in awareness and to play a part in a variety of functions, most of which are associated with emotion or the control of the body’s homeostasis.
Empathy, sympathy, flavor, vision, motor control, self-awareness, brain function, social experience, and knowledge of homeostatic emotions like appetite, anxiety, and weariness are examples of these capabilities.
The insular cortex is split into two sections: the anterior insula and the posterior insula, each of which contains many field sections.
The operculum is the cortical area that extends from the insula to the brain’s lateral surface (meaning lid).
Portions of the encompassing frontal, parietal, and temporal lobes are used to build the opercula.
The insula cortex is a centre of convergence for sensory experiences, autonomic regulation, and afferents from areas of the brain in emotion regulation, such as the amygdala.
Anxiety disorders, schizophrenia, PTSD, autism spectrum disorder are all complications that involve the insular cortex.
Location:
In primates, such as human beings, the insula is tucked down inside each hemisphere’s lateral sulcus, concealed beneath sections of the frontal, parietal, and temporal lobes that comprise the opercula, or “lids.” This one-of-a-kind locale inspired the labels ‘insula’ (in Latin it stands for ‘island,’ ‘hidden fifth lobe,’ and ‘The Island of Reil.'(Figure 1)
The central insular sulcus divides the adult human being’s insula into anterior and posterior portions. The endpoints of these two portions have significantly different interactions with other parts of the brain, but a midway ‘middle’ insular zone has a mix of anterior and posterior connectedness properties.
The insula is revealed on the lateral surface of the hemisphere in lissencephalic species including such as mice and rats, usually well above rhinal fissure.
Structure:
The insula is divided into three sections that differ in their cytoarchitecture across organisms: the granular, dysgranular, and agranular sections. This concept refers to the gradual disappearance of the granular layer four. The granular insular cortex has a traditional structure that is made up of six layers, and layer 4 grows smaller in the dysgranular insula; and the agranular insula is tri-laminar, with no layer 4. The three sections are tightly linked all along the dorsoventral and rostrocaudal axes.
The existence of a specific cell kind in layer 5 of the insular cortex of human beings, big apes, whales, and elephants is a distinguishing trait of their insular cortex. The massive, bipolar ‘von Economo neurons’ were first observed by Ramon y Cajal in the earlier part of the 20th century and more officially described by Constantin von Economo.
Although the particular tasks of this unique type of cell are unknown, von Economo nerve cells are preferentially damaged in frontotemporal dementia and are only seen in animals with big brains and sophisticated socializing capabilities. These findings have prompted scientists to propose that they could play a unique role in complicated social – communication skills.
The insular cortex is a clear physical integrative centre, with strong connections to a massive array of cortical and subcortical brain areas in perceptual, affective, motivational, and cognitive processes. It gets the highest volume of sensory information from all modes.
Straight thalamic and horizontal cortical afferents transport data to the insula from within and without the body (olfactory, somatosensory, auditory, sight, and gustatory information) (interoceptive data). Many of these stimuli extend to topographically structured insular sense areas, eventually leading to the ‘visceral insular cortex, the ‘gustatory cortex’ (the primary taste cortex), and the insular auditory and somatosensory fields.
It is worth noting that all of these sense areas do not analyze solely their primary sensory information: all insula areas achieve a large volume of cross-modal afferents and are best conceived of as a multimodal integration region.
The insula has reciprocal linkages with the limbic system along with sensory afferents. The lateral and basolateral amygdala, for example, significantly extend towards the granular and dysgranular regions of the insula that transmit dense efferents to the basolateral, lateral, and central amygdala nuclei. The insula also links to the stria termina’s lateral section of the bed nucleus, the lateral hypothalamus, the parahippocampal areas, such as the perirhinal and lateral entorhinal cortices, and, the thalamus’s mediodorsal nucleus.
The insula attaches to the regions of the frontal brain including the orbitofrontal, the anterior cingulate, and medial prefrontal cortices, which are involved in executive functions, cognitive functions, and emotional tasks and extends to areas of the brain involved in encouragement and incentive, including the nucleus accumbens and caudate putamen.
Generally, dopaminergic, cholinergic, noradrenergic, and serotonergic afferents provide substantial neuromodulatory feedback to the insular cortex.
Function:
If you choose to see the people you admire deeply, try and listen to your pulse, have a migraine, or desire a cheesecake, one portion of your brain will undoubtedly boost its action and it is none other than the insular cortex. The insular cortex, often known as the insula, is a portion of the cerebral cortex.
In the earlier part of the nineteenth century, J.C. Reil, who was a German specialist who was concerned with diseases of the brain and spinal cord, peripheral nerves, and muscles, was the first to label this brain region. Later study results have linked the insula in a wide range of processes, including sensory processing, emotion representation, autonomic and motor regulation, danger prediction and judgment, bodily- and self-awareness, and sophisticated social engagements such as compassion.
With an expanding number of relevant diagnostic tests revealing the insula as a key region impacted across many mental and neurological illnesses, there has lately been an increasing interest in the adult human being’s insular cortex. In the meantime, new methods for dissecting operational microcircuits in animal studies, particularly rodents, have put the insular cortex on the map.
This particularly interests neuroscience experts who are excited about understanding the neural processes underpinning feelings and motivated behaviour.
Some suggest that the human individual’s insular cortex is distinctive and underpins distinctly human actions and cognitive abilities. Even though human beings have by far the most complex brains and behaviours in the animal kingdom, numerous anatomical and physiological aspects of the insula are similar in rodents and humans, and that these parallels could provide a starting point for investigating the fundamental neurological foundations of insular activities and abnormalities in a tractable animal study.
To achieve this goal, data from animal and human studies that emphasize the comparable traits, as well as offer a few issues that have to be answered to get a better mechanistic knowledge of the insular cortex should be explained. This article is intended to serve as a jumping-off point for further exploration of this interesting, extremely essential, yet still unknown brain region.
How is it possible for one brain region to be engaged in several various tasks? Is the insula divided into operating units? What is the connection between these? And, are there any similar patterns behind the insula’s seemingly disparate duties?
Afferents from limbic, autonomic, sensory, and frontal brain areas merge and intertwine inside the insular cortex, laying the groundwork for cross-modal and cross-functional connection and perhaps anchoring. Whereas the neural foundations of such connectivity remain unknown, it is described hereunder how the integration of multiple systems directly leads to diverse functional responsibilities of the insula.(Figure 2)
The posterior (granular) insula obtains sensory information from vestibular, pain, temperature, visceral, and other routes; this multimodal sensory description has been further developed in the midinsular (dysgranular) cortex and then communicated to the anterior (agranular) insula that also analyzes this data and communicates with regions involved in cognitive and emotional command.
As a result, the insula serves as a bridge between bodily experience and sentiments, and it may play an important role in perceptual awareness, social behaviour, and strategic planning. fMRI and cortical microstimulation research in patients receiving an assessment for diagnosis and treatment have added to our understanding of the insula’s functional complicatedness in people.
The awareness of body states, known as interoception, is an important component of the insular cortex. The insula collects topographically structured afferents from separate thalamic relay nuclei and combines data that concerns blood pressure and circulation, gastrointestinal movement, heartbeat rhythm and intensity, discomfort, appetite, sickness, itching sensation, and many other body sensations.
Additionally to reviewing the state of the body, the insula maintains strong top-down regulation of autonomic activity, such as blood pressure, heartbeat, stomach movement, most probably by direct connections to the lateral hypothalamic region, the parabrachial nucleus, and the solitary tract nucleus.
Surprisingly, microstimulation investigations in rodents and diagnostic methods found neighbouring ‘pressor’ and ‘depressor’ areas with opposite impacts on blood pressure and heart rate, implying that distinct circuits with antagonistic functions reside in the insula.
The insula cortex is thus a centre of convergence for sensory experiences, autonomic regulation, and afferents from areas of the brain in emotion regulation, such as the amygdala.
Prior studies of emotions, dating back to Descartes and James-Lange, highlight the connection between interoception and feelings, suggesting that emotional responses are elicited by observations of body reactions and therefore cannot arise without the awareness of bodily sensations.
Likewise, both human and animal research has linked the insula cortex to the interpretation of good and negative emotions such as anger, despair, nervousness, contempt, joy or delight, confidence, astonishment, and social emotions. Surprisingly, the insula’s participation in emotion regulation is cross-modal — feelings triggered through many modalities such as speech, music, sights, and even contact through touch involve insula.
The insula gets sensory data from the outside in addition to sensory impulses from the inside body. However, unlike the insula, which contains multiple discrete sensory regions that re-map the outside perceptions, insula lesions influence the identification or valence of certain sensory information or collection of stimuli rather than the sensory perception levels.
Insula lesions, for instance, may impair flavour perception, which is aided by the combination of many sensory modalities such as taste, vision, and olfaction. Insula lesions can also interfere with the learning and manifestation of conditioned taste dislike.
Conditioned taste dislike is a long-lasting dislike that develops when a new flavour is accompanied by visceral malaise, and it is thus dependent on learning and memorizing the correlation between the consumption of certain food and its bad impact on health.
The neural mechanisms behind conditioned taste dislike have been widely investigated in the research using mice. These investigations indicated that cholinergic and dopaminergic transmission play significant roles in controlling insular plasticity and that interaction between the basolateral amygdala and the insular cortex underpins the generation of conditioned taste aversion memories.
A noteworthy instance of sensory–emotion connection impairments is ‘pain asymbolia,’ a condition in which individuals ailing from an insular lesion may identify pain but lack an adequate emotional reaction and do not ascribe a negative effect to this generally unpleasant encounter.
Intriguingly, rats were shown to display pain asymbolia after anterior insula lesions, signifying that the insula facilitates a discomfort-connected negative impact in both rodents and human beings. Social touch is a different analogy that emphasized the importance of the insula in allocating a positive valence to a sensation.
The insula receives C tactile (CT) fibre afferents, which enhance firing when the skin is caressed at a gentle, delicate speed. The rate of CT fibre discharge coincides with the subjective hedonic feeling of caress. In people, the insula cortex responds even when the caress is not directly experienced but is seen to affect others.
Complications involving Insular Cortex:
Autism spectrum disorders (ASD):
Autism spectrum disorders (ASD) are a group of complicated neurological conditions with uncertain causes. The insula has repeatedly been found as a location of hypoactivity and defective connection in Autism, and the insula’s brain functional structure can be utilized to distinguish individuals with ASD from generally developing youngsters.
The insula is directly engaged in multisensory and affective handling, and also social engagements such as compassion and understanding, which are all severely impaired in autistic patients. Rodent models of autism exhibit sensory hyper-reactivity and deficiencies in multisensory integration as a consequence of alterations in inhibitory circuits inside the insula, eerily similar to clinical observations.
Anxiety disorders:
The insula changes function and structure in anxiety disorders such as particular and social phobia, nonspecific anxiety disorder, post-traumatic stress disorder (PTSD), and panic disorder.
Although the insula’s mechanism of active involvement in anxiety and fear is unknown, it has been suggested that people who may be more conscious of or centred on their bodily emotions may demonstrate higher interoceptive prediction messages: that is, enhanced prediction of future aversive physical states may stimulate anxiety, fear, and behavioral problems.
This second point is supported by the discovery that self-reported measurements of anxiety are linked to heartbeat detection precision and interaction in the right anterior insular cortex. Modifications in insular-mediated expectancy and forecasts of future occurrences may also increase anxiety.
Analyzing the correlation in the roles of the normal person and rodent insulas in interoception, anticipation, and fear mediation, the rodent prototype may portray a once-in-a-lifetime opportunity to explore the exact neuronal processes involved in the insula’s position in healthy and pathological anxiety and fear.
Schizophrenia:
Magnetic resonance imaging research findings reveal reduced grey matter volume and cortical size in the insula of schizophrenia patients, which worsen with the advancement of the disease. Post-mortem investigations have revealed less cellular heterogeneity in the insula’s higher layers, with fewer nerve cells and smaller glial and neuronal soma sizes.
Pain indifference, sensory-emotional integration deficiencies including such disadvantaged emotional detection in facial gestures, emotionally brutally honest speech, disabilities in differentiating self from non-self, and the incidence of delusions have all been witnessed in schizophrenic patients and are probably due to modified insula function.
Conclusion:
The insula serves as a bridge between bodily experience and sentiments, and it may play an important role in perceptual awareness, social behaviour, and strategic planning. The insula gets sensory data from the outside in addition to sensory impulses from the inside body. Both human and animal research has linked the insula cortex to the interpretation of good and negative emotions such as anger, despair, nervousness, contempt, joy or delight, confidence, astonishment, and social emotions. The insula cortex is a centre of convergence for sensory experiences, autonomic regulation, and afferents from areas of the brain in emotion regulation, such as the amygdala. Anxiety disorders, schizophrenia, PTSD, autism spectrum disorder are all complications that involve the insular cortex. Modifications in insular-mediated expectancy and forecasts of future occurrences may also increase anxiety.
Reference list
- Benarroch, E.E. (2019) ‘Insular cortex: Functional complexity and clinical correlations’, Neurology, 93(21), pp. 932–938. doi: 10.1212/WNL.0000000000008525
- Gogolla, N. (2017) ‘The insular cortex’, Current Biology : CB, 27(12), R580-R586. doi: 10.1016/j.cub.2017.05.010
