The word “ insula ” is the latin word for the “island”. Which is the island of the human brain and which is its functions? Today the insula is considered a separate lobe of the human brain. Among all of the lobes, it seems that the insula is often marginalized. For that reason, it is considered to be very mystical among medical students because there is so little information about this structure.
For that reason, this specific article will clarify everything that you need to know about the anatomy and the function of the insular lobe.
|Opercula||Three in total (frontal, parietal, temporal) formed by the portions of the lobes covering the insula|
Short gyri: anterior, middle, posterior
Long gyri: anterior, posterior
|Cytoarchitecture||Central agranular, intermediate dysgranular, and outermost granular cortices|
|Connections||Neocortex, basal ganglia, thalamus, limbic system|
|Functions||Desires, cravings, and addiction
The insular lobe is a part of the cerebral cortex located in both hemispheres. The insula forms the floor of the lateral sulcus, so in order to visualize it macroscopically, parts of the frontal, parietal and the temporal lobe must be removed. The portions of these lobes that cover the insula are therefore called the opercula (the plural of “operculum” which in latin means a little lid). When the opercula of the insula are removed, notable gyri and sulci can be seen.
The insula is a lobe that has a triangular shape. It is surrounded by the anterior, superior and medial limiting sulci that are used to distinguish the three parts of the operculum:
- The frontal operculum
- The parietal operculum
- The temporal operculum
When the insular operculum is opened, the first thing to be seen is the major central sulcus of the insula that divides it into an anterior and a posterior part.
The anterior part includes three short gyri:
- The anterior short gyrus
- The middle short gyrus
- The posterior short gyrus
Additionally, on the ventral margin of the anterior part, the accessory gyrus is seen.
The posterior part includes two long gyri:
- The anterior long gyrus
- The posterior long gyrus
As we said at the beginning of this article, insula means island; and like every island, the insula has its own port. The port of the insula is called limen insula (latin name for the “port of the island”). The limen is the most anteroinferior part of the insular cortical surface. It forms the lateral limit of the anterior perforated substance and represents the level at the which the middle cerebral artery typically bifurcates/trifurcates.
From an embryology perspective, the insular lobe is the structure between the neocortex and paleocortex. Three or more cytoarchitectonic cortical areas have been described in the insula, depending on the pattern of lamination. The insular cortex is categorized based on the degree of granularity into three portions:
- Central agranular cortex
- Intermediate dysgranular cortex
- The outermost granular cortex
The level of granularity increases from the central region, that has no granular cells at all, through the intermediate that shows the presence of some granular cells, to the outermost cortex that has the fully developed granular layer.
The insula is connected with the following structures:
These findings disprove the theory that the insular lobe is an isolated island, but instead, they suggest that the insular lobe possesses multiple functions.
Functional neuroimaging studies have linked the insula to desires, cravings, and addiction. Also, it has been proved that the insula plays an important role in a wide range of psychiatric disorders, such are schizophrenia, mood, panic, post-traumatic stress and obsessive-compulsive disorders.
It is not difficult to see that the insular lobe cortex has wide connections with surrounding brain structures and is involved in many important activities of the central nervous system.
By definition, epilepsy is a group of neurological disorders characterized by epileptic seizures. Seizures originate from a group of neurons that start firing the action potentials in an abnormal and excessive way. The epilepsies differentiate by the place of their origin, which significantly changes the electroencephalographic (EEG) record of the patient.
An interesting field of neuroscientific research nowadays is on insular epilepsies. Generally, they have the similar symptomatology as the temporal epilepsies, and it is the fact that divides the experts to the group that believes that the seizures originate from the insula, and to the others that believe that it’s only a temporal seizure that transmits to the insular cortex.
Anyway, some researchers suggest that the insula has the epileptogenic potential. The difficulties to prove it originate from the fact that the insula is located deep within the brain which makes it hard to get an EEG from the scalp surface.
The stimulation of the insula solely causes the following symptoms:
- Consciousness during the seizure
- A sensory symptom before the seizure usually manifested as a sensation of burning heat restricted in the perioral area
- An ever-present abnormal sensation of retrosternal pain, abdominal elongation and distension, nausea and vomiting and dyspnea
- Pharyngeal symptoms of movement and sensation of paroxysm, accompanied with contralateral hands of the discharge side and extending to the cervix, to grab and scratch
- Movement symptoms in the ipsilateral or contralateral discharge side, such as facial or upper limb spasm, rotating head or eyes, and systemic dystonia