Insula

Anatomy

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.

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Cytoarchitectonics

From an embryological 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.

Connections

The insula is connected with the following structures:

• Neocortex
• Basal ganglia
• Thalamus
• Limbic system and the olfactory cortex

These findings disprove the theory that the insular lobe is an isolated island, but instead, they suggest that the insular lobe possesses multiple functions.

Functions

The insula has extensive connections with neighbouring brain structures, playing an important role in many critical functions of the nervous system.

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.

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