Insula

The insula is a hidden region within the brain that can be found deep within the lateral sulcus of each cerebral hemisphere. It is considered hidden because the cortical opercula, or the lips of the lateral sulcus, cover the insular cortex and need to be moved aside in order to see it. Additionally, blood vessels like the middle cerebral artery and its branches, which are situated in the lateral sulcus, further obscure the surface of the insular cortex. These factors make it quite challenging to access the insular cortex through the lateral sulcus.

The insula plays a wide range of functions, including sensory processing, representation of emotions and feelings, autonomic and motor control, prediction of risks and decision-making, bodily and self-awareness, as well as complex social functions like empathy.

Cortical opercula: The distinct cortical regions in the upper and lower lips of lateral sulcus are called cortical opercula. They are formed by the lateral sulcus and its rami. These include the orbital, frontal, and parietal (or frontoparietal) opercula, as well as the temporal opercula. The first three make up the upper lip of the lateral sulcus, while the temporal operculum forms the lower lip.

• The orbital operculum (sometimes considered part of the frontal operculum) is located beneath the anterior horizontal ramus of the lateral sulcus and belongs to the inferior frontal gyrus.

• The frontal operculum refers to the area between the anterior horizontal and ascending rami of the lateral sulcus, also belonging to the inferior frontal gyrus. Both orbital and frontal opercula are associated with Broca's motor speech area.

• The region behind the anterior ascending ramus includes the frontoparietal operculum. The frontal section is related to the lower part of the precentral gyrus, which controls the facial muscles and the head and neck region. The parietal section of the frontoparietal operculum contains the primary sensory area in the postcentral gyrus.

• The temporal operculum is formed by the superior temporal gyrus and contains the Wernicke’s speech region for processing sensory speech perception.

Peri-insular circular sulcus of insula: The peri-insular circular sulcus of insula is a groove or a sulcus encircling the insula and acting as a boundary between the insula and the temporal, frontal, and parietal lobes. It is not easily observable as it lies hidden beneath the opercula of the frontal, parietal, and temporal lobes.

The peri-insular circular sulcus of insula of three parts or rami: the anterior, superior, and inferior rami or sulci. The anterior limiting sulcus separates the insula from the frontal cortex, the superior limiting sulcus separates it from the fronto-parietal cortex, and the inferior limiting sulcus separates it from the temporal cortex.

The anterior and superior limiting sulci meet at a point called the anterior insular point, and the superior and inferior sulci meet at another point called the posterior insular point. This creates a triangular shape for the insular cortex. The base of the triangle is between the anterior and posterior insular points, while the apex points downward and outward where the anterior inferior and posterior limiting sulci attempt to meet. However, the union between the anterior inferior and posterior limiting sulci is not complete, and the region between them, located within the lateral sulcus and facing the lesser wing of the sphenoid bone, is known as the limen insulae.

Gyri of insular cortex: The insular cortex contains a central sulcus that runs from the apex to the base and continues as the central sulcus between the pre- and post-central gyri of the cerebral hemisphere. In the insular cortex, there are three short gyri (anterior, middle, and posterior) located in front of the central sulcus. Behind the central sulcus, there are two long gyri (anterior and posterior) in the insular cortex.

Peri-insular sub-cortical white matter: Understanding the deep anatomical relations of the insular cortex is important. Within the hemisphere, the insular cortical region is surrounded by peri-insular subcortical white matter association fibers. Superiorly, this includes the arcuate fasciculus and the superior longitudinal fasciculus. Inferiorly, this includes the inferior-fronto-occipital fasciculus and the uncinate fasciculus.

Deep medial relations of the insular cortex: Deep and medial to the insular cortex, you can find the extreme capsule, followed by the claustrum and external capsule. Further medially, there are the lentiform nucleus and the internal capsule. The association fibers are located outside to the corticospinal tract fibers and the corona radiata projection fibers. Even further medially, we can observe the caudate nucleus and thalamus, which are located inner to the internal capsule.