Insula & Sylvian Fissure

Insula & Sylvian Fissure

Here, we will draw the anatomy of the Insula and the Sylvian fissure.

Insula

Brain Atlas: Insula

Coronal View

First, to get a good look at the insula, draw a coronal view of a cerebral hemisphere.

Indicate that the insula (aka the island of Reil) lies deep within the brain under the cerebral folds (the opercula).
Denote that the superior opercula comprise the frontal and parietal opercula.
Then, indicate that the inferior operculum is the temporal operculum.

Sagittal View

Now, let's look at the insula in sagittal view.

Draw a lateral face of the cerebral hemisphere, but carve out the center of the hemisphere.
Indicate that the circular sulcus circumscribes the insula (except in the antero-inferior region at the limen insulae — the insular apex).
Next, show that the central sulcus extends through the insula.

Lobules

Short Insular Gyri

Show that the insula divides into an anterior lobule, which we shade in green and a posterior lobule, which we shade in purple.
Indicate that the anterior lobule contains three short insular gyri.

Then, denote that the posterior lobule contains two long insular gyri.
Within the anterior lobule, draw the precentral sulcus and anterior to it, draw the short insular sulcus.
In between the precentral and central sulci, label the posterior short insular gyrus.
In between the precentral and short insular sulci, label the middle short insular gyrus.
Anterior to the short insular sulcus, label the anterior short insular gyrus.
Now, within the posterior lobule, draw the postcentral sulcus.
In between it and the central sulcus, label the anterior long insular gyrus, and posterior to it, label the posterior long insular gyrus.

Functional Roles of the Insula

The insula has widespread intracortical connectivity, including connections to the hypothalamus, thalamus, cortical sensory association areas, auditory cortex, and limbic system.
It has been shown to play a role in pain modulation, appetite, awareness of visceral sensation, anxiety and emotion, socialization, and auditory processing, and much more.

Clinical Correlation: Insular Stroke

Sylvian Fissure

Brain Atlas: Sylvian Fissure

Next, let's learn the detailed anatomy of the Sylvian fissure and its related structures.

Inferior Brain

First, draw the undersurface of the brain but exclude the anterior temporal lobe.

Include the optic chiasm, pituitary bodies, and midbrain in midline.
Next, further define the region surrounding the optic chiasm as the anterior perforated substance, which is perforated by short branches of the proximal middle cerebral artery.
Next, label the internal carotid artery just lateral to the optic chiasm.
Then, show that the proximal middle cerebral artery (the M1 branch) originates from the internal carotid artery within the cistern of the vallecula cerebri (aka the carotid cistern) and passes laterally within this cistern through the deep (or basal) portion of the Sylvian fissure between the frontal and temporal lobes.
As the Sylvian fissure wraps around to the lateral convexity of the brain, the related cisternal space becomes the cistern of the lateral cerebral fossa; exactly where this cisternal transition occurs is inconsistently defined.
Now, let's continue with the lateral face of the Sylvian fissure.
Draw an outline of the lateral cerebral hemisphere.
Include the central sulcus for orientational purposes.

Inferior Frontal Gyrus

First, draw the anterior-inferior division of the Sylvian fissure.
At its midpoint, draw a "V"; for the rami extensions from the Sylvian fissure.
Label the lower arm of the "V"; as the anterior horizontal ramus and the upper arm as the anterior ascending ramus.
The anterior horizontal ramus runs roughly horizontally and the anterior ascending ramus runs roughly vertically.
Indicate that these rami subdivide the inferior frontal gyrus as follows:
Beneath the anterior horizontal ramus, label the pars orbitalis.
Then, in between the two rami, label the triangular-shaped pars triangularis.
Lastly, posterior to the anterior ascending ramus, label the pars opercularis.
Next, let's draw the posterior-superior division of the Sylvian fissure.
At the distal end of the central sulcus, draw the horizontally directed posterior horizontal ramus.
Then complete, the Sylvian fissure with the posterior ascending ramus.

Posterior, Superior Temporal Gyrus

Indicate that the supramarginal gyrus caps this ramus.
Note that a posterior descending ramus also exists, which lies along the vertical plane of the posterior ascending ramus (we leave it out for simplicity).
With this Sylvian fissure anatomy defined, return to the lateral face of the insula to further illustrate the various cerebral opercula (in accordance with Nieuwenhuys' description).
Label the orbital operculum, which lies inferior to the anterior horizontal ramus of the Sylvian fissure.
Label the frontal operculum, which lies in between the anterior horizontal and anterior ascending Sylvian fissure rami.
Then, label the parietal operculum, which lies in between the anterior ascending and posterior Sylvian fissure rami.
Label the temporal operculum, which lies inferior to the posterior ramus of the Sylvian fissure.
Lastly, include the transverse temporal gyri (Heschl's gyri).
The temporal operculum is formed from both them and the superior temporal gyrus.