The insula is the only cortical part of the brain which is not visible on the surface of the hemisphere. This is due to the fact that it is totally covered by the fronto-parietal and temporal operculum.
The insula has a triangular shape, and is separated from the opercula by the anterior, superior, and inferior periinsular sulci. The limen insulae is the antero-inferiorly located insular cortical surface which conjoins the inferior insular point, the anterior perforated surface, and the temporo-mesial surface. The insula is morphologically divided into two parts by the central insular sulcus. The anterior part of the insula bears 3 gyri: the anterior, middle, and posterior short insular gyri, separated by the anterior and precentral insular sulcus. The posterior part of the insula contains the anterior and posterior long insular gyri, separated by the postcentral insular sulcus.
The insular cortex is divided into two parts: the larger anterior insula and the smaller posterior insula in which more than a dozen field areas have been identified. The cortical area overlying the insula toward the lateral surface of the brain is the operculum (meaning lid). The opercula are formed from parts of the enclosing frontal, temporal, and parietal lobes.
Each insula has a trapezoid shape, surrounded by four periinsular sulcus (anterior, superior, posterior, and inferior).
Afif and Mertens differentiated the posterior peri-insular sulcus from the inferior peri-insular sulcus. These two sulci have two different axes separated by a clear angle. The central insular sulcus divides the insula into two parts. The anterior insula includes three short gyri and the anterior insular pole. The posterior insula includes two long gyri and the posterior insular pole. This structure defines two intra-insular opercula. In 60% of cases, the superior extremity of the central insular sulcus is in direct continuity with the inferior extremity of the cerebral central sulcus. The superior branch of the middle cerebral artery supplies the majority of the anterior insular gyri, and the inferior branch supplies the majority of the posterior insular gyri 1).
A photograph of the left insular cortex of a human patient. For a comparison with the insulae of other patients, see Ref. 114. The human insular cortex is a distinct but hidden lobe of the brain. It is disproportionately (approx30%) enlarged in the human relative to the macaque monkey109. It has 5–7 oblique gyri, but its morphology is quite variable, even between the two sides114, 115, 116, 117, 118. A comprehensive hodological description in the macaque is lacking, and few connectivity analyses of the insula have been made in humans. Primary interoceptive representations are located in the dorsal posterior insula and re-represented in a polymodal integrative zone in the mid-insula and again in the anterior insular cortex (AIC)2, 7, 119, 120. The primary interoceptive, gustatory and vagal representations extend to the anterior limit of the insula in macaques but only to the middle of the insula in humans98, 121, 122, 123, 124, which suggests that the AIC of humans has no equivalent in the monkey. The most anterior and ventral (inferior) portion of the human insula that adjoins the frontal operculum is probably the most recently evolved, because this part (as well as the anterior cingulate cortex; Box 1) contains von Economo neurons (Box 2).
as, anterior short insular gyrus;
al, anterior long insular gyrus;
ac, accessory gyrus;
APS, anterior peri-insular sulcus;
H, Heschl's gyrus;
IPS, inferior peri-insular sulcus;
ms, middle short insular gyrus;
ps, posterior short insular gyrus;
pl, posterior long insular gyrus;
SPS, superior peri-insular sulcus. Photograph is courtesy of Professor Thomas P. Naidich, Mount Sinai Medical Center, New York.
The vascular supply of the insula is predominantly provided by the M2 segment of the middle cerebral artery, which constitutes a substantial obstacle to any open or stereotactic procedure aiming at the insular region.
The superior branch of the middle cerebral artery supplies the majority of the anterior insular gyri, and the inferior branch supplies the majority of the posterior insular gyri 2).
It seems that all the arteries supplying the insula come from the middle cerebral artery (MCA), and predominantly from the M2 segment. Some of them arise from the M1 segment and from the M3 segment. Among the insular arteries, there are short and medium perforating arteries that supply the extreme capsule, the claustrum and the external capsule.
Some insular long perforating arteries that are as important as the lenticulostriates arteries arise from the MCA trunks, the early branches and the cortical arteries, as they cross the insula.
They most commonly penetrate into the posterior insular region and vascularize the corona radiata 3).
Arterial vascularization of the insula was studied in 20 human cadaver brains (40 hemispheres). The cerebral arteries were perfused with red latex to enhance their visibility, and they were dissected with the aid of an operating microscope. Arteries supplying the insula numbered an average of 96 (range 77-112). Their mean diameter measured 0.23 mm (range 0.1-0.8 mm), and the origin of each artery could be traced to the middle cerebral artery (MCA), predominantly the M2 segment. In 22 hemispheres (55%), one to six insular arteries arose from the M1 segment of the MCA and supplied the region of the limen insulae. In an additional 10 hemispheres (25%), one or two insular arteries arose from the M3 segment of the MCA and supplied the region of either the superior or inferior periinsular sulcus. The insular arteries primarily supply the insular cortex, extreme capsule, and, occasionally, the claustrum and external capsule, but not the putamen, globus pallidus, or internal capsule, which are vascularized by the lateral lenticulostriate arteries (LLAs). However, an average of 9.9 (range four-14) insular arteries in each hemisphere, mostly in the posterior insular region, were similar to perforating arteries and some of these supplied the corona radiata. Larger, more prominent insular arteries (insuloopercular arteries) were also observed (an average of 3.5 per hemisphere, range one-seven). These coursed across the surface of the insula and then looped laterally, extending branches to the medial surfaces of the opercula.
Complete comprehension of the intricate vascularization patterns associated with the insula, as well as proficiency in insular anatomy, are prerequisites to accomplishing appropriate surgical planning and, ultimately, to completing successful exploration and removal of pathological lesions in this region 4).
Tanriover found a total of 194 insular perforating arteries, equal to or larger than 0.3 mm in diameter, arising from the middle cerebral artery trunks, the early branches and the cortical and stem arteries as they crossed the insula. In Tanriover’s study, the branches larger than 0.3 mm most commonly arose from the central, angular and posterior parietal arteries and penetrated into the posterior half of the central insular and inferior limiting sulci and the long gyri. Twenty percent of the 194 larger insular perforating arteries had a diameter greater than 0.5 mm and were directed predominantly to the posterosuperior part of the long gyri 5).
All studies agree that there are insular perforators arising from the M2 segment and destined for the motor tract of the corona radiata. They explain that these perforators most commonly penetrate into the posterosuperior part of the insular long gyri.
But contrary to Tanriover, these arteries arise mainly from the precentral, central, anterior parietal and posterior parietal arteries, but only 3.3 % arise from the angular artery 6).
Histologically, the insula is a part of the paralimbic cortex, as it bears in its antero-inferior part an allo and mesocortical area. The insula is functionally involved in cardiac rhythm and arterial blood pressure control, as well as in viscero-motor control and in viscero-sensitive functions. There is considerable evidence for the involvement of the insula as a somesthetic area, including a major role in the processing of nociceptive inputs.
The insulae are believed to be involved in consciousness and play a role in diverse functions usually linked to emotion or the regulation of the body's homeostasis. These functions include perception, motor control, self-awareness, cognitive functioning, and interpersonal experience. In relation to these, it is involved in psychopathology.
Recent neuroimaging studies have demonstrated that anterior insular cortex activation is associated with accessing interoceptive information and underpinning the subjective experience of emotional state. Only a small number of studies have focused on the influence of insular damage on emotion processing and interoceptive awareness. Moreover, disparate hypotheses have been proposed for the alteration of emotion processing by insular lesions. Some studies show that insular lesions yield an inability for understanding and representing disgust exclusively, but other studies suggest that such lesions modulate arousal and valence judgments for both positive and negative emotions.
In a study, Terasawa et al. examined the alteration in emotion recognition in three right insular and adjacent area damaged cases with well-preserved higher cognitive function. Participants performed an experimental task using morphed photos that ranged between neutral and emotional facial expressions (i.e., anger, sadness, disgust, and happiness). Recognition rates of particular emotions were calculated to measure emotional sensitivity. In addition, they performed heartbeat perception task for measuring interoceptive accuracy. The cases identified emotions that have high arousal level (e.g., anger) as less aroused emotions (e.g., sadness) and a case showed remarkably low interoceptive accuracy. The current results show that insular lesions lead to attenuated emotional sensitivity across emotions, rather than category-specific impairments such as to disgust. Despite the small number of cases, our findings suggest that the insular cortex modulates recognition of emotional saliency and mediates interoceptive and emotional awareness 8).
see Insular tumor
Transcortical (TC) and transsylvian approach (TS) corridors have been described as the main surgical approaches to the insula, but there is insufficient evidence to support one approach versus the other.
Overall, the TC approach to the insula provided better insula exposure and surgical freedom compared with the TS with the superficial sylvian bridging veins cut. Cortical and subcortical mapping is critical during the TC approach to the posterior zones (II and III), as the facial motor and somatosensory functions (Zone II) and language areas (Zone III) may be involved 9).