Abstract
Historically, the insular cortex has attracted only minimal research interest, perhaps due to its inconspicuous position in the brain. Although its cytoarchitectural organization and circuitry have recently been documented, no quantitative data on the dendritic characteristics of the insula have yet been reported. The present study quantified the dendritic and spine extent of layer III pyramidal neurons across three regions of human insular cortex. The dendritic properties of the insula were also compared with those of previously quantified human neocortex. Tissue was removed from the left insula of five adult males (Mage = 43±6 years), specifically from the secondary gyrus breves, the precentral insular gyrus, and the postcentral insular gyrus. Sampled tissue was then stained with a modified rapid Golgi technique. Ten neurons from each gyrus (N = 150) were quantified. A nested ANOVA revealed significant differences across the three regions of the insula for total dendritic length and dendritic spine number. As predicted, both measures significantly increased from the postcentral gyrus to secondary gyrus breves, which is consistent with the general caudal to rostral dendritic progression observed in the neocortex. Moreover, comparison of dendritic morphology between the insula and the neocortex demonstrated that insular pyramidal neurons have longer but less branchy dendrites than those of the neocortex. The results suggest potential differences in dendritic systems between insular and neocortical neurons that may provide clues for understanding the functional characteristics of insular cortex.