agranular sector of the insula
Acronym: Ia-p
The term agranular sector of the insula refers to a portion of the insula defined on the basis of internal structure: it lacks granular cell layers. It is found in the human ( Zilles-2012 ) and the macaque ( Mesulam-1982 ). It is bounded dorsally by the dysgranular insula and laterally by the inferior limiting sulcus. Classified as peripaleocortex, it is located between the piriform cortex ( paleocortex ) and the dysgranular insula ( proisocortex ).
     It is distinguished from surrounding areas by the absence of the external granular layer (II) and the internal granular layer (IV). It has a superficial layer of pyramidal cells, which is continuous with the pyramidal layer of the piriform cortex, and a deeper layer that is continuous with the claustrum. An intermediate layer of pyramidal cells appears near the boundary with, and continues through, the dysgranular insula. It contains a few granule cells, but they are scattered and do not form clusters or layers as in the other two histologically defined parts of the insula: the dysgranular insula and granular insula.
     It is the insular component of a three-part agranular area. The others, with which it is continuous, are the agranular orbitofrontal cortex and the medial part of the temporal pole ( Mesulam-1984 ).
     Note that the agranular sector of the insula is not the same as agranular insular cortex (Roberts), which is illustrated in many textbooks and brain atlases ( Paxinos-2009a ).

Also known as: agranular insulaNeuroNames ID : 3316

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