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Neural Features and Subdivision of the Medial Temporal Region

Surgical Correlation

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Neural Features and Subdivision of the Medial Temporal Region. A, Mediobasal surface of the right cerebral hemisphere. The MTR can be divided into 3 segments: anterior, middle, and posterior. The anterior segment extends posteriorly from where the rhinal sulcus turns upward at the posterior edge of the temporal pole (yellow interrupted line) to a vertical line crossing the posterior edge of the uncus (green interrupted line). The middle segment extends posteriorly from the posterior edge of the uncus to a vertical line (red interrupted line) at the level of the quadrigeminal plate. The posterior segment extends from the quadrigeminal plate to a vertical line (blue interrupted line) passing through the calcarine point located at the junction of the parieto-occipital and calcarine sulci. B, Enlarged view of the anterior and middle segments of the MTR. The anterior segment of the MTR is formed by the uncus and the entorhinal area. The uncus is divided into anterior and posterior segments that meet at a medially directed apex. The anterior segment of the uncus belongs to the parahippocampal gyrus and presents 2 small gyri: the semilunar gyrus and the ambient gyrus, separated by the semiannular sulcus. The posterior segment of the uncus is divided into an upper and lower part by the uncal sulcus. The part above the uncal sulcus is formed by the medially folded extraventricular (cisternal) head of the hippocampus. The lower part is formed by the anterior part of the parahippocampal gyrus. The entorhinal area occupies this part of the parahippocampal gyrus and is separated from the occipitotemporal gyrus by the rhinal sulcus. The posterior demarcation of the entorhinal area is approximately at the level of the posterior end of the uncus. The parahippocampal gyrus forms the cortical component of the middle segment of the MTR. It has 3 surfaces: medial, superior, and inferior. The medial surface of the middle segment of the MTR is formed by the medial (presubicular) surface of the parahippocampal gyrus. The superior (subicular) surface faces the lower surface of the pulvinar across the upper part of the ambient cistern and is formed by the upper surface of the parahippocampal gyrus, the dentate gyrus, and the fimbria of the fornix. The inferior surface is formed by the lower surface of the parahippocampal gyrus and faces the tentorium. C, Cortical anatomy of the anterior segment of the MTR. The semilunar gyrus is a round prominence on the upper surface of the anterior part of the uncus that covers the cortical nucleus of the amygdala, and is separated from the anterior perforated substance and the optic tract by the entorhinal sulcus. The ambient gyrus embraces the semilunar gyrus from which it is separated medially and anteriorly by the semiannular sulcus, and is continuous inferiorly with the entorhinal area and anterolaterally with the lateral orbital gyrus and the limen insula. The posterior segment of the uncus is separated from the parahippocampal gyrus by the uncal sulcus, also called the uncal notch, and has 3 surfaces: superior, inferior, and medial. The superior surface is formed, from anterior to posterior, by 3 small gyri: the uncinate gyrus, the band of Giacomini, and the intralimbic gyrus. The uncinate gyrus merges into the ambient gyrus at the level of the uncal apex. The band of Giacomini is the continuation of the dentate gyrus in the hippocampal head, and extends from the inferior to the superior surface of the posterior segment of the uncus. The intralimbic gyrus forms the posterior end of the uncus and is the site of attachment of the fimbria. The inferior surface of the posterior segment of the uncus is visible only after removal of the subjacent parahippocampal gyrus (see part J). D, Superior view of the right MTR. An incision at the level of the limen insula, entorhinal sulcus, and choroidal fissure has separated the MTR from the central core of the hemisphere, and exposes both the ventricular and the cisternal components of the MTR. The semilunar gyrus overlies the amygdala, which forms the anterior wall and the anterior part of the roof of the temporal horn. The uncal apex, the most medially prominent point of the uncus, is positioned medial to the uncal recess located in the anterior part of the temporal horn between the hippocampal head or ventricle surface of the amygdala. The intralimbic gyrus, at the posterior end of the uncus, is positioned adjacent the inferior choroidal point at the lower end of the choroidal fissure and choroid plexus in the temporal horn. The posterior part of the rhinal sulcus and anterior part of the collateral sulcus lie lateral to the entorhinal area and underlie the anterior part of the collateral eminence. (Images courtesy of AL Rhoton, Jr.)

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