Central nervous system disorders

II/1.2: The brain’s convex surface

The surface is segmented by several convolutions (gyrus) and grooves (sulcus). The convolutions are often not uniform, they might be crossed by small additional grooves.

The frontal pole is located at the anterior end of the frontal lobe. The lobe’s posterior border is the central sulcus (fissure of Rolando). Parallel to, and in front of the central sulcus lies the precentral gyrus which deviates from the frontal plane in posterior direction. It is bordered by the precentral sulcus from the front, and two grooves which are almost perpendicular to it, the superior and inferior frontal sulcus divide the anterior part of the frontal lobe into superior, medial and inferior gyri.

Of the frontal cortical Brodmann areas a few needs to be emphasized in a functional point of view. These are Br.4 (primary motor area) located at the precentral gyrus; Br.6 (premotor and supplementray motor area) at the anterior part of the precentral gyrus and at the superior and medial frontal gyrus; and Br.8 (cortical gaze center). Broca’s motor speech area (Br. 44;45) is located at the frontal operculum (posterior part) and triangular part of the inferior frontal gyrus in the dominant (ussually the left) hemisphere. Exner’s area or writing center is located between the Broca area and the premotor cortex.

1. fotó: Az agy konvex felszíne elölnézetben – Molnár Attila és Balogh Attila

The parietal lobe starts behind the central sulcus (fissure of Rolando) with the postcentral gyrus. The intraparietal sulcus is roughly perpendicular to it, with the superior parietal lobule on its upper medial side and the inferior parietal lobule on its lower lateral side. The angular gyrus which is behind the supramarginal gyrus and the superior temporal gyrus around the posterior end of lateral cerebral sulcus (Sylvian fissure) are also parietal convolutions. Postcentral gyrus represents the primary sensory cortex as Br.3, 1, 2 areas. The whole body’s somatosensory representation is located here except for the lower limbs. Innervation of the face and the tongue below have especially large areas. The supramarginal gyrus (Br.40) and the angular gyrus (Br.39) are significant associative areas located at the border of the parietal, temporal and occipital lobes. The parietal lobules correspond to associative areas Br 5 and 7. Together with the primary sensory cortex they are also important inputs for movement planning.

The border between the parietal and occipital lobe is the artificially assigned parietooccipital line which connects the preoccipital notch where the convex and basal surfaces meet with the upper end of the parietooccipital sulcus where the convex and medial surfaces meet. If we connect the middle of this line with the posterior end of the lateral cerebral fissure, we get the parietotemporal line which separates the parietal and temporal lobes.

3. fotó: Az agy konvex felszíne felülnézetben – Molnár Attila és Balogh Attila

The posterior border of the temporal lobe is therefore the lower part of the parietooccipital line, and the superior border is the parietotemporal line on the rear and the Sylvian fissure in its continuation. Its frontal end is the temporal pole. There are two significant grooves on the surface, called superior and inferior temporal sulcus, separating the superior, medial and inferior temporal gyri. There are short gyri at the upper part of the superior temporal gyrus, these are called Heschl’s transverse gyri, representing primary (Br.41) and secondary (Br.42) auditory areas. The posterior part of the superior temporal gyrus together with the inferior parietal lubule – which belongs to the parietal lobe – includes Wernicke’s sensory speech center (Br.39 and 40).

The occipital lobe which is located at the posterior part of the convexity is divided into superior and inferior occipital gyri by the transverse occipital sulcus. The posterior end of the lobe is the occipital pole. It belongs to the visual center. Most of the fibers arriving from the macula radiate here.