This part of the cerebrum, lying behind the central sulcus and above the sylvian fissure, is the least well demarcated (Fig. 21-1). Its posterior boundary, where it merges with the occipital lobe, is obscure, as is part of the inferior-posterior boundary, where it merges with the temporal lobe. On its medial side, the parietooccipital sulcus marks the posterior border, which is completed by extending the line of the sulcus downward to the preoccipital notch on the inferior border of the hemisphere. Within the parietal lobe, there are two important sulci: the postcentral sulcus, which forms the posterior boundary of the somesthetic cortex, and the interparietal sulcus, which runs anteroposteriorly from the middle of the posterior central sulcus and separates the mass of the parietal lobe into superior and inferior lobules (Fig. 21-1). The inferior parietal lobule is composed of the supramarginal gyrus (Brodmann area 40) and the angular gyrus (area 39). The superior parietal lobule is that remaining part of the lobe that is bounded below by interparietal sulcus, anteriorly by the postcentral sulcus, and extends onto the medial surface of the brain in Brodmann areas 5 and 7 (Fig. 21-2). The architecture of the postcentral convolution is typical of all primary receptive areas (homotypical granular cortex). The rest of the parietal lobe resembles the association cortex, both unimodal and heteromodal, of the frontal and temporal lobes.
The superior and inferior parietal lobules and adjacent parts of the temporal and occipital lobes are relatively much larger in humans than in any of the other primates and are relatively slow in attaining their fully functional state (beyond age 7 years). This area of heteromodal cortex has large fiber connections with the frontal, occipital, and temporal lobes of the same hemisphere and, through the middle part of the corpus callosum, with corresponding parts of the opposite hemisphere.
The postcentral gyrus, or primary somatosensory cortex, receives most of its afferent projections from the ventroposterior thalamic nucleus, which is the terminus of the ascending somatosensory pathways. The contralateral half of the body is represented somatotopically in this gyrus on the posterior bank of the rolandic sulcus. It has been shown in the macaque that spindle afferents project to area 3a, cutaneous afferents to areas 3b and 1, and joint afferents to area 2 (Kaas). Stimulation of the postcentral gyrus elicits a numb, tingling sensation and sense of movement. Penfield (1941) remarked that rarely are these tactile illusions accompanied by pain, warmth, or cold. Stimulation of the motor cortex may produce similar sensations, as do discharging seizure foci from these regions. The primary sensory cortex projects to the superior parietal lobule (area 5), which is the somatosensory association cortex. Some parts of areas 1, 3, and 5 (except the hand and foot representations) probably connect, via the corpus callosum, with the opposite somatosensory cortex. There is some uncertainty as to whether area 7 (which lies posterior to area 5) is unimodal somatosensory or heteromodal visual and somatosensory; certainly, it receives a large contingent of fibers from the occipital lobe.
In humans, electrical stimulation of the cortex of the superior and inferior parietal lobules evokes no specific motor or sensory effects. Overlapping here, however, are the integrative zones for vision, hearing, and somatic sensation, the supramodal integration of which is essential to our awareness of space and person and certain aspects of language and calculation (apperception), as described below.
The parietal lobe is supplied by the middle cerebral artery, the inferior and superior divisions supplying the inferior and superior lobules, respectively, although the demarcation between the areas of supply of these two divisions is quite variable.