Parietal lobe

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Introduction

The Parietal lobe located posterior to the frontal lobe occupies a fifth of the neocortex, and is responsible for processing sensory information vital for spatial perceptual awareness. It receives somatosensory information related to touch, temperature, pressure, and pain from the periphery. It is intimately connected with motor and sensory areas, including visual, auditory, smell and taste, and synthesizes inputs from these different sensory modalities to form a coherent world view. The parietal lobe plays a key role in somatosensory perception, visuospatial integration, navigation, speech comprehension - including reading and writing ability, and computation.

Structural and Functional Anatomy

The Parietal lobe lies posterior to the central sulcus, and its internal structural and functional divisions are delineated by the sulci and gyri of the cortical surface. On the lateral surface, the parietal lobe is separated from the frontal lobe by the central suclus or fissure of Rolando immediately behind which is the postcentral gyrus that harbors the somatosensory cortex . The remainder of the lobe posterior to the somatosensory cortex constitutes the association areas on account of the processing of unimodal, or multimodal sensory information. The intraparietal sulcus extents from the the postcentral sulcus parallel to the cortical surface and further demarcates the superior and inferior parietal lobules, the latter containing the supramarginal layer at its anterior end and the angular gyrus more posteriorly. The lateral sulcus or Sylvian fissure separates the parietal lobe at its inferior margin from the temporal lobe. The medial longitudinal, sagittal sulcus defines the left and right parietal lobes of each hemisphere. In the medial aspect of the lobes, the cingulate sulcus separates the lobe from the limbic lobe posteriorly and the parieto-occipital sulcus designates the boundary with the occipital lobe. The preoccipital notch defines this boundary with the occipital lobe at the posterior tip on the lateral surface. The area in the medial surface known as the posterior paracentral lobule abuts the postcentral gyrus and continues to the marginal end of the cingulate sulcus, posterior to which lies the precuneus. The precuneus is more distinguished in humans compared to non-human primates, and is contiguous with superior parietal lobule of the lateral surface and extends to the parietooccipital sulcus.

The primary somatosensory cortex (SI) occupies the postcentral gyrus and is divided cytoarchitectonically into Brodmann's areas 3a, 3b, 1, and 2. The more anterior 3a and the 3b areas receive strong input from the ventral posterior nucleus of the thalamus and they in turn project to area's 1 and 2. Sensory input from the largest organ in the body, the skin is conveyed to area's 3b and 1, whereas proprioceptive information from the joints about load bearing tendon's and ligaments, movement-associated muscles, and skin concerning the mechanical state and position of body is transmitted to area's 3a and 2. The thalamic afferents arrive in distinct columns that represent the contralateral side of the body. These columns follow the same pattern in all cortical areas, spanning the 6 cortical layers and extending from the surface of the brain to the boundary with the white matter. The neurons in each column share common features relating to sensory modality, function, and region of body represented, and target specific cortical and subcortical regions. The neurons in SI send axons to higher order processing areas of the cortex that integrate somatosensory with input from other sensory and motor modalities, in particular it projects to the secondary somatosensory cortex (SII, Brodmann's area 43), posterior parietal cortex, and motor cortex.

The primary somatosensory cortex Layer 4 neurons receive afferents from the thalamus.

The parietal lobe is organized to accommodate and integrate spatial information for informing visually guided motor behavior. The primary visual cortex processes rudimentary retinal information that is then conveyed to higher-order parietal association areas in what is referred to as the dorsal stream projection. The dorsal stream represents the composite 'where-how' of visual classification and corresponds to the spatial-temporal aspects of objects, i.e., location and trajectory that is key for steering behavior in relation to the object.

Somatosensory Cortex

The somatosensory cortex is tasked with processing and integrating inputs from the entire body to constantly be aware of its physical condition.