Parietal lobe

Topography

The parietal lobe is located between the frontal and occipital lobe and above the temporal lobe on each cerebral hemisphere. Its borders are as follows:

• Anterior border - formed by the central sulcus (of Rolando)
• Posterior border - formed by the imaginary line extending between the parieto-occipital sulcus (superiorly) and the preoccipital notch (inferiorly).
• Inferior border - formed by the lateral fissure (of Sylvius)
• Superior border - formed by the medial longitudinal fissure that separates the two hemispheres

Running almost parallel with the central sulcus is the post-central sulcus. Both sulci demarcate the post-central gyrus, located about 6.5 cm posterior to the bregma of the skull. Because the marginal sulcus (or ascending band of the cingulate) points directly to the post-central gyrus on the superior surface of the hemisphere, it serves as an important landmark for identifying the gyrus (particularly on MRIs).

The post-central gyrus is usually connected with the pre-central gyrus of the frontal lobe with a tiny, horizontal gyrus at the base of the central sulcus, called the subcentral gyrus.  Together, all three of these gyri which surround the central sulcus, are referred to as the central lobe.

Just posterior to the post-central gyrus, the parietal lobe is divided into a superior and inferior parietal lobule with the intraparietal sulcus. This sulcus originates about at the midpoint of the post-central sulcus and extends posteriorly, parallel to the medial longitudinal fissure. The inferior parietal lobule continues to the parieto-temporal intersection, and is formed by the supramarginal gyrus and the angular gyrus. The “U”-shaped supramarginal gyrus surrounds the posterior end of the lateral fissure, while the angular gyrus is found at the posterior tip of the superior temporal sulcus.

On the medial surface of the hemisphere, the parietal lobe forms the posterior part of the paracentral lobule, which is demarcated by the pre-central sulcus anteriorly and the marginal sulcus posteriorly. Just posterior to the paracentral lobule is the precuneus, which extends from the supramarginal sulcus to the parieto-occipital sulcus.

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Cytoarchitecture

The primary sensory areas, such as the post-central gyrus, of the parietal lobe show a granular-type histology. In these areas the normal 6-layers of the cortex are not evident, as the layers II and IV that contain predominantly sensory granular cells (external and internal granular layer) are much more pronounced, compared to the layers III and V which contain predominantly motor pyramidal cells (external and internal pyramidal layer). The association cortical areas of the parietal lobe, however, demonstrate all 6 cell layers of the cortex.

Blood supply

The lateral surface of the parietal lobe is supplied by the middle cerebral artery (one of the three branches of the internal carotid artery). Another of the internal carotid artery branches is the anterior cerebral artery, which supplies the medial surface of the parietal lobe. The posterior cerebral artery supplies the posterior surface of the medial parietal lobe.

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Primary somatosensory area

Postcentral gyrus forms the primary somatosensory cortex and is referred to as Brodmann area 3, 1, 2. This gyrus receives the sensory information from all the sensory receptors that provide information related to temperature, pain (spinothalamic pathway), vibration, proprioception and fine touch (dorsal column pathway).

The parts of the body are neurologically mapped on the somatosensory cortex. This pictorial, somatotopic representation of the human body on the post-central gyrus, is referred to as the cortical sensory homunculus devised by Wilder Penfield. The sensory map consists of an upside-down representation of the body, running superior-inferiorly along the post-central gyrus. The point-for-point correspondence of the body on the gyrus, results in a grotesque disproportionate figure, with large hands, lips and face, compared to the rest of the body. This is because that areas that are finely controlled or have acute sensation, have larger portions of the somatosensory cortex. 

Parietal association cortex

The superior parietal lobule forms the association cortex of the parietal lobe, and plays an important role in planned movements, spatial reasoning and attention. The intraparietal sulcus can be further divided into a lateral, medial, ventral and anterior area. The lateral area is responsible for our eye movements in response to a stimulus in space. The medial area helps us to determine how far and where we need to reach in relation to our nose. The ventral area is an area that receives a number of sensory modalities; these include auditory, visual, vestibular and somatosensory information. Finally, the anterior area enables us to interpret the size, shape and position of objects we are about to grasp. The anterior and ventral areas work together to enable visual motor coordination of hand movements.  

Sensory speech area of Wernicke

Wernicke’s area is important for language development and for comprehension of speech. It functions in language comprehension, semantic processing, language recognition and language interpretation. Wernicke’s area is classically found in the posterior part of the superior temporal gyrus usually in the left cerebral hemisphere (Brodmann area 22), an area which also encircles the auditory cortex. Most neuroscientists also include regions of the inferior parietal lobule, particularly the supramarginal gyrus (Brodmann area 40) and the angular gyrus (Brodmann area 39) in Wernicke’s area. The supramarginal gyrus forms the auditory area of speech, while the angular gyrus, the visual area of speech.   

Baum’s loop

In the optic radiations from the lateral geniculate nucleus of the thalamus, two loops of fibers carry information back to the visual area: Meyer’s loop and the Baum’s loop. Meyer’s loop carries information for the superior part of the visual field, while Baum’s loop carries information from the inferior part of the visual field. Meyer’s loop (superior visual field, inferior retinal field)  runs through the temporal lobe. Baum’s loop or parietal optic radiation runs through the parietal lobe to terminate on the upper bank of the calcarine sulcus in the cuneus of the occipital lobe.  

Dominant vs. non-dominant hemispheres

The parietal lobes control calculation and language on the dominant side, and the sensory visuospatial processing on the non-dominant hemisphere side.