Parietal Lobe

The parietal lobe is one of the four major lobes of the cerebral cortex, positioned posterior to the frontal lobe and superior to the occipital lobe. It plays a central role in processing somatosensory information, spatial orientation, proprioception, and attention. It is also involved in higher-order functions such as language comprehension and mathematical reasoning.

Location

The parietal lobe is located posterior to the central sulcus and anterior to the parieto-occipital sulcus. It lies superior to the lateral sulcus and forms the upper part of the cerebral hemisphere. The lobe is bordered:

• Anteriorly: By the central sulcus (separating it from the frontal lobe)
• Posteriorly: By the parieto-occipital sulcus (separating it from the occipital lobe)
• Inferiorly: By the lateral sulcus (separating it from the temporal lobe)
• Medially: By the longitudinal fissure separating the two hemispheres

Major Gyri and Functional Areas

• Postcentral gyrus (Primary Somatosensory Cortex, Brodmann areas 1, 2, 3): Receives and processes tactile, pressure, vibration, pain, temperature, and proprioceptive input from the contralateral side of the body in a somatotopically organized fashion.
• Superior parietal lobule: Involved in spatial orientation, body awareness, and integration of visual and sensory input.
• Inferior parietal lobule: Includes the supramarginal gyrus and angular gyrus; plays roles in language processing, mathematical cognition, and body schema.
• Precuneus (medial surface): Involved in visuospatial imagery, episodic memory retrieval, and aspects of consciousness.

Functional Roles

• Somatosensory processing: The primary somatosensory cortex interprets tactile and proprioceptive input from skin, joints, and muscles.
• Spatial awareness: Integration of sensory input for the perception of body position and orientation in space.
• Visual-spatial attention: Helps direct attention to different parts of the visual field; the right parietal lobe is dominant in this function.
• Language and reading: The angular gyrus contributes to reading, comprehension, and writing, particularly in the dominant hemisphere.
• Mathematical ability: Regions within the inferior parietal lobule support arithmetic calculation and numerical understanding.
• Tool use and object manipulation: Parietal areas help coordinate hand-eye actions and interpret the function of tools.

White Matter Connections

The parietal lobe connects with other brain regions via association fibers (e.g., superior longitudinal fasciculus), allowing communication with the frontal lobe, occipital lobe, and temporal lobe. These pathways are essential for integrated sensory-motor coordination and cognition.

Blood Supply

• Middle cerebral artery (MCA): Supplies the lateral surface of the parietal lobe, including the postcentral gyrus and inferior parietal lobule.
• Anterior cerebral artery (ACA): Supplies the medial portion of the superior parietal lobule.
• Posterior cerebral artery (PCA): May supply parts of the posterior parietal lobe near the parieto-occipital junction.

Venous drainage is primarily via the superior sagittal sinus through cortical veins on the lateral and medial surfaces.

Development

The parietal lobe originates from the dorsal part of the embryonic telencephalon. As the brain matures, the lobe becomes highly folded, forming distinct sulci and gyri. Functional specialization of the parietal cortex, particularly for spatial and sensory processing, continues to develop into adolescence.

Clinical Significance

• Contralateral sensory loss: Lesions in the primary somatosensory cortex result in decreased tactile discrimination and proprioception.
• Astereognosis: Inability to recognize objects by touch alone due to parietal lobe damage.
• Hemispatial neglect: Damage to the right parietal lobe can cause left-sided spatial neglect, where the patient ignores stimuli on the left side of space.
• Gerstmann syndrome: Typically caused by lesions of the dominant inferior parietal lobule; features include agraphia, acalculia, finger agnosia, and left-right disorientation.
• Apraxia: Difficulty performing purposeful movements despite intact motor and sensory systems, due to parietal dysfunction.
• Balint syndrome: Bilateral parietal lesions leading to optic ataxia, oculomotor apraxia, and simultanagnosia.
• Dyslexia and dyscalculia: Impairments in reading and calculation have been associated with altered function in the angular and supramarginal gyri.

Imaging

MRI and CT imaging of the parietal lobe are used to detect strokes, tumors, traumatic injuries, and cortical atrophy. Functional MRI (fMRI) studies have identified parietal lobe activation in attention, number processing, and spatial memory tasks. Diffusion tensor imaging (DTI) can assess white matter tracts connecting the parietal lobe to other brain regions.