Brain Anatomy Part II
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Section V. Parietal Lobe Overview
The parietal lobe consists largely of the postcentral gyrus and the two lobules-the superior and inferior parietal lobules. The medial projection of the superior parietal lobule is referred to as the precuneus. The functional topology of the parietal lobe is roughly a mirror image of that of the frontal lobe. That is, there is a strip of somatosensory cortex in a homunculus along the bank of the central sulcus and a larger region of heteromodal association cortex elsewhere. Accordingly, lesions in the association cortex are associated with more complex clinical deficits such as neglect, difficulty reading, writing, math, counting, and so forth. This association cortex also has a role in language, although more difficult to localize than Broca's area of the frontal lobe.
Again, there are some key anatomic signs that help define surface anatomy and facilitate identification of the central sulcus
3D surface anatomy demonstrate the major borders, gyri, sulci, and signs.
Major Borders
Major borders of the parietal lobe, lateral view. The major borders include the central sulcus, Sylvian fissure, and 'imaginary' lines. Localization of the parieto-occipito-temporal region on cross-sectional imaging can imprecise.
Key Structures
The parietal lobe largely consists of the postcentral gyrus and two large regions: the superior and inferior parietal lobules.
The medial projection of the superior parietal lobule is the precuneous. The inferior parietal lobule is primarily comprised of two gyri: the supramarginal and angular gyri.
Key Signs
The postcentral sulcus termination is often, but not always, bifid (bifid postcentral sulcus sign). The intraparietal sulcus can extend anteriorly as far as the postcentral sulcus (intra-parietal sulcus intersection sign) and does not violate the postcentral gyrus; this extension is often superficial along the brain surface and is not always seen.
Key Structures
The postcentral gyrus is bridged to the precentral gyrus by the subcentral gyrus. The inferior parietal lobule is dived into the supramarginal gyrus (orange) and angular gyrus (blue). The supramarginal gyrus is draped over the blind termination of the posterior ascending ramus of the Sylvian fissure while the angular gyrus is centered over the posterior extend of the superior temporal sulcus. The intra-parietal sulcus terminates at the postcentral sulcus.
Key Signs
The triple sulcus, intra-parietal intersection, and subcentral U signs are evident.
Major Boundaries, Structures, and Signs
The posterior extent is defined by the parieto-occipital fissure and imaginary line from this fissure to the pre-occipital notch (which is hard to specifically identify). The intra-parietal sulcus becomes two parallel lines that surround the posterior extent of the parieto-occipital fissure (intraparietal sulcus goalpost sign) and continues as the intra-occipital sulcus, which ends as a T-bone termination. Note that the occipital lobe is arbitrarily divided into a superior portion superior to the calcarine sulcus (green) and an inferior portion (light blue) for simplification.
The most specific method to learn cross-sectional anatomy is viewing actual cross-sectional images.
Axial Image, Near Vertex
Key anatomy of the parietal lobe, axial view near vertex. The postcentral gyrus and superior parietal lobule are seen. The pars marginalis has upturned edges.
Axial Image, Near Vertex
Key sign of the parietal lobe, axial view near vertex. Note that the postcentral sulcus is at the AP level of the pars marginalis of the cingulate sulcus, terminates lateral to the pars marginalis (in distinction to the central sulcus), and ends in a bifid configuration (bifid postcentral sulcus sign). These features are common, but variable.
Axial Image, Superior Level
The postcentral gyrus, superior parietal lobule, and inferior parietal lobule are seen. The pars marginalis edges have flattened out. The postcentral gyrus is not as thick as the precentral gyrus. The termination of the central sulcus remains medial to that of the postcentral sulcus on the right.
Axial Image, Superior Level
Key sign of the parietal lobe axial view superior level. The intraparietal sulcus can extend as far anteriorly as the postcentral sulcus (intra-parietal sulcus intersection sign) but not further (does not violate the postcentral gyrus). It may only extend this far anteriorly superficially (such as on a single axial slice).
Intra-Parietal Sulcus Baseball Seam Sign
Key sign of the parietal lobe, mid axial level. The intra-parietal sulcus has a medial convex configuration at this level, leading to a baseball seam appearance. This sulcus is discontinuous and the sulcus can be a little tricky to identify, but there are no other major sulci in this region. Note that the intra-parietal sulcus does not extend anteriorly to the postcentral sulcus in all locations. More inferiorly, the intra-parietal sulcus continues as a parasagittal goalpost configuration (see occipital lobe section).
Midline Sagittal Image
Note the relationship of the precuneus with the posterior cingulate gyrus-both of which are typically involved early on with certain dementia, such as Alzheimer dementia.
Midline Sagittal Signs of the Paracentral Lobule and Precuneus
Key signs of the parietal lobe, midline sagittal view. The Y-shaped paracentral lobule is defined by the paracentral sulcus and the pars marginalis of the cingulate sulcus. The precuneus is defined by the parallel pars marginalis and the parieto-occipital fissure (quadrilateral precuneus sign).
Anterior Shift Sign
The parieto-occipital fissure anterior shift sign. On the sagittal images above, note that the parieto-occipital fissure is obliquely oriented, parallel to the pars marginalis of the cingulate sulcus. Like the pars marginalis of the cingulate sulcus, the location of the fissure on axial images shifts anteriorly as you scroll inferiorly.
Anterior Shift Sign
The parieto-occipital fissure is located at a relative anterior position relative to the prior image. This anterior shifting sulcus sign in this location is a reliable method to identify the parieto-occipital fissure, and hence the medial posterior border of the parietal lobe, on axial images.
Lesion Localization
Clinical Correlate. Where is this lesion? This lesion is located along the posterior bank of the pars marginalis of the cingulate sulcus, placing it within the precuneus region. This bubbly T2 hyperintense lesion did not enhance, most suggestive of a DNET.
Summary of Key Clinical Correlates of the Parietal lobe
The primary somatosensory cortex lines the posterior bank of the central sulcus within the postcentral gyrus. The functional distribution is in a homunculus pattern similar to the somatomotor cortex.
There is a large region of multimodal association cortex posterior to the somatosensory region with contributions to numerous functions including attention, reading, writing, math.
Hence, lesions may result in impairment of these functions depending on specific location/side. For example left hemi-neglect can result from a lesion on the right (attention).
The medial extent of this heteromodal functional cortex is the precuneus, a key component of the default mode network (brain region more active when daydreaming/not engaging in active thought and involved early with certain forms of dementia.
The large area of heteromodal cortex can be thought of as a mirror image of the large region of hetermodal cortex of the prefrontal cortex within a cerebral hemisphere; in fact these two areas are tightly connected with white matter tracts.
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