Parietal lobe

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Cortex

General

• The cerebral hemispheres have their greatest transverse diameter across the parietal lobes.

• The parietal lobe borders only two cerebral surfaces: the lateral and medial surfaces.

Boundaries

• Anteriorly (Lateral Surface):
• Bounded by the central sulcus.

• Superiorly:
• Limited by the interhemispheric fissure.

• Inferiorly (Lateral Surface):
• Bounded by the posterior end of the sylvian fissure and the extended sylvian line (a line extending backward along the long axis of the sylvian fissure).

• Posteriorly (Lateral Surface):
• Limited by the upper half of the parietotemporal line, which runs from the impression of the upper end of the parieto-occipital sulcus on the lateral surface to the preoccipital notch.

• Medial Boundary (Posterior):
• Separated from the occipital lobe by the parieto-occipital sulcus.

Surfaces

Lateral Surface Anatomy (Convexity)

• Composed of
• Postcentral Gyrus (Anterior Part):
• It is the anterior convolution of the parietal lobe.
• It is situated behind and parallel to the central sulcus.
• It is separated from the large posterior parietal area by the postcentral sulcus.
• The postcentral sulcus is frequently broken into several discontinuous parts by gyral bridges.
• It is located lateral to deep structures, including the anterior edge of the atrium and the body of the ventricle.
• The postcentral gyrus forms part of the upper lip of the opercular cleft of the sylvian fissure.
• It often meets Heschl’s gyrus across the sylvian fissure.
• Posterior Part of the Lateral Surface:
• This large area, situated behind the postcentral sulcus
• Subdivided by the horizontal intraparietal sulcus into two lobules.
• Superior Parietal Lobule:
• Extends from the intraparietal sulcus to the superior margin of the hemisphere.
• Inferior Parietal Lobule:
• This is the larger of the two posterior lobules.
• It contains two significant gyri:
• Supramarginal Gyrus: Forms the anterior part of the inferior parietal lobule.
• It arches over the upturned end of the posterior ramus of the sylvian fissure.
• It forms the most posterior opercular lips of the sylvian fissure.
• It is located superficial to the atrium of the lateral ventricle.
• The part above the sylvian fissure is continuous in front with the lower end of the postcentral sulcus.
• The supramarginal gyrus serves as an intermediate station in the superior longitudinal fasciculus, which is an anatomic substrate for high-order multisensory associative systems like language in the dominant hemisphere.
• Angular Gyrus: Forms the posterior part of the inferior parietal lobule.
• It arches over the upturned posterior end of the superior temporal sulcus.
• It blends posteriorly into the anterior part of the occipital lobe.

• Two main sulci:
• Postcentral sulcus
• Intraparietal sulcus
• Oriented anteroposteriorly
• Approximately 2 to 3 cm lateral to the superior border of the hemisphere.
• It is commonly, but regularly, interrupted.
• The depth of the intraparietal sulcus is directed toward the roof of the atrium and the occipital horn.

Medial Surface Anatomy

• The medial parietal surface is situated between
• Line extending from the upper end of the central sulcus to the corpus callosum anteriorly,
• Parieto-occipital sulcus posteriorly.

• Component Structures
• Precuneus
• It is a quadrilateral area located between the ascending ramus of the cingulate sulcus (anteriorly) and the parieto-occipital sulcus (posteriorly).
• It is separated inferiorly from the cingulate gyrus by the subparietal sulcus.
• The precuneus is the medial extension of the superior parietal lobule.
• Posterior part of the cingulate gyrus
• Paracentral Lobule
• Represents the extension of the pre- and postcentral gyri that wraps around the extension of the central sulcus onto the medial surface.
• The posterior part of the paracentral lobule is considered part of the parietal lobe and is a medial extension of the postcentral gyrus.
• The paracentral lobule is located above the posterior half of the corpus callosum.
• It is separated posteriorly from the precuneus by the marginal or ascending ramus of the cingulate sulcus.

Functional White Matter Tracts

• Superior Longitudinal Fasciculus (SLF):
• This fasciculus surrounds the insula and connects the frontal, temporal, and parietal lobes.
• The frontal fibers of the SLF often terminate in the inferior parietal lobule region, forming the frontoparietal or horizontal segment.
• A temporoparietal or vertical segment of the SLF travels between the posterior temporal gyri and the inferior parietal lobule.
• In the dominant hemisphere, the inferior parietal lobule and its connecting white matter (like the frontoparietal segment of the SLF) play a role in the dorsal phonological pathway (motor language) and the integration of multiple modalities for language function.
• In the non-dominant hemisphere, the SLF (specifically the frontoparietal segment) is associated with spatial awareness.

Vascular Supply

• Middle Cerebral Artery (MCA):
• Supplies the majority of the lateral surface of the hemisphere, extending around the lateral convexity.
• Specific MCA branches supply the
• Anterior parietal area (including the postcentral gyrus),
• Posterior parietal area (including the superior and inferior parietal lobules and supramarginal gyrus)
• Angular area (angular gyrus and superior parts of the lateral occipital gyri).
• The MCA territory is narrowest in the superior parietal area.

• Anterior Cerebral Artery (ACA):
• Supplies the cortex and adjacent white matter of the medial surface.
• The ACA supplies the superior parts of the precentral, central, and postcentral gyri on the lateral surface.
• The cortical branch supplying the parietal area is divided into the superior and inferior parietal arteries, which supply the ACA distribution posterior to the paracentral lobule.
• The superior parietal artery supplies the superior portion of the precuneus.
• The inferior parietal artery supplies the posteroinferior part of the precuneus and adjacent portions of the cuneus (it is the least frequent cortical branch of the ACA).

 

Examination

Dominant

Gerstmann's syndrome

• https://academic.oup.com/brain/article/133/2/320/287068

• Lesion of
• Angular gyrus
• Supramarginal gyrus

• Theory
• Gerstmann postulated that knowledge of both fingers and right/left orientation are supported by knowledge of the body (a shared network for body schema).
• Critchley proposed that counting and calculating are first learned on the fingers. Thus, finger knowledge and calculation ability may be supported by a shared neuronal network

• Finger Agnosia:
• The examiner should cover the patient eyes and ask him to stretch his fingers, then the examiner should touch the patient fingers lightly and the patient should respond by identifying the finger as soon as it was touched, this examination should be done firstly with opened eyes to eliminate the misunderstanding or lack of alertness, this test may be affected if the patient has tactile insensitivity, a minimum 20% false recognition by the patient is needed to diagnose the patient with finger agnosia.

• Right-left disorientation:
• Physician should give the patient a card with written instruction such as “ place left hand to right ear” and the patient should be asked to read it loudly, if the patient read it incorrectly, the physician should contact with the patient verbally, then the patient should be asked to do similar instruction.

• Agraphia:
• Ask patient to write a simple sentence
• Without Alexia
• Read out what examiner has written
• VS Alexia without agraphia (loss in ability to communicated through writing
• Splenium damage + occipital lobe primary visual lobe damage

• Dyscalculia:
• A patient should be given a card which has this simple equation written on it “85-27” and he should be asked to write it and calculate it, then the patient should be asked to do a multiplying equation in his head without writing it or seeing it.

Non dominant

Unilateral spatial neglect

Draw a clock

• Put a cross in a middle of the line

Crossed response test

• Ask patient to move the limb opposite the one that is touched (motor neglect).

Dressing apraxia

• Ask the patient to take off a jumper or other item of clothing Turn it in-

• Side out and ask the patient to put it back on

Ideamotor apraxia

• Paper cutting: Ask patient to pantomine cutting a paper but not using his finger as if it is scissor

Constructional apraxia

• Necker's cube
• Ask patient to copy a 3D drawing

Both sides

Sensory inattention:

• Ask patient to hold out arms with eyes shut. Touch either one or both sides of the corresponding part of the body and ask where he has been touched. Extinction occurs when the patient says that only one side is being touched when in fact it is both.

• Inattention to one side

• Extinction: When both side are touch one side is not detected but if each side is touched separately they are both sensed

Asteroagnosia

• With the patient's eyes closed, place a familiar object in their hand and ask them to identify it.

• Coins of different denominations may be used.

Dysgraphaesthesia

• Ask the patient to identify the number or letter that you trace on their palm. It should be agreed which way is up before starting.

2 point discrimination

• Using callipers or a bent paper clip, ask the patient if they can feel one or two points.

• Normal should be able to feel difference of
• On the fingertips: 2-4 mm apart,
• On the palm: 8-15 mm.

Qudrantanopia

• Examine for homonymous inferior quadrantanopia.