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Cerebrum - Grey & White Matter: All 5 Questions Answered
Q1. Association fibres are all EXCEPT:
Answer: d. Forceps major
Explanation:
White matter fibres of the cerebrum are classified into 3 types:
- Association fibres - connect cortical areas within the same hemisphere
- Commissural fibres - connect the two hemispheres
- Projection fibres - connect cortex with subcortical structures (e.g., internal capsule)
Association fibres include:
- Uncinate fasciculus - connects frontal lobe with temporal pole
- Cingulum - runs beneath the cingulate gyrus, connects frontal/parietal cortex to hippocampal formation
- Superior longitudinal fasciculus - connects frontal to parietal/occipital/temporal lobes
- Inferior longitudinal fasciculus - connects temporal to occipital lobe
- Arcuate fasciculus - connects Broca's and Wernicke's areas
Forceps major is NOT an association fibre. It is the posterior radiation of the corpus callosum (a commissural fibre), formed by the splenium of the corpus callosum as it fans out into the occipital lobes to connect the two occipital cortices. Similarly, forceps minor is the anterior radiation of the corpus callosum from the genu.
Q2. Marked structure in the image connects which of the following?
Answer: d. Dentate nucleus
Explanation:
The brain image (medial/sagittal section) shows the superior cerebellar peduncle (brachium conjunctivum), which is the principal efferent pathway from the cerebellum. The arrow points to this tract emerging from the cerebellum.
The dentate nucleus is the largest and most lateral of the deep cerebellar nuclei. Its efferent fibres form the superior cerebellar peduncle, which:
- Exits the cerebellum
- Crosses the midline at the level of the inferior colliculus (decussation of Wernekinck)
- Projects to the contralateral red nucleus and ventral lateral (VL) nucleus of the thalamus
- From the thalamus, it relays to the motor cortex
So the marked structure (superior cerebellar peduncle) originates from and connects the dentate nucleus to the thalamus and red nucleus.
Note: The image likely shows the medial surface of the cerebral hemisphere with the cerebellum visible; the marked structure with the blue arrow appears to be the superior cerebellar peduncle or a related commissure connecting the dentate nucleus via the brachium conjunctivum.
Q3. Vascular lesion of posterior half of posterior limb of internal capsule is most likely to involve:
Answer: d. Lower limb muscles
Explanation:
The internal capsule posterior limb carries the corticospinal tract with a strict somatotopic arrangement:
- Anterior part of posterior limb: face and corticobulbar fibres (most anterior)
- Middle part: upper limb (arm) fibres
- Posterior part (posterior half of posterior limb): lower limb (leg) fibres
As confirmed by Neuroanatomy through Clinical Cases (Figure 6.10A): "motor fibers for the face are most anterior, and those for the arm and leg are progressively more posterior."
Therefore, a vascular lesion of the posterior half of the posterior limb would preferentially damage lower limb fibres, causing contralateral leg weakness/paralysis.
The blood supply to this region is via the anterior choroidal artery and lenticulostriate arteries.
Q4. 40-year-old with transient lesion to marked white matter fibres - clinical presentation?
Answer: c. Loss in communication of somatosensory information between occipital cortex
Explanation:
The MRI image (sagittal view) shows a lesion in the splenium of the corpus callosum (the posterior thick part visible in the midline). The splenium carries:
- Visual (occipital) commissural fibres - connecting the two occipital cortices (visual association areas)
- Posterior parietal interhemispheric fibres
- Its posterior fibres form the forceps major
A lesion of the splenium disrupts communication of visual/somatosensory information between the two occipital cortices. This can cause:
- Alexia without agraphia (disconnection of right visual cortex from left language areas)
- Loss of interhemispheric transfer of visual and somatosensory information from the occipital cortex
Option c ("Loss in communication of somatosensory information between occipital cortex") best describes this disconnection syndrome - the posterior corpus callosum/splenium transmits information between bilateral occipital (and posterior parietal) cortices.
Q5. Marked structure (in coronal section) originates from which of the following?
Answer: c. Hippocampus
Explanation:
The coronal brain section shows the fornix, the large white C-shaped bundle visible beneath the corpus callosum. The fornix is the principal efferent pathway of the hippocampal formation.
The fornix:
- Originates from the subiculum and hippocampus (CA1 and subiculum neurons)
- Its fibres travel as the alveus → fimbria → crus of fornix → body of fornix → columns of fornix
- It projects primarily to the mammillary bodies (postcommissural fornix) and to the septal nuclei (precommissural fornix), with some fibres reaching the anterior thalamic nucleus
As noted in Neuroanatomy through Clinical Cases (3rd Ed.): "The fornix connects the hippocampal formation with the mammillary bodies and septal nuclei, as well as with the anterior thalamic nuclei."
So the marked structure = fornix, which originates from the hippocampus.
Summary Table
| Q | Correct Answer | Key Concept |
|---|---|---|
| 1 | d. Forceps major | Forceps major = commissural fibre (corpus callosum), NOT association fibre |
| 2 | d. Dentate nucleus | Marked structure = superior cerebellar peduncle, originates from dentate nucleus |
| 3 | d. Lower limb muscles | Posterior part of posterior limb of IC = leg fibres (somatotopic arrangement) |
| 4 | c. Loss between occipital cortex | Splenium of corpus callosum - connects bilateral occipital cortices |
| 5 | c. Hippocampus | Fornix = efferent pathway from hippocampus to mammillary bodies |
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