Neuroanatomy, NEET PG & INI-CET Anatomy MCQs

Explanation

The lenticulostriate arteries are small perforating branches of the middle cerebral artery that supply much of the internal capsule and basal ganglia. They arise at right angles from a large vessel, which exposes them to high pressure and makes them the classic site of hypertensive haemorrhage and lacunar infarction. Because corticospinal fibres are densely packed there, a tiny lesion can cause a dense contralateral hemiplegia.

Explanation

About 85 per cent of corticospinal fibres cross in the pyramidal decussation at the lower medulla and descend as the lateral corticospinal tract. The uncrossed remainder forms the anterior corticospinal tract and decussates segmentally. Because the crossing is above the cord, a cortical or capsular lesion causes weakness on the opposite side of the body while a cord lesion causes it on the same side.

Explanation

The dorsal columns carry fine touch, vibration and proprioception without synapsing until the medulla. Fibres from below T6 travel in the medially placed fasciculus gracilis and relay in the nucleus gracilis, while those from above T6 use the laterally placed fasciculus cuneatus. Second order fibres then cross as the internal arcuate fibres to form the medial lemniscus.

Explanation

The oculomotor nerve runs beside the posterior communicating artery, so an aneurysm there compresses it. The parasympathetic pupilloconstrictor fibres travel on the outside of the nerve, which is why compression blows the pupil early while ischaemic third nerve palsies from diabetes typically spare it. Unopposed lateral rectus and superior oblique action leaves the eye down and out.