Neuroanatomy and Neurophysiology
Core concepts
Candidates must command the functional geography of the cortex, white-matter tracts, cranial nerve nuclei and courses, brainstem cross-sectional anatomy, ventricular and cisternal anatomy, and the vascular territories. Know the descending and ascending pathways (corticospinal, spinothalamic, dorsal columns, corticobulbar) and where each decussates, because lesion-localization questions hinge on these crossings. Understand cerebrospinal fluid production and absorption, the blood-brain barrier, and the physiology of intracranial pressure (Monro-Kellie doctrine, cerebral perfusion pressure = MAP − ICP, autoregulation curve).
Traps
- Confusing the level of the corticospinal decussation (medulla) with the spinothalamic crossing (spinal cord, several segments above entry).
- Assuming a single cranial nerve palsy localizes to the nerve when a nuclear or fascicular lesion produces the same deficit plus a long-tract sign.
- Forgetting that the abducens nerve has the longest intracranial course and is the most vulnerable to raised ICP.
- Mixing up the anterior choroidal artery territory (posterior limb internal capsule, globus pallidus, hippocampus) with lenticulostriate territory.
Worked example / rule of thumb
For brainstem syndromes, use the "crossed" rule: a cranial nerve deficit on one side with a contralateral long-tract deficit places the lesion at that cranial nerve's nucleus or fascicle. A pupil-involving third nerve palsy with contralateral hemiparesis suggests midbrain (Weber) syndrome. A rule of thumb: the side of the cranial nerve deficit is the side of the lesion.