Basic Neuroscience and Neuroanatomy: Localization as a Reasoning Skill
Build every answer from four data points
Most neuroanatomy items are localization puzzles dressed as trivia. For any vignette, extract: side of the lesion, level of the neuraxis, upper versus lower motor neuron pattern, and which cranial nerve nucleus or long tract is involved. Commit to a level before naming a disease.
Cortex and tracts. Know the motor homunculus (leg in the interhemispheric fissure, face lateral), the middle cerebral artery as the classic source of dense hemiparesis with aphasia or neglect, and the internal capsule as the convergence point where small lesions produce large deficits. Corticospinal fibers decussate at the medullary pyramid, so weakness is contralateral below that point. Dorsal column fibers cross in the medulla; spinothalamic fibers cross within one or two segments of entry. That dissociation explains Brown-Sequard syndrome (ipsilateral weakness and vibration loss, contralateral pain loss) and anterior cord syndrome (preserved vibration and proprioception with bilateral pain loss and weakness).
Brainstem. Learn each level as a stack of nuclei plus long tracts. Medial medullary injury produces contralateral hemiparesis and contralateral vibration loss with ipsilateral tongue deviation. Lateral medullary injury produces crossed pain and temperature loss, ataxia, Horner syndrome, hoarseness, and dysphagia with preserved strength.
Nerve and muscle. Reflex asymmetry, fasciculations, and atrophy place the lesion at the anterior horn or peripheral nerve; spasticity, hyperreflexia, and a Babinski sign place it central.
Trap: mixed upper and lower motor neuron findings in the same limb usually indicate an anterior horn cell process such as ALS, not two separate lesions.
Rule of thumb: decide level first, side second, etiology last. Never reverse that order.