Neurology and Neuroanatomy: High-Yield Concepts

Step 1 neurology rewards localization logic over rote recall. If you know where each tract runs, whether it has crossed yet, and which neurotransmitter system fails in which disease, most vignettes become a map-reading exercise: match the deficit to the level, side, and pathway, then name the lesion.

Core Idea

• Every sensory and motor pathway either has crossed or has not crossed at the level of the lesion — the pattern of ipsilateral versus contralateral deficits tells you exactly where the damage is.
• Upper motor neuron (UMN) and lower motor neuron (LMN) signs are opposites, and their combination points to a specific level of the neuraxis.
• Named diseases are neurotransmitter stories: Parkinson's is falling dopamine, Alzheimer's is falling acetylcholine, and knowing the deficient system predicts both symptoms and treatment logic.

The Three Major Spinal Cord Tracts

• Dorsal columns carry fine touch, vibration, and proprioception. Fibers ascend ipsilaterally and cross in the medulla. A cord lesion causes ipsilateral loss below the level.
• Spinothalamic tract carries pain and temperature. Fibers cross within 1–2 levels of entering the cord, so a cord lesion causes contralateral loss beginning a couple of levels below the lesion.
• Corticospinal (lateral) tract carries voluntary motor output. It crossed in the medulla (pyramidal decussation), so a cord lesion causes ipsilateral UMN weakness below the level.

This anatomy generates classic patterns. Brown-Séquard (cord hemisection) gives ipsilateral motor and dorsal-column loss with contralateral pain/temperature loss. Anterior spinal artery infarct spares dorsal columns but knocks out corticospinal and spinothalamic bilaterally. Syringomyelia interrupts crossing spinothalamic fibers, giving a "cape" of bilateral pain/temperature loss.

Upper vs. Lower Motor Neuron Signs

• UMN lesion (cortex, internal capsule, brainstem, or cord tract): spasticity, hyperreflexia, upgoing Babinski, no atrophy (disuse only).
• LMN lesion (anterior horn, nerve root, peripheral nerve): flaccidity, hyporeflexia, fasciculations, and marked atrophy.
• Amyotrophic lateral sclerosis (ALS) is the board favorite because it produces both UMN and LMN signs with no sensory loss.

Brainstem: The Rule of 4

A quick organizing scheme: 4 midline structures start with M (Motor corticospinal, Medial lemniscus, Medial longitudinal fasciculus, Motor cranial nuclei) and 4 lateral structures start with S (Spinothalamic, Spinocerebellar, Sensory of CN V, Sympathetic). The motor cranial nerve nuclei divide evenly by 12: nuclei of nerves that divide into 12 (3, 4, 6, 12) sit medially; the rest (mainly 5, 7, 9, 10) sit laterally. A medial brainstem stroke gives contralateral body weakness plus an ipsilateral cranial-nerve motor deficit; a lateral stroke gives crossed sensory findings (ipsilateral face, contralateral body). Lateral medullary (Wallenberg) syndrome is the archetype.

Cerebellum and Basal Ganglia

• Cerebellum coordinates movement; lesions cause ipsilateral ataxia, dysmetria, and intention tremor. The lateral hemispheres affect the limbs; the vermis affects the trunk/gait.
• Basal ganglia modulate movement through dopamine. Parkinson's (loss of substantia nigra dopamine) is hypokinetic: resting tremor, rigidity, bradykinesia. Huntington's (loss of GABAergic striatal neurons) and hemiballismus (subthalamic nucleus) are hyperkinetic.

Neurotransmitters in Disease

• Dopamine: decreased in Parkinson's (nigrostriatal); increased activity implicated in schizophrenia and Huntington's.
• Acetylcholine: decreased in Alzheimer's (basal nucleus of Meynert) and at the neuromuscular junction in myasthenia gravis.
• GABA: decreased in Huntington's; the main inhibitory transmitter.
• Serotonin and norepinephrine: decreased in depression and anxiety states.

Stroke Syndromes by Vascular Territory

• Anterior cerebral artery (ACA): contralateral leg > arm weakness/sensory loss (medial homunculus).
• Middle cerebral artery (MCA): contralateral face and arm > leg, plus aphasia (dominant) or neglect (non-dominant).
• Posterior cerebral artery (PCA): contralateral homonymous hemianopia with macular sparing.

High-Yield Exam Patterns

• Ipsilateral vs. contralateral is the single best clue — decide whether the affected tract had crossed at the lesion level.
• A "crossed" deficit (one side of the face, the other side of the body) localizes to the brainstem, not the cord or cortex.
• UMN + LMN signs with intact sensation = ALS.
• Leg-dominant weakness points to ACA; face/arm-dominant points to MCA.
• Match the movement disorder to the transmitter: hypokinetic/rigid = low dopamine (Parkinson's); memory loss = low acetylcholine (Alzheimer's).
• Cerebellar signs are ipsilateral — a right-sided intention tremor means a right cerebellar lesion.

Common Traps to Avoid

• Forgetting that the spinothalamic deficit appears contralateral and a level or two below the cord lesion, not at the exact level.
• Assuming all weakness is UMN — fasciculations and atrophy mean an LMN process.
• Confusing cerebellar (ipsilateral, coordination) with basal ganglia (tremor/rigidity) signs.
• Calling any tremor "Parkinsonian" — an intention tremor is cerebellar, a resting tremor is basal ganglia.
• Mixing up ACA (leg) and MCA (face/arm) homuncular territories.