5.1 Neuroanatomy Localization & Pathways

Cortex Lobes and Homunculus Somatotopy

Clinical localization begins with cortical function. The frontal lobe contains primary motor cortex (precentral gyrus), premotor and supplementary motor areas, frontal eye fields, and prefrontal circuits for executive function, personality, and working memory. Broca’s area (typically dominant inferior frontal gyrus) supports speech production and fluency. The parietal lobe houses primary somatosensory cortex (postcentral gyrus), association cortex for spatial attention and multimodal integration, and (in the dominant hemisphere) circuits contributing to calculation, left–right orientation, and language-related praxis. The temporal lobe includes primary auditory cortex, Wernicke’s area (dominant superior temporal/posterior language zone) for comprehension, medial temporal memory circuits (hippocampus), and limbic emotion/memory interfaces. The occipital lobe is primarily visual: primary visual cortex (calcarine/V1) and surrounding association areas for form, motion, and color processing.

The homunculus maps body parts onto precentral and postcentral gyri with disproportionate representation of face, tongue, and hand. The leg and foot occupy the medial cortical surface within the interhemispheric fissure (paracentral lobule), while face and upper extremity sit more laterally on the convexity. This somatotopy explains why anterior cerebral artery lesions preferentially affect the contralateral lower extremity, whereas middle cerebral artery lesions preferentially affect face and arm and language or neglect networks.

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Long Tracts: Motor and Sensory Crossings

Corticospinal Tract

Upper motor neurons in primary motor and premotor cortex send axons through the corona radiata, internal capsule (posterior limb), cerebral peduncle, basis pontis, and medullary pyramids. The majority of fibers decussate at the pyramidal (motor) decussation at the caudal medulla to form the lateral corticospinal tract in the spinal cord, which synapses on lower motor neurons in the anterior horn. Because of this caudal medullary crossing, a lesion above the decussation produces contralateral hemiparesis (body), whereas a cord lesion below the decussation produces ipsilateral paresis at and below the level.

Upper motor neuron signs (after acute shock resolves) include weakness with spasticity, hyperreflexia, and Babinski sign. Lower motor neuron lesions produce flaccid weakness, hyporeflexia/areflexia, fasciculations, and atrophy—reflecting denervation of the muscle unit.

Dorsal Column–Medial Lemniscus vs Anterolateral (Spinothalamic) System

Dorsal columns carry fine touch, vibration, and proprioception. Primary afferents ascend ipsilaterally in fasciculus gracilis (lower body) and cuneatus (upper body), synapse in nucleus gracilis/cuneatus in the medulla, then decussate as internal arcuate fibers to form the medial lemniscus, which ascends to the contralateral thalamus (VPL) and then to parietal cortex.

Spinothalamic (anterolateral) fibers carry pain and temperature (and crude touch). Primary afferents enter the cord, synapse in the dorsal horn, and second-order neurons cross within one or two segments in the anterior white commissure, then ascend contralaterally to thalamus and cortex.

These three rules generate classic cord and brainstem patterns without memorizing every fasciculus name.

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Brainstem Syndromes: Medial vs Lateral Patterns

The brainstem is organized so that medial structures (corticospinal fibers, medial lemniscus, medial longitudinal fasciculus in places, and motor nuclei of CN III, VI, XII) cluster near the midline, whereas lateral structures include spinothalamic tract, sympathetic fibers, spinal trigeminal nucleus/tract, vestibular nuclei, and cerebellar peduncles, plus cranial nerves with lateral nuclei (V, VII, VIII, IX, X depending on level).

Medial midbrain (Weber-type pattern): ipsilateral CN III palsy (eye “down and out,” ptosis, pupil often involved if fascicles compressed) plus contralateral hemiparesis from cerebral peduncle involvement.

Medial pontine patterns: ipsilateral CN VI (and sometimes VII fascicles) with contralateral hemiparesis; may include internuclear ophthalmoplegia if MLF is hit.

Medial medullary syndrome: ipsilateral tongue weakness (CN XII) and contralateral hemiparesis plus contralateral lemniscal sensory loss (pyramid + medial lemniscus + hypoglossal).

Lateral medullary (Wallenberg) syndrome is the prototype lateral pattern: vestibular nuclei (vertigo, nystagmus), inferior cerebellar peduncle (ataxia), spinal trigeminal nucleus (ipsilateral facial pain/temp loss), spinothalamic tract (contralateral body pain/temp loss), nucleus ambiguus (dysphagia, hoarseness), and descending sympathetics (ipsilateral Horner syndrome). Notably, pure lateral medullary lesions often spare corticospinal fibers (which are medial/ventral), so dense hemiparesis is not required.

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Cranial Nerves I–XII: High-Yield Lesion Logic

• CN I (olfactory): special sensory smell; fracture of cribriform plate or frontal base trauma → anosmia; not a true brainstem cranial nerve in the same nuclear sense.
• CN II (optic): retinal ganglion axons form optic nerve → chiasm → tract; field defects localize along the visual pathway (monocular loss, bitemporal hemianopia at chiasm, homonymous defects retrochiasmal).
• CN III (oculomotor): midbrain; innervates most extraocular muscles, levator palpebrae, and parasympathetic pupil constriction (Edinger–Westphal). Compressive lesions often dilate the pupil early; ischemic microvascular lesions may spare the pupil relatively more often in teaching vignettes.
• CN IV (trochlear): only nerve to decussate and exit dorsally; superior oblique palsy → difficulty depressing the adducted eye; head tilt compensation is a classic clue.
• CN V (trigeminal): sensory face (V1–V3) and muscles of mastication; brainstem spinal tract/nucleus lesions cause ipsilateral facial sensory loss in onion-skin or division patterns.
• CN VI (abducens): long intracranial course; lateral rectus palsy → inability to abduct; false localizing sixths appear with raised ICP stretching the nerve.
• CN VII (facial): wraps around CN VI nucleus (facial colliculus); LMN lesion paralyzes entire ipsilateral face including forehead; UMN lesion relatively spares forehead (bilateral cortical innervation concept).
• CN VIII (vestibulocochlear): hearing and balance; CPA lesions (for example, vestibular schwannoma concepts) can add V and VII signs.
• CN IX/X: palate, pharynx, larynx motor/sensory and autonomic; nucleus ambiguus lesions → dysphagia, absent gag, uvula deviation away from weak side (teaching convention).
• CN XI (accessory): ipsilateral shoulder shrug/head turn weakness (trapezius/SCM).
• CN XII (hypoglossal): tongue; LMN lesion → ipsilateral atrophy and deviation toward the weak side on protrusion.

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Circle of Willis Territories

The circle of Willis anastomoses internal carotid and vertebrobasilar systems. Functional territories matter more than every communicating artery variant:

• Anterior cerebral artery (ACA): medial frontal/parietal cortex → contralateral leg-predominant weakness and sensory loss; may include frontal behavioral changes and urinary issues with bilateral or large lesions.
• Middle cerebral artery (MCA): lateral convexity → contralateral face/arm > leg weakness and sensory loss; aphasia if dominant hemisphere; hemineglect if nondominant; gaze preference toward the lesion early; homonymous field cuts with deeper/optic radiation involvement.
• Posterior cerebral artery (PCA): occipital lobe → contralateral homonymous hemianopia, often with macular sparing if occipital pole collateralized; thalamic branches may add sensory syndromes.
• Lenticulostriates / deep penetrators: supply internal capsule and basal ganglia; pure motor or sensorimotor lacunar patterns when small vessels are diseased.
• Vertebrobasilar: brainstem and cerebellum; crossed findings and cranial nerve signs dominate localization.

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Spinal Cord Tracts and Brown-Séquard

In spinal cord hemisection (Brown-Séquard), combine ipsilateral corticospinal weakness (and ipsilateral dorsal-column loss of vibration/proprioception) with contralateral pain/temperature loss beginning a level or two below the lesion because spinothalamic fibers have already crossed. Segmental LMN signs and a band of sensory change can mark the level of entry-root or horn damage. Complete transection yields bilateral loss of motor and sensory modalities below the lesion with an acute spinal shock phase (flaccid areflexia) before UMN signs emerge.

Central cord syndromes preferentially affect crossing spinothalamic fibers in the anterior commissure (cape-like pain/temp loss) and can involve medial arm/hand fibers of the corticospinal tract in the cervical enlargement—classic after hyperextension injury in stenotic canals, taught as mechanism rather than a clinical management algorithm.

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Autonomic Nervous System: Receptor Overview

Sympathetic preganglionic neurons exit thoracolumbar cord, synapse in paravertebral or prevertebral ganglia using acetylcholine on nicotinic receptors. Postganglionic sympathetics release norepinephrine onto α1, α2, β1, β2 adrenergic receptors on most effectors (vasoconstriction α1, cardiac stimulation β1, bronchodilation β2, etc.). Sweat glands are the major exception: postganglionic sympathetic fibers are cholinergic, acting on muscarinic receptors.

Parasympathetic preganglionic fibers travel with cranial nerves (III, VII, IX, X) and sacral roots, synapse near targets with ACh–nicotinic transmission, then postganglionic ACh–muscarinic (M) effectors: miosis and accommodation (CN III), salivation/lacrimation, bradycardia, enhanced gut motility/secretions, detrusor contraction, and erectile vasodilation pathways.

Adrenal medulla is a modified sympathetic ganglion releasing epinephrine/norepinephrine into blood after preganglionic ACh–nicotinic stimulation—useful for linking autonomic pharmacology to systemic catecholamine surges.

Localization Algorithm for CBSE

When a vignette offers crossed findings, think brainstem. When modalities split by laterality in the body after trauma, think cord hemisection. When face and arm exceed leg on the convexity side with language or neglect, think MCA cortex/operculum. When only the leg is weak with frontal features, think ACA/medial homunculus. Pair every cranial nerve deficit with the expected neighboring long tract. That combinatorial method is what NBME-style basic science items reward.