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AN62.1-6 | Cranial nerve nuclei & Cerebral hemispheres — Part 2
Part 4: Basal Ganglia & Limbic Lobe (AN62.4)
Basal Ganglia Components:
• Striatum = Caudate nucleus + Putamen (connected anteriorly)
• Lenticular nucleus = Putamen + Globus pallidus (external GPe + internal GPi)
• Corpus striatum = Striatum + Lenticular nucleus
• Basal ganglia (functional group) = Corpus striatum + Subthalamic nucleus + Substantia nigra
Direct and Indirect Pathways:
Direct pathway (facilitatory — promotes movement):
Cortex → Striatum → GPi (inhibited) → Thalamus disinhibited → Motor cortex activated → MOVEMENT
Indirect pathway (inhibitory — suppresses movement):
Cortex → Striatum → GPe (inhibited) → Subthalamic nucleus (disinhibited) → GPi (activated) → Thalamus inhibited → Motor cortex suppressed → NO MOVEMENT
Balance between direct and indirect = normal smooth movement
Pathophysiology of Movement Disorders:
| Disorder | Pathology | Dominant Pathway | Clinical |
|---|---|---|---|
| Parkinson's disease | Loss of SN dopamine → reduces direct pathway, enhances indirect | Indirect dominant | Bradykinesia, rigidity, resting tremor |
| Huntington's disease | Loss of striatal neurons (indirect pathway first) | Direct dominant | Chorea (choreiform movements) |
| Hemiballismus | Damage to subthalamic nucleus (Patient C) | Indirect pathway loses STN inhibition | Violent flinging movements of contralateral limb |
| Athetosis | Basal ganglia lesion (often perinatal) | — | Slow writhing movements |
Limbic Lobe:
• Components: Cingulate gyrus, Parahippocampal gyrus, Hippocampus + dentate gyrus, Amygdala, Fornix → mammillary bodies → thalamus (anterior) → cingulate gyrus (Papez circuit)
• Hippocampus — memory consolidation (HM patient landmark); bilateral hippocampal damage → anterograde amnesia
• Amygdala — fear, emotion, aggression; lesion → Klüver-Bucy syndrome (placidity, hypersexuality, oral exploration)
• Fornix — main output of hippocampus → mammillary bodies → via mammillothalamic tract → anterior thalamus → cingulate (Papez circuit)
• TLE (temporal lobe epilepsy) = most common focal epilepsy in India; hippocampal sclerosis; can present with déjà vu, automatisms, fear
Part 5: Thalamus, Hypothalamus & Diencephalon (AN62.5)
Thalamus (Dorsal Thalamus):
• Egg-shaped, bilateral, flanking the 3rd ventricle
• "Gateway to consciousness" — all sensory modalities (except smell) relay here before cortex
• Key nuclei:
| Nucleus | Input | Output | Function |
|---|---|---|---|
| VPL (ventroposterolateral) | Medial lemniscus, spinothalamic (body) | Somatosensory cortex (areas 1,2,3) | Body sensation |
| VPM (ventroposteromedial) | Trigeminal lemniscus (face) | Somatosensory cortex | Face sensation |
| VL (ventrolateral) | Cerebellum (dentate), basal ganglia | Motor cortex (area 4,6) | Motor coordination |
| LGN (lateral geniculate) | Optic tract (visual) | Primary visual cortex (area 17) | Vision |
| MGN (medial geniculate) | Lateral lemniscus (auditory) | Primary auditory cortex | Hearing |
| Anterior | Mammillothalamic tract | Cingulate gyrus | Limbic (Papez circuit) |
| CM (centromedian) | Basal ganglia, reticular formation | Striatum, cortex | Consciousness, arousal |
Hypothalamus:
• Floor and lower walls of 3rd ventricle; above pituitary stalk
• Controls: pituitary, autonomic NS, temperature, hunger/satiety, water balance (ADH), circadian rhythm, sleep-wake
• Key nuclei:
- Supraoptic (SON) and Paraventricular (PVN) → synthesise ADH (vasopressin) and oxytocin → released from posterior pituitary
- Anterior → parasympathetic, heat dissipation; Posterior → sympathetic, heat conservation
- Lateral → hunger centre (destroyed → starvation); Ventromedial → satiety centre (destroyed → hyperphagia)
Epithalamus: Pineal gland (melatonin, circadian rhythm) + habenular nuclei
Metathalamus: LGN + MGN (sensory relay for vision and hearing)
Subthalamus: Subthalamic nucleus (STN) — part of basal ganglia indirect pathway; damage → hemiballismus (Patient C in hook)
Part 6: Circle of Willis (AN62.6)
Formation:
• Formed by anastomosis of the anterior and posterior circulations at the base of the brain in the interpeduncular cistern
Vessels forming the circle (anterior → posterior, each side):
1. Anterior cerebral artery (ACA) — from ICA; supplies medial surface of hemisphere (motor/sensory for leg)
2. Anterior communicating artery (AComm) — connects left and right ACA; most common site of berry aneurysm
3. Middle cerebral artery (MCA) — largest branch of ICA; lateral surface (arm, face, Broca, Wernicke); most common stroke territory in India
4. Posterior communicating artery (PComm) — connects ICA to PCA; 2nd most common aneurysm site; compresses CN III (oculomotor)
5. Posterior cerebral artery (PCA) — from basilar artery; occipital lobe (visual cortex), thalamus, medial temporal
6. Basilar artery — from union of two vertebral arteries; gives anterior inferior cerebellar (AICA), posterior inferior cerebellar (PICA via vertebral), superior cerebellar (SCA)
Common aneurysm sites (berry aneurysms = congenital, at arterial bifurcations):
| Site | % |
|---|---|
| Anterior communicating artery | 30–35% |
| Posterior communicating artery | 25–30% |
| MCA bifurcation | 20% |
| Basilar tip | 5% |
| Others | <10% |
Circle of Willis Strokes:
• ACA territory: Leg weakness + sensory loss (contralateral) + frontal lobe signs; abulia, urinary incontinence (paracentral lobule)
• MCA territory: Arm + face weakness + sensory loss + aphasia (dominant hemisphere); contralateral homonymous hemianopia
• PCA territory: Contralateral homonymous hemianopia (occipital cortex) + thalamic pain syndrome; memory impairment (temporal);
• Lacunar infarcts — small perforating arteries (lenticulostriate from MCA, thalamoperforators from PCA) → pure motor or sensory strokes; most common in Indian hypertensives