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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

Figure: Part 6: Circle of Willis (AN62.6)

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