SN2
SHORT NOTE
5M
★ High-yield
Describe the circle of Willis with its formation and significance. Add a note on berry aneurysm.
Definition
The circle of Willis (circulus arteriosus) is an arterial anastomosis at the base of the brain, in the interpeduncular fossa, joining the internal carotid and vertebrobasilar systems.
Formation
| Position | Vessels |
|---|---|
| Anteriorly | One anterior communicating artery joining the two anterior cerebral arteries |
| Laterally | Two internal carotid arteries and two posterior communicating arteries |
| Posteriorly | Two posterior cerebral arteries, from the bifurcation of the basilar artery |
- Nine arteries in all — three unpaired contributions and three paired
- Central (perforating) branches arise from the circle to supply the internal capsule, basal ganglia, thalamus and hypothalamus — all end arteries
Function and its Limits
- It equalises blood flow and provides a collateral route if one vessel is narrowed slowly
- A gradual internal carotid occlusion may cause no infarct at all
- But it is anatomically complete and functionally efficient in fewer than half of people — hypoplastic communicating arteries are common
- It offers little protection against sudden occlusion, since collaterals need time to open
Berry Aneurysms
| Site | Frequency |
|---|---|
| Anterior communicating artery | About 30–35% — the commonest |
| Posterior communicating / internal carotid junction | About 30% |
| Middle cerebral bifurcation | About 20% |
| Basilar tip and posterior circulation | About 10% |
Applied Aspects
- Subarachnoid haemorrhage — rupture of a berry aneurysm; sudden "worst headache of my life", neck stiffness, photophobia, often loss of consciousness. CT first; if negative, lumbar puncture after 12 hours for xanthochromia
- Associations — polycystic kidney disease, coarctation of the aorta, Ehlers–Danlos and Marfan syndromes, and a family history
- Vasospasm is the major cause of delayed deterioration after subarachnoid haemorrhage; treated with nimodipine
- Treatment — endovascular coiling or surgical clipping, to prevent the high mortality of a rebleed
- The circle explains why carotid endarterectomy is tolerated, and why cross-clamping is possible with monitoring
All questions in Neuroanatomy
- Q1Describe the external features and internal structure of the spinal cord. Describe its ascending and descending tracts and add a note on its lesions. ★
- Q2Describe the functional areas of the cerebral cortex and the blood supply of the cerebrum. Add a note on aphasia. ★
- Q3Describe the cerebellum under divisions, peduncles, connections and functions. Add a note on the signs of cerebellar disease. ★
- Q4Describe the parts of the brainstem and the arrangement of the cranial nerve nuclei. Add a note on the brainstem syndromes. ★
- Q5Describe the meninges and the ventricular system. Explain the formation, circulation and absorption of cerebrospinal fluid and add a note on hydrocephalus. ★
- SN1Describe the internal capsule with its parts, contents and blood supply. Add a note on capsular stroke. ★
- SN2Describe the circle of Willis with its formation and significance. Add a note on berry aneurysm. ★
- SN3Describe the basal ganglia with their connections and functions. Add a note on their disorders. ★
- SN4Describe the thalamus with its nuclei and connections. Add a note on the thalamic syndrome. ★
- SN5Describe the visual pathway and the effects of lesions at different levels. Add a note on the light reflex. ★
- SN6Describe the hypothalamus with its nuclei, connections and functions. ★
- SN7Compare the sympathetic and parasympathetic nervous systems. Add a note on Horner syndrome.