QS Study

Circle of Willis:

It is a free anastomosis of arteries around the interpeduncular fossa. It begins to form when the right and left internal carotid artery (ICA) enters the cranial cavity and each one divides into two main branches: the anterior cerebral artery (ACA) and middle cerebral artery (MCA).

Formation –

  • Anteriorly – by an anterior communicating artery.
  • Posteriorly – by a posterior cerebral artery.
  • On each side – Anterior cerebral, internal carotid, posterior cerebral & posterior communicating arteries.

Situation – It lies in the interpeduncular subarachnoid cistern. Its location is near to the pituitary stalk, optic chiasm, and hypothalamus.

Circle of Willis 1

Fig: Branches and area of the brain supplied by Circle of Willis

Branches and area of the brain supplied by Circle of Willis:

  1. Anterior cerebral artery; this is the smaller terminal branch of the internal carotid artery. It runs along the optic nerve and enters the longitudinal fissure of the cerebrum.
  2. Middle cerebral artery; though it is not part of the circle of Willis, it is the largest branch of the internal carotid, runs laterally in the lateral cerebral sulcus.
  3. Posterior cerebral artery; this is a small artery that originates from the internal carotid artery close to its terminal bifurcation.
  4. Anterior communicating artery; central branches helps to supply parts of the lentiform and caudate nuclei and the internal capsule.
  5. Posterior communicating artery; the posterior communicating artery runs posteriorly above the oculomotor nerve and joins the posterior cerebral artery, thus forming part of the circle of Willis.
  6. Anterior choroidal artery; choroidal artery is a small branch which originates from the internal carotid artery close to its terminal bifurcation.
  7. Posterior choroidal artery; the choroidal artery passes close to the optic tract and ends in the choroid plexus.

Anterior, Middle & posterior cerebral arteries give rise to two sets of branches:

(a) Cortical branches,

(b) Central branches – Consists of 4 main groups:

  1. Anteromedial group.
  2. Anterolateral group.
  3. Posteromedial group.
  4. Posterolateral group.

Supply –

  1. Cortical branches supply the cerebral cortex.
  2. Central branches supply the deep structure of cerebral hemisphere.

Functions: Circle of Willis is a very important anastomotic structure in the brain that provides blood supply to the fore-brain and hind-brain.

  1. It equalizes blood pressure between the two sides of the brain.
  2. It furnishes collateral circulation.
  3. It allows blood to pass blood if an artery is occluded on either side.
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