Arterial Supply of the Brain

Summary

• Overview of blood supply to the brain
• The Circle of Willis
• Source arteries of the cerebral blood supply
  • Internal carotid artery
  • Vertebral and Basilar arteries
• Arteries supplying the brain
  • Cerebral arteries
  • Cerebellar arteries

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Overview

• Brain has high oxygen demands
  • 20% of cardiac output at rest
  • Grey matter has higher oxygen requirements

• Arteries supplying the brain lie in the subarachnoid space
  • Surrounded by CSF
  • Surrounded by astrocytic processes
    • Maintain BBB by inducing tight junction formation in endothelial cells

Blood supply to brain

• Overall, the brain is supplied by two pairs of arteries:
  1. Two Internal carotid arteries  
  2. Two Vertebral arteries
  • It is important to have a detailed knowledge of both of these arteries
  • These are joined at the Circle of Willis from where they anastamose to form the arteries supplying the brain

  • 

• Vertebral arteries:
  • Supply cerebellum, brainstem, and the cerebrum underside
  • Combine to form basilar artery

• Basilar artery communicates with ICA
  • Allows collateral blood flow in case of blockage of an artery

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Circle of Willis

• This is an anastomosis of the basilar artery and ICA to form the arteries supplying the brain

• The circle is made up of cerebral and communicating arteries only
  • In the centre the circle overlies the optic chiasm
    • (therefore it lies superior to the pituitary fossa)

    • 

 

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Sources of cranial blood supply

 

• 4 arteries input into CofW
  1. Internal carotid arteries
  2. Vertebral arteries

Internal carotid artery

• Classified into 4 segments based on the path it takes:
  1. Cervical part
     • Arises from the CCA
     • No branches in the neck until it reaches the cranium
  2. Petrous part –
     • Enters through the carotid canal into the petrous part of the temporal bone
     • Gives off the caroticotympanic branches
  3. Cavernous part –
     • Emerges at the foramen lacerum into the cavernous sinus
     • Travels within the cavernous sinus (towards forehead).
     • Gives off inferior hypohyseal artery and marginal branches
  4. Cerebral part –
     • Becomes cerebral part when passing the anterior clinoid process
     • It then doubles back on itself after coming out of the cavernous sinus superiorly
     • Gives rise to several branches  before terminating as the MCA:
       1. Ophthalmic artery – immediately after emerging near the optic chiasm
       2. Superior hyophysial branch
       3. Anterior choroidal branch
       4. Posterior communicating artery
       5. Anterior cerebral artery
       6. Continues as the middle cerebral artery

          

• Remember, there are 3 key branches in the Circle of Willis : (as cerebral part)
  1. Posterior communicating a.
  2. Anterior cerebral artery
  3. Middle cerebral artery (terminal)

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Vertebral artery

• Path:
  • Paired arteries branching off the subclavian artery
  • Ascend with the vertebral column (C6-C1) in transverse foramen
  • Pass through foramen magnum to enter subarachnoid space
    • Pierce dura

• Branches – only really one branch before becoming the basilar:
  • Posterior inferior cerebellar a.

• Unite at the pontomedullary border to form basilar artery
  • Travels along anterior aspect of pons (see below)

Basilar artery

• Travels along the anterior aspect of pons

• Gives off several branches at the posterior aspect of the circle of willis
  1. Anterior inferior cerebellar artery
  2. Pontine arteries – branch straight off the basilar into the pons
  3. Labyrinthine artery (to ear)
  4. Superior cerebellar a
  5. Posterior cerebral a
     • Joins onto the posterior cerebral artery which branches from ICA

       

 

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Arteries to the Brain

Consider the cerebrum and cerebellum separately

Terminal and preterminal branches in the Circle of Willis

• The arterial brnaches in the CoW can be classified in the following way:

• Terminal branches:
  1. Cerebral arteries
     1. Anterior
     2. Middle
     3. Posterior
  2. Pontine arteries
  3. Cerebellar arteries
     • Superior cerebellar artery
     • Posterior inferior cerebellar artery
     • Anterior inferior cerebellar artery

• Preterminal branches:
  1. Posterior communicating artery
  2. Anterior communicating artery

Cerebral supply

• Supplied by 3 arteries:
  • All 3 are interconnected by the anterior and posterior communicating arteries
  • Different arteries supply different territories

1. Anterior cerebral artery
   • Branches off ICA (which then continues as the MCA)
   • Both ACAs are connected by the anterior communicating artery
   • Runs in the longitudinal fissure to supply the medial hemisphere of the brain which includes:
     1. Medial frontal lobe – Supplies lower limb sensory/motor function
     2. Superior/medial parietal lobe
2. Middle cerebral artery
   • Continuation of ICA
   • Runs in the lateral sulcus to supply the lateral hemisphere of the brain
     1. Lateral frontal lobe = Supplies sensorimotor of whole body except lower limb
     2. Parietal lobe – all of this is supplied by the MCA
     3. Temporal lobe – all of this is supplied by the MCA
   • Branch into narrow striate arteries that often cause stroke
3. Posterior cerebral artery
   • Comes from vertebral and basilar arteries as well as ICA (via post. communicating artery)
   • Supplies occipital lobe + some of the parietal lobe

 

• Remember to consider the symptoms of stroke with anterior vs middle vs posterior cerebral artery occlusion
  • Anterior will affect lower limbs only
  • Middle will affect the rest of the body
  • Posterior will affect contralateral vision etc

• The homunculus is very useful here, it describes proportionately where in the motor/sensory cortex each part of the body is represented
  • Note how the medial part of the cortex is where the lower limb is represented. Hence, infarction to this area (as in ACA infarction) causes lower limb symptoms.

Cerebellar supply

• 3 arteries

1. Posterior inferior cerebellar artery (aka posterior cerebellar artery)
   • Comes off vertebral artery
2. Anterior inferior cerebellar artery
   • Branch of basilar a.at the pontomedullary junction (i.e. near the origin of the basilar artery)
3. Superior cerebellar artery (aka anterior superior)
4. • Branch of basilar a. (just before it’s bifurcation in PCAs)