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

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
    • Diagram showing the origin of the arteries supplying the brain (i.e. the ICA from the common carotid arteries and the vertebral arteries originating from the subclavian arteries).
  • 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

Circle of Willis

  • This is an anastomosis of the basilar artery and ICA to form the arteries supplying the brain
A useful, colour-coded diagram of the Circle of Willis
The location of the CoW relative to the brain. From https://commons.wikimedia.org/wiki/File:2123_Arteries_of_the_Brain.jpg.
  • 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)

 


Sources of cranial blood supply

 

  • 4 arteries input into CofW
    1. Internal carotid arteries
    2. Vertebral arteries
Internal carotid artery
Path of the internal carotid artery. (From Moore & Dalley).
  • 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

          There are six right-angle turns of the ICA. This schematic shows these as well as the parts of the carotid 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)

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

 


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
Territories supplied by the cerebral arteries.
  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)
    • Branch of basilar a. (just before it’s bifurcation in PCAs)

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