Brain – Arterial Supply

Introduction

Brain is one of the most metabolically active organs of the body. Although it constitutes only 2% of the total body weight, it receives  17-20%  of the total cardiac output and consumes 20% of the total O₂ used by the body. It is supplied by Four arteries:

• Two internal carotid arteries.
• Two vertebral arteries. 

Describe the origin and course of vertebral artery.

•  It arises from the 1st part of subclavian artery, ascends up in the neck.
• It passes through the foramen transversarium of all the cervical vertebrae EXCEPT seventh cervical vertebra.
• It  enters the cranial cavity via the foramen magnum.
• Ascends up on the ventral surface of medulla oblongata.
• The  arteries of both the sides unite to form basilar artery at the lower border of the pons.
• The basilar artery ascends in the midline on the ventral surface of the pons and at its upper border ends by dividing into left and right posterior cerebral arteries.

Describe origin and course of internal carotid artery.

• Internal carotid arteries arise from common carotid arteries at the level of superior border of thyroid cartilage (C4 vertebral level). It ascends in the neck without  giving any branches.
• It enters the cranial cavity via carotid canal.
• It then takes a sinuous course via the cavernous sinus, pierces the dural roof of the sinus and  ends at below the cerebral hemisphere lateral to the optic chiasma by dividing into its two terminal branches viz. middle cerebral artery and anterior cerebral artery.

Name the branches of the arteries  that supply brain.

Descibe the formation and branches of circle of Willis.

Circle of Willis is formed by anastomosis between the branches of basilar and internal carotid arteries at the base of brain  in the interpeduncular fossa. It is formed:

• Anteriorly: by the anterior communicating artery and anterior cerebral arteries.
• Posteriorly: by the two posterior cerebral arteries .
• Laterally on every side: by the posterior communicating artery connecting the internal carotid artery with the posterior cerebral artery.

What is the function of Circle of Willis?

Normally there is  little or no mixing of blood between the right  and left halves of Circle of Willis. For that reason, right half of the brain is supplied by right vertebral and right internal carotid arteries and left half of the brain is supplied by left vertebral and left internal carotid arteries. But  in case, one of the major arteries forming the circle of Willis is blocked, the circle of Willis allows collateral route for circulation.

Describe the arterial supply of cerebrum.

The cerebrum is supplied by three pairs of cerebral arteries, viz. anterior, middle and posterior. The cerebral arteries give  three types of branches, viz.

• Cortical branches: The cortical branches anastomose in the pia mater on the surface of the cerebral cortex. From this plexus branches pierce the cortex at right angle, these arteries are end arteries.The cortical branches supply the  cerebral cortex.
• Central branches: Central branches are perforating branches, which supply the deeper parts of the cerebral hemisphere such as corpus striatum, internal capsule, etc. The central arteries arise from the arterial circle of Willis and  are end arteries. They are divided into following four groups (see the diagram above):
  • Anteromedial (AM) group.
  • Anterolateral group (AL) (lateral striate arteries).
  • Posteromedial(PM) group.
  • Posterolateral(PL) group.
• Choroidal branches: The choroidal arteries form capillary plexus which is covered by pia mater (choroid plexus) and  project into the ventricles  The branches are:
  • Anterior choroidal artery.
  • Posterior choroidal artery.
• All the three surfaces of the cerebral hemisphere are supplied by the three cerebral arteries, viz. anterior, middle and posterior.
• Most of the superolateral surface is supplied by middle cerebral artery, most of the medial surface by the anterior cerebral artery and most of the inferior surface by the posterior cerebral artery.

Arterial Supply of Superolateral Surface of Cerebral Hemisphere

The superolateral surface of the cerebral hemisphere is supplied by the  three cerebral arteries.

• Middle cerebral artery: Most of  the superolateral surface is supplied by the middle cerebral artery which include most of the  primary motor and sensory functional areas and frontal eye field. In the left (dominant) hemisphere it  includes the Broca and Wernicke’s  speech areas.
• Anterior cerebral artery: A narrow strip of the cerebral cortex (about finger breadth) adjoining superomedial border up to the parieto-occipital sulcus is supplied by anterior cerebral artery. The upper parts of primary motor and sensory areas are located in this region.
• Posterior cerebral artery: A narrow strip along the lower border of temporal lobe (excluding temporal pole) and occipital lobe are supplied by posterior cerebral artery. The posterior parts of visual area is located in this area.

Arterial Supply of Medial Surface of Cerebrum.

The medial surface of the cerebral hemisphere is also supplied by all the three cerebral arteries:

• Anterior cerebral artery:Most of the medial surface is supplied by the anterior cerebral artery. It supplies the parts of motor and sensory regions (paracentral lobule) concerned with perineum, leg and foot.
• Middle cerebral artery: Temporal pole of the temporal lobe is supplied by the middle cerebral artery.
• Posterior cerebral artery:  Medial surface of the occipital lobe is supplied by the posterior cerebral artery. The area supplied contains the visual cortex.

Arterial Supply of Inferior Surface of Cerebrum.

The inferior surface of the cerebral hemisphere is also supplied by all the three cerebral arteries:

• Posterior cerebral artery: Most of the inferior surface with the exception of the temporal pole is supplied by the posterior cerebral artery.
• Middle cerebral artery: Lateral part of the orbital surface of the frontal lobe and temporal pole of the temporal lobe are supplied by the middle cerebral artery.
• Anterior cerebral artery: Medial part of the orbital surface of the frontal lobe is supplied by the anterior cerebral artery.

Applied Aspect