Arterial Supply to the CNS

The brain, like any organ of the body, requires constant oxygenation and nourishment. It has a particularly high oxygen demand – at rest it represents one fifth of the body’s total oxygen consumption. It is also very sensitive to oxygen deprivation, with ischemic cell death resulting within minutes.

In this article, we shall look at the arterial structures supplying the brain.


Overview of the Arterial Supply

Fig 1.0  - Arteriogram of the arterial supply to the CNS.

Fig 1.0 – Arteriogram of the arterial supply to the CNS.

There are two paired arteries which are responsible for the blood supply to the brain; the vertebral arteries, and the internal carotid arteries. These arteries arise in the neck, and ascend to the cranium.

Within the cranial vault, the terminal branches of these arteries form an anastomic circle, called the Circle of Willis. From this circle, branches arise which supply the majority of the cerebrum.

Other parts of the CNS, such as the pons and spinal cord, are supplied by smaller branches from the vertebral arteries.

We shall now look at these individual components in more detail.


Internal Carotid Arteries

The internal carotid arteries (ICA) originate at the bifurcation of the left and right common carotid arteries, at the level of the fourth cervical vertebrae (C4).

They move superiorly within the carotid sheath, and enter the brain via the carotid canal of the temporal bone. They do not supply any branches to the face or neck.

Once in the cranial cavity, the internal carotids pass anteriorly through the cavernous sinus. Distal to the cavernous sinus, each ICA gives rise to:

  • Opthalmic artery – Supplies the structures of the orbit
  • Posterior communicating artery – Acts as an anastamotic ‘connecting vessel’ in the Circle of Willis (see ‘Circle of Willis’ below).
  • Anterior cerebral artery – Supplies part of the cerebellum.

The internal carotids then continue as the middle cerebral arterywhich supplies the the lateral portions of the cerebrum.

Fig 1.1 - Branches of the internal carotid artery.

Fig 1.1 – Branches of the internal carotid artery.


Vertebral Arteries

Fig 1.2 - The right vertebral artery. Superiorly, it converges with the left vertebral artery to form the basilar artery

Fig 1.2 – The right vertebral artery. Superiorly, it converges with the left vertebral artery to form the basilar artery

The right and left vertebral arteries arise from the subclavian arteries, medial to the anterior scalene muscle. They then ascend up the posterior side of the neck, through holes in the transverse processes of the cervical vertebrae, known as foramen transversarium.

The vertebral arteries enter the cranial cavity via the foramen magnum. Within the cranial vault, some branches are given off:

  • Meningeal branch - supplies the falx cerebelli, a sheet of dura mater.
  • Anterior and posterior spinal arteries – supplies the spinal cord, spanning its entire length.
  • Posterior inferior cerebellar artery – supplies the cerebellum.

After this, the two vertebral arteries converge to form the basilar artery. Several branches from the basilar artery originate here, and go onto supply the cerebellum and pons. The basilar artery terminates by bifurcating into the posterior cerebral arteries.


Arterial Circle of Willis

The terminal branches of the vertebral and internal carotid arteries all anastamose to form a circular blood vessel, called the Circle of Willis.

There are three main (paired) constituents of the Circle of Willis:

  • Anterior cerebral arteries: These are terminal branches of the internal carotids.
  • Internal carotid arteries: Present immediately proximal to the origin of the middle cerebral arteries.
  • Posterior cerebral arteries: These are terminal branches of the vertebral arteries.

To complete the circle, two ‘connecting vessels’ are also present:

  • Anterior communicating artery: This artery connects the two anterior cerebral arteries.
  • Posterior communicating artery: A branch of the internal carotid, this artery connects the ICA to the posterior cerebral artery.
Fig 1.3 - Schematic of the blood supply to the brain. Circle of Willis

Fig 1.3 – Schematic of the blood supply to the brain. Circle of Willis


Regional Blood Supply to the Cerebrum

There are three cerebral arteries; anterior, middle and inferior. They each supply a different portion of the cerebrum. Clinically, it is important to be aware of the regional blood supply to the cerebrum – in the event of vascular disruption, a clinician must know what parts of the brain are likely to be affected.

The anterior cerebral arteries supply the anteromedial portion of the cerebrum. The middle cerebral arteries are situated laterally, supplying the majority of the lateral part of the brain. The posterior cerebral arteries supply both the medial and lateral parts of the posterior cerebrum.

Fig 1.4 - Overview of the blood supply to the cerebrum

Fig 1.4 – Overview of the blood supply to the cerebrum

Clinical Relevance: Disorders of Arterial Supply

Stroke

The brain is particularly sensitive to oxygen starvation. A stroke is a acute development of a neurological deficit, due to a disturbance in the blood supply of the brain.

There are four main causes of a cerebrovascular accident:

  • Thrombosis – Obstruction of a blood vessel by a locally forming clot.
  • Embolism – Obstruction of a blood vessel by an emboli formed elsewhere.
  • Hypoperfusion – Underperfusion of the brain, due to systemically low blood pressure (e.g shock).
  • Haemorrhage –  An accumulation of blood within the crainial cavity.

Out of these four, the most common cause is embolism. In many patients, an athlerosclerotic emboli will arise from the vessels of the neck.

Intracerebral Aneurysms

An aneurysm is a dilation of an artery, which is greater than 50% of the normal diameter. They most likely to occur to occur in the vessels contributing to the Circle of Willis. They are particularly dangerous – producing no symptoms until they rupture.

Once the artery wall has ruptured, it is a medical emergency, and the patient is likely to die unless treated swiftly. Treatment of an intracerebral aneurysm is surgical.

 

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