Arteries of the brain

Internal carotid arteries

The internal carotid artery is one of two branches of the common carotid artery. It is responsible for supplying a large portion of the anterior and middle parts of the brain. 

A new classification system divides the internal carotid artery into four parts; cervical in the neck, petrous in the base of the skull, cavernous within the cavernous sinus and intracranial above the cavernous sinus. 

Previously, the Cincinnati Classification (Bouthillier et. al., 1996) classified the internal carotid artery into seven segments; cervical (C1), petrous (C2), lacerum (C3), cavernous (C4), clinoid (C5), ophthalmic or supraclinoid (C6), communicating or terminal (C7). It is undoubtedly easier to remember the new classification. However, here is a quick mnemonic to remember the C2-C7 intracranial segments of the internal carotid artery according to the Cincinnati classification - Please Let Children Consume Our Candy. 

When comparing the Cincinnati classification with the new system, the following differences can be observed:

• The part of the artery that was considered the lacerum segment is now referred to as a continuation of the petrous segment.
• The intracranial part involves the clinoid, ophthalmic and communicating portions (i.e. C5, C6, and C7)

The petrous part (C2) gives off the caroticotympanic and Vidian arteries. The cavernous segment (C4) gives numerous branches to the walls of the cavernous sinus and the surrounding nerves and dura mater. Of significance, the inferior hypophyseal artery also originates from this segment.

The ophthalmic segment (C6) gives of the ophthalmic artery and the superior hypophyseal artery. The communicating segment (C7) gives off the anterior cerebral (ACA), middle cerebral (MCA) and the anterior choroidal (AChA) arteries. The AChA supplies mesencephalic, diencephalic, and telencephalic derivatives.

Anterior cerebral artery

The anterior cerebral artery (ACA) is a much smaller branch of the internal carotid artery (when compared to the middle cerebral artery). It begins at the terminal portion of the internal carotid artery (after the ophthalmic branch is given off) on the medial part of the Sylvian fissure. It travels in an anteromedial course, superior to the optic nerve (CN II) towards the longitudinal cerebral fissure. Here it anastomoses with the contralateral counterpart via the short anterior communicating artery (AComm). The paired arteries then travel through the longitudinal cerebral fissure along the genu of the corpus callosum. 

The anterior cerebral artery also gives off central and cortical branches. Central branches arise from the AComm to perfuse the optic chiasma, lamina terminalis, hypothalamus, para-olfactory areas, cingulate gyrus, and anterior columns of the fornix. 

The cortical branches are named for the regions they supply. They are responsible for the somatosensory and motor cortices of the lower limbs. 

• Frontal arteries supply the paracentral lobule, medial frontal and cingulate gyri, and the corpus callosum. 
• Parietal branches perfuse the precuneus 
• Orbital branches supply the frontal lobe (olfactory cortex, medial orbital gyrus, and gyrus rectus)

Anterior communicating artery

The anterior communicating artery (AComm) is a short, slender vessel that runs horizontally between the anterior cerebral arteries. The vessel crosses the ventral aspect of the median longitudinal fissure and is located anterior to the optic chiasm and posteromedial to the olfactory tracts. This vessel forms the anterior bridge between the left and right halves of the anterior circuit. It also completes the anterior part of the anastomotic ring known as the circle of Willis.

Middle cerebral artery

The middle cerebral artery (MCA) is the largest terminal branch of the internal carotid artery. It travels through the Sylvian (lateral) fissure before coursing in a posterosuperior direction on the island of Reil (insula). It subsequently divides to supply the lateral cortical surfaces along with the insula. 

The vessel gives numerous tributaries to both central and cortical regions of the brain. The central branches are relatively small and include the lenticulostriate arteries that pass through the anterior perforated substance to supply the lentiform nucleus and the posterior limb of the internal capsule. 

The cortical branches include the frontal, orbital, parietal, and temporal branches:

• The frontal arteries perfuse the inferior frontal, middle, and precentral gyri. 
• The lateral orbital parts of the frontal lobe, as well as the frontal gyrus, are supplied by the orbital branches. 
• The inferior parietal lobe, the inferior part of the superior parietal lobe, and the postcentral gyrus receive blood from the parietal branch. 
• Several temporal arteries then go on to perfuse the lateral aspect of the temporal lobe.

Why don't you check what you've learned so far and solidify that knowledge with the following quiz:

Vertebral arteries

The vertebral arteries gain access to the cranial vault via the foramen magnum anterolateral to the brainstem. Concerning the branches, each vertebral artery:

• Gives off a posterior inferior cerebellar artery
• Contributes to the formation of the anterior spinal artery via tributaries that converge in the midline anterior to the medulla oblongata
• Contributes meningeal branches near the foramen magnum that supplies the falx cerebelli and the surrounding bone
• May give off the posterior spinal artery; although this vessel usually arises from the posterior inferior cerebellar artery
• Gives off medullary arteries that perfuse the medulla oblongata

The vertebral arteries unite in the midline at the pontomedullary junction to form the basilar artery. 

Basilar artery

The basilar artery is an important vessel found in the pontine cistern. It is posterior to the clivus and anterior to the pons, as it ascends in the basilar groove. Its branches are responsible for supplying the pons, cerebellum, internal ear, and other nearby structures. There are three major branches of the basilar artery:

• Anterior inferior cerebellar
• Superior cerebellar
• Internal auditory (Labyrinthine)

There are also smaller pontine and posteromedial (paramedian) arteries that arise from the lateral surface and distal bifurcation of the artery, respectively. The basilar artery ends by dividing into two posterior cerebral arteries. These vessels unite with the posterior communicating arteries to complete the circle of Willis, posteriorly.

Posterior cerebral artery

The posterior cerebral arteries (PCA) are terminal branches arising from the bifurcation of the basilar artery. The division takes place behind the dorsum sellae. It is separated from the superior cerebellar artery by the oculomotor nerve (CN III). The artery continues in a course lateral to the midbrain (adjacent to the trochlear nerve, CN IV). It gives off the posterior communicating artery, which completes the circle of Willis. The vessel then continues to course around the cerebral peduncles toward the tentorial aspect of the cerebrum. Here, it supplies the occipital and temporal lobes.

The branches of the posterior cerebral artery bring oxygenated blood to the following areas:

• Anterior thalamus and subthalamus
• Lateral wall of the third ventricle and inferior horn of the lateral ventricle
• Choroid plexus of third and lateral ventricles
• Globus pallidus
• Lateral and medial geniculate bodies

Posterior communicating artery

The posterior communicating artery (PComm) is a long, slender vessel originating from the posterior cerebral artery. It is much longer than its anterior counterpart - the anterior communicating artery. The vessel is medial to the uncus of the temporal lobe and lateral to the mammillary bodies of the hypothalamus. The distal part of the vessel may overlap the proximal part of the optic tract.

The posterior communicating artery completes the circle of Willis posteriorly. Additionally, it gives tributaries to the optic tract, cerebral peduncles, internal capsule, and thalamus.