Arteries of the Brain
Arteries supply the brain with oxygenated blood. They include those that have close connections with one another to form networks, for example the circle of Willis. This arterial circle provides an alternative route of blood supply to an area of the brain in the event of an occlusion to a major artery supplying that area, as well as others that only give-off branches to supply parts of the brain.
Arteries of the brain include the branches of the left and right carotid arteries and two vertebral arteries which ascend on the anterolateral aspect of the medulla. They give off branches and take part in formation of the circle of Willis.
The internal carotid arteries arise in the neck from the common carotid arteries, ascend vertically to the base of the skull, and enter the cranial cavity through the carotid canals in the temporal bones. Within the cranial cavity, the internal carotid arteries terminate by dividing into the anterior and middle cerebral arteries. Because the internal carotid and those terminal branches mainly supply blood to the anterior portions of the brain and course to the base of the brain from the anterior direction, they are clinically regarded as the anterior circulation of the brain.
Similarly, the two vertebral arteries and their branches anastomose with those forming the anterior circulation of the brain by coursing through the posterior aspects, thus constituting the posterior circulation of the brain. The two vertebral arteries begin in the root of the neck as the first branches of the first part of the subclavian arteries, and course upwards.
They ascend through the transverse foramina of the atlas (C1 vertebral) and perforate the dura and arachnoid matas of the meninges. They pass through the foramen magnum and course further on the anterolateral aspect of the medulla. They unite at the caudal border of the pons to form the basilar artery, thus forming the vertebrobasilar arterial system. The basilar artery is so-named because of its close relationship to the base of the skull.
Arteries of the brain supply blood to the brainstem, cerebellum, and cerebrum, and those arteries will be discussed in relation to these brain regions after describing the circle of Willis.
Circle of Willis
The circle of Willis, also called Circulus arteriosus or the polygon of Willis, is formed at the base of the brain around the optic chiasm, infundibulum of the pituitary stalk and the hypothalamus, as a ring of interconnecting arteries. The interconnecting arteries are from the anterior and posterior circulation of the brain.
Two vertebral arteries, originating from the subclavian artery, course upwards through the neck and ascend on the anterolateral aspect of the medulla. They unite at the lower border of the pons, forming a single artery called the basilar artery. The vertebral arteries also give-off the following branches which run somewhat downwards:
- the posterior inferior cerebellar arteries
- anterior spinal artery
- posterior spinal arteries
The basilar artery runs upwards in the midline and ventral to the pons, from the level of the lower border of the pons, giving off the:
- pontine arteries
- superior cerebellar arteries
- anterior inferior cerebellar arteries
- right and left posterior cerebral arteries (terminal branches obtained from the bifurcation at the level of the upper border of the pons or lower border of midbrain)
From the anterior circulation, the internal carotid artery, originating from the common carotid artery at the level of C4 vertebra, courses upwards within the carotid sheath, and enters the cranial cavity through the carotid canal of the temporal bone. Once in the cranial cavity, it runs anteriorly through the cavernous sinus, giving off a branch called the ophthalmic artery, and then curls back to lie in the front half of the roof of the cavernous sinus.
The artery then turns vertically upwards to the anterior perforated substance where it divides into the middle cerebral branch (middle cerebral artery) and the anterior cerebral branch (anterior cerebral artery). At the anterior perforated substance, it also gives off other branches to complete its half of the polygon of Willis. It gives off the:
- striate arteries
- anterior choroidal artery,
- anterior communicating artery (which is a very short artery connecting the right and left anterior cerebral arteries)
- posterior communicating arteries
The posterior communicating arteries anastomose with the posterior cerebral arteries from the vertebrobasilar system, thus connecting the internal carotid arteries to the vertebrobasilar system to complete the circle of Willis.
Arteries of the Cerebellum
The cerebellum is supplied by the superior cerebellar arteries, anterior inferior cerebellar arteries, and the posterior inferior cerebellar arteries.
Superior cerebellar arteries: The left and right superior cerebellar arteries are formed just before the basilar artery bifurcates into the posterior cerebral arteries. They curl around the upper margin of the pons, supplying the pons, midbrain and superior surface of the cerebellum.
Anterior inferior cerebellar arteries: They arise from the basilar artery at the lower part of the pons and pass back on the inferior surface of the cerebellar hemisphere, supplying this surface and the adjacent flocculus.
Posterior inferior cerebellar arteries: They are the largest branches of the vertebral artery. They arise ventrally from the vertebral artery, near the lower end of the olive. The arteries spiral back around the medulla below the hypoglossal rootlets and then between the rootlets of the glossopharyngeal nerve and vagus nerve. They supply the choroid plexus of the 4th ventricle, the adjacent part of the medulla and the posterior part of the cerebellar hemispheres, including the inferior cerebellar peduncle.
Arteries of the Brainstem
Arteries of this region include those arteries supplying the pons, medulla and midbrain. They include various branches of the vertebral and basilar arteries, such as:
- anterior spinal artery
- posterior spinal arteries
- posterior inferior cerebellar artery
- direct bulbar branches (of the vertebral arteries)
- paramedian, short and long circumferential branches of the basilar artery
- posterior cerebral artery
- superior cerebellar artery
- posterior communicating arteries
- anterior choroidal arteries
Arteries of this part of the brainstem include the anterior and posterior spinal arteries, posterior inferior cerebellar artery, and direct bulbar branches of the vertebral arteries.
Anterior spinal artery: This is a single artery formed by the two vertebral arteries, and runs inferiorly in the anterior median fissure of the spinal cord. It is formed by union of the anterior spinal branch from each vertebral artery. It supplies the whole spinal cord anterior to the posterior grey column, a triangular area which includes the pyramid, the medial lemniscus, and the hypoglossal nucleus of the medulla situated next to the midline.
Posterior spinal artery: This artery runs inferiorly on the posterior aspect of the spinal cord. It supplies the grey and white posterior columns of its own side (posterior side of the spinal cord), and a small area of the medulla which includes the gracile and cuneate nuclei.
The posterior inferior cerebellar artery described above also supplies the retro-olivary region. This region contains the spinothalamic tracts, the rubrospinal tract, the nucleus ambiguous, the dorsal vagal nucleus, and descending autonomic fibres, all of which are supplied by that artery.
Arteries of this part of the brainstem include the paramedian, short and long circumferential branches of the basilar artery. The branches of the basilar artery are referred to as pontine arteries.
Pontine arteries: The pontine arteries are several small branches from the basilar artery. They run in groups to supply the medial portion of the ventral part of the pons as paramedian branches; the lateral portion of the ventral part of the pons as short circumferential branches; and the dorsal part of the pons as long circumferential branches.
Arteries of this part of the brainstem include the posterior cerebral artery, superior cerebellar artery, posterior communicating arteries, and the anterior choroidal arteries, most of which are branches of the basilar artery.
Posterior communicating arteries: It connects the internal carotid artery to the posterior cerebral artery. That is, it joins the anterior half of the Willis circle (contributed primarily, by the internal carotid arteries) to the posterior half (from the vertebrobasilar system). The posterior communicating arteries send branches to the midbrain which are either paramedian or circumferential. They wind round the midbrain to supply lateral and dorsal parts. One of such branches is called the quadrigeminal artery. This artery is the main source of blood to the colliculi. The posterior communicating arteries also send branches to the internal capsule.
Posterior cerebral artery: This artery curls back around the cerebral peduncle, supplying it (cerebral peduncle) and the optic tract. It then passes back above the tentorium to supply the inferomedial surface of the temporal, occipital lobes and distribute cortical branches. Branches of this artery are also sent to the midbrain. The visual area for the opposite field of vision lies wholly within the supply of the posterior cerebral artery. The posterior cerebral artery also sends perforating branches to the thalamus. Some of these branches are grouped and named thalamo-perforating arteries and the thalamo-geniculate branches.
Anterior choroidal arteries: They supply the choroid plexus, passe below the optic tract to enter the inferior extremity of the choroid fissure. Through its branches, it also supplies the optic chiasm, optic tract, lateral geniculate body and the internal capsule. Like the posterior communicating arteries, branches of the anterior choroidal are also sent to the midbrain and those branches are either paramedian or circumferential.
Arteries of the Cerebrum
The cerebrum is chiefly supplied by the:
- internal carotid
- posterior communicating arteries
- anterior and posterior choroidal arteries
- anterior, middle and posterior cerebral arteries
Arteries of this part of the cerebrum include the cortical branches of the anterior, middle and posterior cerebral arteries. The superolateral surface is chiefly supplied by cortical branches of the middle cerebral artery. A tiny part of about one inch wide along the superomedial border, extending from the frontal lobe to the parieto-occipital sulcus is supplied by cortical branches of the anterior cerebral artery. Also, on the superolateral surface, the inferior temporal gyrus and the area belonging to the occipital lobe are supplied by cortical branches of the posterior cerebral artery.
On the medial surface, the cerebral cortex is mainly supplied by cortical branches of the anterior cerebral artery. Cortical branches of the middle cerebral artery supply only a small portion related to the temporal pole, while the cortical branches from the posterior cerebral artery supply the area belonging to the occipital lobe and the rest of the temporal lobe.
The tentorial part of the inferior surface is mainly supplied by cortical branches of the posterior cerebral artery. The medial half of the orbital surface (orbital part of inferior surface) is supplied by cortical branches of the anterior cerebral artery. The remaining parts of the inferior surface, which includes the lateral half of the orbital surface and a small portion of the tentorial surface is supplied by cortical branches of the middle cerebral artery.
Anterior cerebral artery: This branch leaves the internal carotid at the anterior perforated substance and passes forwards above the optic nerve, running in the interhemispheric fissure or medial longitudinal fissure. It supplies the whole of the medial surface of the cerebrum above the corpus callosum as far back as the parieto-occipital sulcus and the orbital part or orbital surface of the frontal lobe. The arterial supply to those surfaces is through its cortical branches. Its distribution extends over the superior border to meet the area supplied by the middle cerebral artery.
Middle cerebral artery: The middle cerebral artery is the largest and most direct branch of the internal carotid artery, and therefore, it is the most susceptible to embolism (obstruction of an artery by an embolus like air-bubble, or clot of blood). It runs deep into the lateral sulcus (lateral fissure) to supply the cortex of the insula, overlying opercula, and send terminal cortical branches to other parts of the cortex as stated above.
Interior Of The Cerebrum
Most arteries of this region have already been described above. However, the following are additional arteries supplying some of the deep structures of the cerebrum:
Anterior communicating artery: This is a very short single artery. It connects the left and right anterior cerebral arteries to each other.
Recurrent branches of the anterior cerebral artery: These arteries supply the internal capsule and the basal nuclei.
Obstruction of blood-flow to brain tissues leads to oxygen and nutrient starvation, and may ultimately result in conditions like stroke, paralysis, personality changes, dulling of sensations, aphasia, etc. depending on the degree and site of occlusion. Some of these conditions are described as follows:
Generally, an ischemic stroke is caused by an embolism in a major cerebral artery. Strokes are the most common neurologic disorders affecting most adults in the United States. An ischemic stroke denotes the sudden development of focal neurological deficits that are usually related to impaired cerebral blood flow.
They are more often disabling than fatal. This is mostly because the arterial circle of Willis serves to provide alternative route of blood supply to the region supplied by occluded arteries. Common causes of strokes include spontaneous cerebrovascular accidents, such as cerebral thrombosis, cerebral hemorrhage, cerebral embolism, and subarachnoid haemorrhage.
Anterior Cerebral Artery Thrombosis
An anterior cerebral artery thrombosis happening beyond the anterior communicating artery will lead to paralysis or weakness of muscles of the leg and foot of the opposite side (that is, complete contralateral hemiplegia and hemianaesthesia – leg, arm and face). The upper parts of the “Sensory area” corresponding to area 1,2,3 of Brodmann is also affected, leading to loss or dulling of sensations from the leg and foot of the opposite side. The sense of stereognosis is impaired, by involvement of the parietal lobe, and also, personality changes occur, by involvement of the frontal lobe.
Variations of the Cerebral Arterial Circle
The size and formation of the arterial circle of Willis varies among individuals. In about 1 in 3 persons, one posterior cerebral artery is a major branch of the internal carotid. In some individuals, the posterior communicating arteries are absent, while in some others, two anterior communicating arteries are present.
Middle Cerebral Artery Thrombosis
This causes hemiplegia and loss of sensations on the opposite half of the body, affecting the face and arms the most. Also, because the middle cerebral artery is the supply to the Broca's area, also called “Motor speech area (of Broca)” which corresponds to areas 44 and 45 of Brodmann, and to the Wernicke's area, aphasia (inability to speak) results, especially if the thrombosis is in the left hemisphere in a right handed person. Cerebral oedema may also result, causing compression of the optic radiation leading to homonymous hemianopia on the opposite side. Hearing may also be affected, but this may be compensated by the opposite hemisphere.
Transient Ischemic Attacks
This condition refers to neurologic symptoms resulting from ischemia. Most transient ischemic attacks (TIAs) last only a few minutes, but some persist for up to one hour. With stenosis (progressive narrowing) of major arteries of the brain, the TIAs tend to last longer and causes distal closure of intracranial vessels. Symptoms of TIAs include dizziness, staggering, and fainting.
Posterior Cerebral Artery Thrombosis
This condition leads mainly to visual effects, causing contralateral hemianopia (homonymous hemianopia), hemianaesthesia, and cerebral oedema. This means areas 17, 18 and 19 of Brodmann are affected, but the macular area is often spared (this is referred to as “macular sparing”).
Other clinical conditions involving arteries of the brain are:
- Complete occlusion of the anterior choroidal artery may lead to contralateral hemiplegia, hemianaesthesia and hemianopia.
- Thrombosis in the anterior spinal artery leads to medial medullary syndrome.
- Thrombosis in the posterior inferior cerebellar artery leads to lateral medullary syndrome.