The Neuroscience on the Web Series:
SPPA 636 Neuropathologies of Language and Cognition

CSU, Chico, Patrick McCaffrey, Ph.D.

Chapter 4. Medical Aspects: Blood Supply to the Brain

Blood Supply to the Brain

The consequences of stroke depend upon which areas of the brain are affected by ischemia, hemorrhage, hematoma or aneurysm. Therefore, it is important to understand the organization of the cerebral blood supply system.

Two arterial systems supply the brain with blood: the internal carotid and basilar. The internal carotids and basilar arteries are connected via the circle of Willis, which allows blood to pass from one system to another in the event of blockage.

The Internal Carotid Arteries

There are two internal carotid arteries. They ascend from the aortic arch toward the brain along each side of the neck. They pass behind the ear into the temporal lobe and enter the subarachnoid space. They then go to the circle of Willis where each bifurcates to form two main branches: the anterior cerebral artery and the middle cerebral artery.

The course of the internal carotid arteries and their branches is tortuous. Because of their many twists and turns these arteries are all subject to blockages caused by cholesterol build-up. For this reason, the internal carotids, and the middle and anterior cerebral arteries are more vulnerable to ischemic rather than hemorrhagic stroke. According to FitzGerald, 1996, the contralateral internal carotid may use the circle of Willis/anterior communicating artery to supply blood to both pairs of anterior and middle cerebral arteries. Complete interruption of blood flow in the anterior cerebral artery is not very likely because the opposite anterior cerebral can use the anterior communicating artery to make up for the deficiency. Even the facial artery (external carotid) can shunt blood to the anterior and middle cerebral arteries via the ophthalmic artery.

The Anterior Cerebral Arteries

The anterior cerebral arteries supply blood to the medial cortex, including the medial aspects of the motor and sensory strips. Due to the upside-down representation of the homunculus in both of these area, a blockage in an anterior cerebral artery could cause paralysis or sensory deficits which affect the opposite side of the body from the hips on down. For example, a patient who has suffered such a blockage might have a paralyzed leg or be incontinent of bowel and/or bladder. Since the parts of the motor and sensory strips connected to the arm receive some blood from the anterior cerebral artery, the arm may also be somewhat affected by this kind of blockage. Finally, apraxia of gait may also occur if an anterior cerebral artery fails to supply an adequate amount of blood to the medial part of the motor strip.

The anterior cerebral arteries also supply blood to the anterior aspects of the frontal lobes. As these areas are involved in higher level cognition such as reasoning and judgment, a condition called cerebral dementia may result from anterior cerebral artery blockages. Confused language, or a language indicative of cognitive impairment, may also occur.

The Middle Cerebral Arteries

These large arteries have tree-like branches that bring blood to most of the lateral cortex
of each cerebral hemisphere. This means that the middle cerebral arteries supply blood to the cortical areas involved in speech, language and swallowing. The left middle cerebral artery provides Broca's area, Wernicke's area, Heschl's gyrus, and the angular gyrus with blood. Also, the "head" and "neck" areas of the motor and sensory strips in both hemispheres receive their blood supply from the middle cerebral arteries. Damage to these cortical areas on either side of the brain can impair motor speech and swallowing functions.
In addition, the middle cerebral arteries provide most of the blood supply to the corpus striatum. The striatas, which are arterial branches of the middle cerebral arteries, are known as the arteries of stroke as they are the main source of blood for the internal capsule. A rupture of the lenticulo-striate artery results in bleeding usually in the region of the internal capsule (Steadman, 1997). When one of these arteries is damaged, the bottleneck of fibers within the internal capsule , including the pyramidal tract, can be affected, causing many disabilities. The striatas have relatively thin walls and pressure within them is high. For this reason, they are more vulnerable to hemorrhages than to blockages.

Because the middle cerebral arteries supply blood to so many areas of the brain, blockages in this part of the circulatory system can cause numerous types of disorders.

If the left middle cerebral artery is blocked, aphasia and apraxia may occur.

Blockages of the right middle cerebral artery can cause left side neglect, which is an impairment in the ability to recognize and respond to stimuli on the left side of the body, prosopagnosia or the inability to recognize faces, and various cognitive problems, including agnosia. All of these are part of the right hemisphere syndrome.

Blockages of either middle cerebral artery can cause contralateral hemiplegia or hemiparesis as well as contralateral hyposthesia. Paralysis and sensory deficits will affect the head and neck most. Because the "hand" and "arm" areas are superior to the "head" and "neck" areas of the pre and postcentral gyri, they receive some blood supply from the anterior cerebral arteries. For this reason, they may not suffer as much after a middle cerebral artery blockage.

Damage to the internal capsule caused by blockage or hemorrhage of the striata (branch of middle cerebral) can cause upper motor neuron dysarthria and dysphagia.

In addition, damage to either middle cerebral artery can cause hemianopsia or the loss of sight in one half of the visual field. This blindness affects the contralateral aspect of both visual fields. For example, a blockage in the right middle cerebral artery will cause left hemianopsia or blindness in the left visual field of both eyes.

The Vertebro-Basilar System

Both of the vertebral arteries ascend through the spinal column and enter the brain through the magnum foramen. Once in the brain, they continue to ascend, traveling beside the brain stem. At the lower border of the pons the two vertebral arteries join together to form the basilar artery or vertebro-basilar artery.

The vertebral arteries and the basilar are straight arteries and therefore not as subject to blockages due to the build up of cholesterol as are the internal carotids.

The posterior inferior cerebellar not only supply the cerebellum but take blood to the lateral medulla. Anterior and posterior spinal arteries supply the ventral and dorsal medulla, respectively (FitzGerald 1996). The three arteries are branches of the vertebral.

The side of the pons and the cerebellum receive blood from the anterior inferior cerebellar artery and the superior cerebellar artery. These arteries are branches of the basilar. The anterior inferior cerebellar artery also has a branch, the labyrinthine artery, that supplies the inner ear. The basilar also gives off about twelve pontine arteries that supply the medial pons (FitzGerald, 1996).

At the superior border of the pons, the basilar artery divides to form the two posterior cerebral arteries.

Before the basilar artery divides, several other arteries arise from it. These include the anterior, inferior, and posterior cerebellar arteries as well as pontine branches. So, the cerebellum and pons are mainly supplied by branches of the basilar.

The Posterior Cerebral Arteries

At the superior border of the pons, the basilar artery divides to form the two posterior cerebral arteries. These arteries supply blood to the part of the brain that lies in the posterior fossa of the skull, including the medial aspects of the occipital lobes, the inferior portions of the temporal lobes, the brainstem, and the cerebellum. They also deliver blood to the thalamus and some other subcortical structures. A vascular lesion may result in Thalamic Aphasia or Thalamic Syndrome where the posterior thalamic nucleus is disconnected from the sensory cortex. This rare syndrome causes complete contralateral sensory loss alternating with bouts of severe pain (FitzGerald, 1997).

As the vertebral and basilar are somewhat wide arteries and their course is relatively straight, they are not as subject to blockages caused by the build-up of cholesterol as the internal carotids. This means that, overall, cerebrovascular accidents effect the basilar system less frequently than the internal carotid system. However, the basilar and posterior cerebral arteries may hemorrhage due to shearing injuries caused by accidents or pressure due to edema of the brain.

Since they provide blood to the occipital lobes, damage to these arteries can cause a variety of visual problems, including cortical blindness and alexia, which is an inability to read and visual agnosia, or the inability to recognize stimuli presented visually. Because these arteries supply blood to the cerebellum and to the brain stem, blockages or hemorrhaging there can cause either ataxic (cerebellar)
or flaccid (lower motor neuron) dysarthria..

It is important to note that the effects of ischemic stroke are almost always contralateral and therefore unilateral. On the other hand, hemorrhagic strokes may damage both hemispheres, depending upon the size and location of the resulting hematoma.

One exception to this pattern is alternating hemiplegia. This is a paralysis that can be caused by ischemic stroke and affects different structures on each side of the body. It results from a brain stem lesion that damages cranial nerve nuclei and one side of the pyramidal tract in the same area. Lesions of cranial nerve nuclei will cause a paralysis of the structures served by that nerve on the same side of the body as the injury. Because the pyramidal tract provides only contralateral innervation to the spinal nerves, damage to these fibers will cause a paralysis of different structures on the other side of the body. For example, a lesion that affects the right nucleus of the trigeminal nerve and the right half of the pyramidal tract will cause paralysis of the right side of the jaw and of the skeletal muscles on the left side of the body.

The Circle of Willis

The Circle of Willis or the Circulus Arteriosus is the main arterial anastomatic trunk of the brain. According to Bhatnagar and Andy (1995), anastomosis occurs when vessels bring blood to one spot and then redistribute it. The two internal carotid arteries and the basilar arteries feed into the Circle of Willis. The blood is then redistributed by the anterior, middle and posterior cerebral arteries. Thus, the ends of the internal carotids and the basilar artery enter the Circle while the three cerebral arteries arise and exit from it. (Neither the internal carotids nor the basilar arteries can be considered "cerebral" arteries since they do not begin within the brain).

In the Circle of Willis the two internal carotids are joined together by the anterior communicating artery while the posterior communicating artery links the internal carotid system with the basilar artery. These connections make collateral circulation, which Love and Webb (1992, p. 40) define as "the flow of blood through an alternate route," possible. This is a safety mechanism, allowing brain areas to continue receiving adequate blood supply even when there is a blockage somewhere in an arterial system. When all arteries are functioning normally, their blood supplies will not mix where they meet in the Circle because the pressure of their streams will be equal.

As long as the Circle of Willis can maintain blood pressure at fifty percent of its normal level, no infarction or death of tissue will occur in an area where a blockage exists. If collateral circulation is good, sometimes a blockage will have no permanent effects. Sometimes, an adjustment time is required before collateral circulation can reach a level that supports normal functioning; the communicating arteries will enlarge as blood flow through them increases. In such cases, a transient ischemic attack may occur, meaning that parts of the brain are temporarily deprived of oxygen.

Collateral circulation is not always sufficient to prevent a stroke. Some people lack one or more of the communicating arteries in the Circle of Willis. This means that, if a blockage develops, blood cannot be redistributed from another arterial system and stroke will occur. FitzGerald (1996), writes that 25% of people get blood to their posterior cerebral arteries from the internal carotids, rather than from the vertebro-basilar system. Also, even when all the communicating arteries exist, the influence of several extraneural factors including abnormal blood pressure, cerebrovascular resistance, and arteriosclerosis may impede collateral circulation. Low blood pressure, which can result from surgical shock, may prevent blood from traveling through the smaller diameter arteries. Pressure or hypertension can cause stenosis of the arteries by pushing plaque up against their walls. Cerebrovascular resistance makes it more difficult for blood to flow from one area to another. It can be caused by arterial spasm, a high level of tri-glycerides in the blood which increases its viscosity, or by elevated levels of cerebral spinal fluid. Arteriosclerosis or the formation of "plaque" consisting of muscle cells and blood cells, as well as fats on arterial walls also makes it more difficult for blood to flow through communicating arteries.

Watershed Areas

Parts of the brain located at the ends of the distribution areas of the vascular systems are called the watershed areas. According to Bhatnager and Andy (1995, p.345), a watershed area is a "tertiary brain area located peripheral to primary distribution areas for anterior, posterior, and middle cerebral arteries; it derives blood via the branches of all three cortical arteries." Because they are at the extreme ends of arterial distribution they are particularly vulnerable to ischemia and infarction in those who have circulation problems. Lesions in the watershed areas may cause transcortical sensory aphasia.

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Other courses in the Neuroscience on the Web series:
CMSD 320 Neuroanatomy | CMSD 642 Neuropathologies of Swallowing and Speech

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