Vascular Supply to the Nervous System

Introduction Blood supplies brain cells with essential nutrients such as glucose and oxygen, and removes CO 2 from nerve cells. Nerve cells depend on an uninterrupted supply of oxygenated blood since they do not have any mechanism of storing glucose or oxygen.

Introduction Blood supply to the brain is through an elaborate system of arteries and veins. The brain receives its blood supply from two arterial systems – Carotid and vertebral basilar These two join at the Circle of Willis at the base of the brain.

Vascular supply to the brain Carotid system: Common carotid artery ascends on both sides of the neck. Posterior to the mandible, the common carotid artery splits into Internal and external carotid artery. External carotid artery supplies the facial muscles, forehead, oral, nasal, and orbital cavities. Internal carotid artery – Major source of blood for the brain.

Vascular supply to the brain The Internal carotid artery joins the Circle of Willis and divides into two cortical arteries – Anterior cerebral artery (ACA) Middle cerebral artery (MCA) Vertebral Basilar System: Two vertebral arteries enter the cranium through the foramen magnum and join to form one basilar artery at the level of caudal pons, before joining the Circle of Willis.

Vascular supply to the brain Before joining the Circle of Willis, small branches of the Vertebral artery supplies blood to the spinal chord, brainstem and cerebellum. Vertebral system - Three main divisions: Posterior Spinal – Dorsal medulla & dorsal spinal chord Anterior Spinal – Ventral medulla & ventral spinal chord Posterior Inferior Cerebellar (PICA) – Posterior inferior cerebellum Infarction to PICA may result in Wallenberg’s or PICA syndrome.

Wallenberg’s or PICA syndrome This syndrome is characterized by loss of pain/temperature sensation from side of the face ipsilateral (same side) to the lesion along with loss of pain/temperature from side of body contralateral (opposite side) to the side of the lesion. Other clinical symptoms and findings are ataxic motor disorder, dysarthria (slurred speech), and dysphagia. Also facial pain, vertigo, nystagmus, and diplopia (double vision).

Vascular supply to the brain Just at the junction where the two vertebral arteries join to the form the Basilar artery, emerges the anterior inferior Cerebellar arteries Serves the anterior and lateral aspects of the cerebellum An important branch of the basilar artery is the Internal Auditory Artery that supplies the cochlea and the vestibular apparatus. The basilar artery also forms the Posterior Cerebral Artery (PCA) at its junction with the Circle of Willis

Vascular supply to the brain

Circle of Willis Is wreath-shaped and is located on the ventral surface of the brain connecting the carotid and vertebral basilar system of arteries. The circle of Willis provides a safety mechanism. If one of the arteries gets blocked, the "circle" will still provide the brain with blood. Main parts include: Anterior and posterior connecting arteries Anterior, middle, and posterior cerebral arteries.

Circle of Willis The anterior communicating artery connects the two anterior cerebral arteries. The posterior communicating artery connects the internal carotid artery with the basilar artery. Anterior Cerebral Artery (ACA): Supplies the medial surfaces of the frontal and parietal lobes. Interruption of blood circulation in the ACA results in paralysis of the legs and feet (because it serves the medial aspects of the motor cortex) and prefrontal lobe symptoms (such as disorders of thinking, reasoning, abstraction and planning)

Cerebral Arteries

ACA, PCA & MCA

Cerebral Arteries Middle Cerebral Artery (MCA) – Is a large artery and is a direct extension of the internal carotid artery. Supplies the lateral surfaces of frontal, parietal and temporal lobe. Thus serving sites important for speech-language production and comprehension and a large part of sensori-motor areas. Also supplies some parts of the basal ganglia and diencephalon.

Cerebral Arteries Blockage of the MCA results in contralateral hemiplegia (motor disorder) and sensory impairments, involuntary movements (due to involvement of basal ganglia), aphasia, constructional apraxia, temporospatial deficits, and reading/writing deficits. Posterior Cerebral Artery (PCA) – Supplies the anterior and inferior temporal lobe, and inferior and medial occipital lobe. Occlusion of the PCA will lead to homonymous hemianopia. Loss of half of the field of view on the same side in both eyes.

Constructional Apraxia Constructional apraxia is caused by lesions of the right hemisphere, particularly the right parietal lobe. The primary deficit in constructional apraxia appears to involve the ability to perceive and imagine geometrical relations. Because of this deficit, a person cannot draw a picture, say, of a cube, because he or she cannot imagine what the lines and angles of a cube look like, not because of difficulty controlling the movements of his or her arm and hand. Besides being unable to draw accurately, a person with constructional apraxia invariably has trouble with other tasks involving spatial perception, such as following a map.

Blood Flow The brain is metabolically very active yet there is no way for the brain to effectively store oxygen and glucose. The brain is 2% of total body weight yet accounts for 20% of cardiac output and 20% of oxygen and metabolized glucose consumption. Without oxygen (cerebral hypoxia) damaging effects can happen- 10 secs of brain ischemia, we lose conciousness, Within 20 secs electrical activity ceases, & after a few minutes irreversible brain damage begins. Brain damage can also happen if brain goes without glucose for minutes.

Effects of Blood Supply damage Normal blood flow is 50ml/100g of CNS. Reduction to about 20ml/100g causes neurons to stop generating electrical signals. Reduction to about 10ml/100g results in necrosis in brain tissue called an infarct and eventually a stroke.

Blood-Brain Barrier (BBB) Is an important clinical concept. Refers to the semi permeable barrier of the blood capillaries within the CNS which restricts movement of specific substances such as infectious microorganisms from the blood tissue to the brain tissue. Unlike that of the rest of the body. Is the result of the continuous interior endothelial lining along with outer covering of astrocytes leaving no pores.

Fig. 2-12, p. 37

Blood-Brain Barrier (BBB) The BBB has several important functions: Protects the brain from "foreign substances" in the blood that may injure the brain. Protects the brain from hormones and neurotransmitters in the rest of the body. Maintains a constant environment for the brain. Drawback – Although it prevents harmful micro-organisms out of brain, it also keeps many helpful antibiotics out, making treatment of cerebral infections difficult.

Blood-Brain Barrier (BBB) Some conditions can disrupt the BBB Hypertension (high blood pressure) Development - The BBB is not fully formed at birth Microwaves & Radiation - exposure to microwaves can open the BBB. Infection: Some clinical conditions such as tumors or meningitis can open the BBB. Trauma, Ischemia, Inflammation, Pressure: injury to the brain can open the BBB.

Types of Strokes Ischemic stroke - (those caused by vascular insufficiencies) are most commonly caused by a thrombus (a blood clot formed within the vessel) an embolus (a bit of foreign matter such as a plaque that is carried along in the bloodstream) from somewhere else Stenosis (constriction or narrowing of an artery in the head or neck)

Types of Strokes Coronal section of a brain suffered from an acute MCA stroke

Other Types of Strokes Transient Ischemic attack- is like an ischemic attack but only last a few minutes to a few hours and usually has a complete recovery. Aneurysms-balloon-like swellings of the arterial walls. Can cause damage by compressing brain structures and by rupturing the subarachnoid space.

Other Types of Strokes Hemorrhage-results from the rupture of the small perforating arteries (lenticulostriate arteries). Greatly increased through hypertension.

Label the following in the figure (Vascular Supply to the CNS) below: Internal Carotid Artery Anterior Cerebral Artery Middle Cerebral Artery Posterior Cerebral Artery Anterior Communicating Artery Posterior Communicating Artery Basilar Artery Vertebral Artery (Arteries) Circle of Willis Anterior Spinal Artery Posterior Spinal Artery Posterior Inferior Cerebellar Artery