13.3 Circulation and the Central Nervous System

The central nervous system relies on a complex network of blood vessels to function. Internal carotid and vertebral arteries supply the brain, while spinal arteries nourish the spinal cord. The Circle of Willis provides crucial collateral circulation, ensuring a backup blood supply.

Cerebrospinal fluid, produced in the choroid plexuses, circulates through the brain's ventricles and spinal cord's central canal. It cushions the brain, removes waste, and maintains homeostasis. Disruptions in blood flow or CSF dynamics can lead to serious conditions like stroke.

Circulatory Supply to the Central Nervous System

Blood vessels of central nervous system

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• Arterial supply to the brain consists of internal carotid arteries and vertebral arteries
  • Internal carotid arteries give rise to anterior cerebral artery, middle cerebral artery, and anterior communicating artery supplying the cerebral hemispheres
  • Vertebral arteries join to form the basilar artery, which branches into posterior cerebral arteries and posterior communicating arteries supplying the brainstem and cerebellum
  • Circle of Willis is an anastomosis of cerebral arteries (anterior cerebral, middle cerebral, posterior cerebral, and communicating arteries) providing collateral circulation to the brain
• Venous drainage of the brain occurs through dural venous sinuses and internal jugular veins
  • Dural venous sinuses (superior sagittal, inferior sagittal, straight, transverse, and sigmoid sinuses) collect blood from the brain and drain into the internal jugular veins
  • Internal jugular veins carry blood from the brain and head back to the heart
• Arterial supply to the spinal cord is provided by the anterior spinal artery and posterior spinal arteries
  • Anterior spinal artery supplies the anterior two-thirds of the spinal cord
  • Posterior spinal arteries supply the posterior one-third of the spinal cord
• Venous drainage of the spinal cord occurs through the anterior spinal vein and posterior spinal veins, which drain into the internal vertebral venous plexus

Regulation of cerebral blood flow

• Cerebral autoregulation maintains constant cerebral blood flow despite changes in systemic blood pressure
• Neurovascular coupling ensures that active brain regions receive increased blood flow to meet metabolic demands
• The blood-brain barrier regulates the passage of substances between the bloodstream and the central nervous system, protecting the brain from harmful substances

Ventricular System and Cerebrospinal Fluid

Components of brain ventricular system

• Lateral ventricles are the largest ventricles located in the cerebral hemispheres
  • Interventricular foramen (foramen of Monro) connects each lateral ventricle to the third ventricle allowing cerebrospinal fluid (CSF) flow
• Third ventricle is a narrow, midline cavity located in the diencephalon (thalamus and hypothalamus)
  • Cerebral aqueduct (aqueduct of Sylvius) connects the third ventricle to the fourth ventricle enabling CSF circulation
• Fourth ventricle is a diamond-shaped cavity located in the hindbrain (pons and medulla oblongata)
  • Three openings: two lateral apertures (foramina of Luschka) and a median aperture (foramen of Magendie) allow CSF to exit the ventricular system and enter the subarachnoid space
• Central canal is a narrow, cylindrical space that extends through the center of the spinal cord, continuous with the fourth ventricle

Cerebrospinal fluid dynamics

• Production of CSF occurs primarily in the choroid plexuses, highly vascularized structures located in the ventricles
  • Choroid plexuses secrete CSF by actively transporting ions and water from the blood into the ventricular space
  • Ependymal cells lining the ventricles also contribute to CSF production, although to a lesser extent than the choroid plexuses
• Circulation of CSF follows a unidirectional flow pattern:
  1. CSF is produced in the lateral ventricles
  2. Flows through the interventricular foramina into the third ventricle
  3. Passes through the cerebral aqueduct into the fourth ventricle
  4. Exits the ventricular system through the lateral and median apertures into the subarachnoid space surrounding the brain and spinal cord
  5. Reabsorbed into the venous system via arachnoid villi and granulations, which are projections of the arachnoid mater that protrude into the dural venous sinuses
• Functions of CSF include:
  • Mechanical protection by cushioning and supporting the brain within the skull, reducing the effective weight of the brain and preventing injury
  • Chemical protection by removing metabolic waste products and toxins from the central nervous system
  • Maintaining homeostasis by regulating intracranial pressure and providing buoyancy to the brain, allowing it to float in the CSF
• The glymphatic system facilitates the clearance of metabolic waste products from the brain through CSF-interstitial fluid exchange

Circulatory Disruptions and Stroke

Circulatory disruptions vs stroke occurrence

• Ischemic stroke occurs when blood vessels supplying the brain become blocked, leading to oxygen and nutrient deprivation in the affected brain tissue
  • Thrombotic stroke happens when a blood clot (thrombus) forms within a cerebral artery, often due to atherosclerotic plaque rupture or endothelial damage
  • Embolic stroke occurs when a blood clot (embolus) forms elsewhere in the body (e.g., heart or carotid arteries) and travels through the bloodstream to lodge in a cerebral artery
• Hemorrhagic stroke occurs when blood vessels in the brain rupture, causing bleeding and increased intracranial pressure that compresses brain tissue
  • Intracerebral hemorrhage is bleeding within the brain tissue itself, often due to chronic hypertension or cerebral amyloid angiopathy
  • Subarachnoid hemorrhage is bleeding in the subarachnoid space surrounding the brain, frequently caused by a ruptured cerebral aneurysm
• Risk factors for stroke include:
  • Hypertension, which can damage blood vessels and increase the risk of both ischemic and hemorrhagic strokes
  • Atherosclerosis, the buildup of fatty plaques in the arteries, narrowing the lumen and increasing the risk of thrombosis and embolism
  • Atrial fibrillation, an irregular heart rhythm that can cause blood to pool and clot in the heart, increasing the risk of embolic stroke
  • Diabetes mellitus, which can damage blood vessels and increase the risk of atherosclerosis and thrombosis
  • Smoking, which can damage blood vessels, increase blood pressure, and promote atherosclerosis
  • Age and family history, as the risk of stroke increases with age and a family history of stroke may indicate a genetic predisposition