4. Blood Supply to the Central Nervous System Flashcards
What are the demands of the brain?
- 2% of body weight
- 10-20% of cardiac output
- 20% of body oxygen consumption
- 66% of liver glucose
Because of this, the brain is very vulnerable if its blood supply is impaired
Name the arteries supplying blood to the the brain.
- Two sources:
- Internal carotid arteries (front)
- Vertebral Arteries (back)
- This gives rise to an importnat network of cerebral arteries called the circle of Willis
- You get cerebral arteries coming off the circle
Describe the structure of the carotid artery.
- Bifurcates at the level of the adams apple
- External Carotid - supplies the face
- Internal Carotid - makes its way up into the skull to supply the cerebral hemispheres
Explain the structure of the vertebral artery
- The vertebral artery branches off the subclavian arteries and makes their way through the transverse foramina within the cervical vertebrae and through the foramen magnum into the brain
- Supplies posterior circulation
Describe the arterial supply to the brain (ventral aspect) or the circle of Willis.
On the diagram:
- The two arteries at the bottom are the vertebral arteries –> fuse at the pons
- The vertebral arteries fuse to form the basilar artery
- The basilar artery bifurcates to form the poasterior cerebral arteries
- The vertebral arteries and basilar artery start moving anteriorly before they bifurcate into posterior cerebral arteries
- The internal carotid arteries travel superiorly and then laterally to form the middle cerebral arteries that emmerge through the fissure between the frontal, parietal and temporal lobes
- The internal carotids also branch to form the anterior cerebral arteries that go up in between the two medial surface of the hemispheres and they follow the corpus callosum backwards about 2/3 of the way
- This arrangement of arteries is made into a circle by two posterior communicating arteries and one anterior communicating artery
- It is a circle so that if one side gets occluded there is still another route for blood to travel.
Explain the venous drainage system of the brain.
- There is a huge sustem of large veins that drain the cerebral hemispheres via the jugular system
- Cerebral veins
- Venous sinuses
- Dura mater
- Internal jugular vein
Describe Dural Venous Sinuses.
- Running along the top is the superior sagittal sinus which is housed between the two folds of dura –> this is the largest sinus
- This is also the point at which the CSF is drained back into the venous system
- Running along the bottom of the dural fold is the inferior sagittal sinus
- These sinuses run backwards to form a big space filled with blood called the Confluence of sinuses (where all the sinuses come together)
Define strokes. What causes them?
Rapidly developing focal disturbance of brain function of presumed vascular origin lasting more than 24 hours
- These are 85% due to infarction and 15% due to haemorrhage
- For people who have a blockage in one of their carotid arteries, they have compensatory mechanism from the other carotid arteries however posteriorly, the vertebral artery is too small to compensate for this block.
Define a Transient Ischaemic Attack (TIA).
Rapidly developing focal disturbance of brain function of presumed vascular origin that resolves completely within 24 hours
Define an infarction.
Degenerative changes that occur in tissue following occlusion of an artery.
Define Cerebral Ischaemia.
Lack of sufficient blood supply to nervous tissue resulting in permanent damage if blood flow is not restored quickly - due to hypoxia/anoxia
What are the causes of occlusions?
-
Thrombosis
- Formation of a blood clot (thrombus)
-
Embolism
- Plugging of a small vessel by material carried from larger vessel e.g. thrombi from the heart or atherosclerotic debris from the internal carotid
NOTE: the more proximal the occlusion is in the artey, the more devastating the result
Explain the epidemiology of cerebral ischaemia. What are the risk factors?
- 3rd commonest cause of death
- 100,000 deaths in the UK per annum
- 50% of survivors are permanently disabled
- 70% show an obvious neurological deficit
- Risk factors:
- Smoking
- Hypertension
- Age
- Diabetes Mellitus
- Cardiac Disease
Describe the perfusion fields of the brain.
- The anterior cerebral artery supplies the medial 2/3s of the hemisphere
- The middle cerebral artery extends laterally and emerges through the lateral fissure between the frontal and temporal lobes
- It supplies the front 2/3s of the lateral part of the hemisphere
- The posterior cerebral artery supplies the medial and lateral parts of the poterior 1/3 of the hemisphere
Explain what would occur if there is a disturbance of the anterior cerebral artery.
- Paralysis of the contralateral LEG more so than the arm
- This is because the motor homunculus shows that the part of the motor cortex that controls the lef is more medial to the part controlling the arm
- Disturbance of intellect, executive function and judgement
- abulia - absence of willpower
- Loss of appropriate social behaviour
What is the difference between the dura in the skull and the vertebral column? Describe the meninges in the skull.
- Difference between dura in the skull and in the vertebral column:
- Vertebral column has a SINGLE LAYER of dura with fatbetween the bone and the dura
- In the skull there are TWO LAYERS of dura that are mostly stuck together
- TWO layers of dura in the skull:
- Periosteal
- Meningeal
- These two layers peel apart in some places and are filled with venous blood
- A fold of dura extends between the medial surfaces of the two hemispheres called the FALX CEREBRI
- The peeling apart of the two layers of dura at the top of the falx cerebri forms the superior sagittal sinus
- Within this gap in the dural layers you find arachnoid granulation- these are burst of subarachnoid space that protrude into the superior sagittal sinus
- CSF leaks through holes in the arachnoid membrane and enters the superior sagittal sinus
- Extradural haemorrhages have a rapid onset because it is usually due to the rupture of a meningeal ARTERY
