The CNS is very vulnerable to interupption of blood flow. Why is this?
The CNS has high metabolic requirements
Thus, it requires efficient delivery of nutrients and removal of metabolic byproducts.
What percentage of the cardiac output is directed to the CNS?
Describe the concept of autoregulation of cerebral blood flow?
Cerebral blood flow is maintained at an adequate perfusion even when arterial blood pressure changes or if blood gases change.
What part of the brain has the highest metabolic demand?
Describe the vasculature in this region as a result of this
The cortex and white matter of the brain is very metabolic demanding.
It has a very dense network of um scale vasculature that is fed by a network of larger vessels.
What is the main anastomoses of the vasculature of the brain? Where is it located and what structures does it surround?
The circle of wilis
It provides arterial supply of blood to the brain and is situated on the ventral surface of the brain forming a circle anastomosis around the hypothalamus and optic chiasm
What are the 2 arterial systems that form the circle of willis?
- Vertebrobasilar system
- Internal carotid system
What is the advantage of having this anastomosing structure of vessels in the brain?
If one vessel is damaged, blood is still able to reach other parts of the brain
What are the two main origins of the Circle of Willis? Describe their course through the body to get to the base of the brain
Internal carotid artery: This is a branch off the common carotid artery that ascends through the neck into the carotid canal. It does a sharp bend at the foramen lacerum towards the optic chiasm
The Vertebral artery: is another branch off the subclavian artery that moves through the neck through the transverse foramine of the vertebral column and through teh foramen magnum to the base of the brain
What are the major branches of the circle of willis?
- Anterior cerebral artery
- Middle cerebral artery
- Posterior cerebral artery
- Basilar artery
Describe how the originating vessels form the circle of willis
The Internal carotid artery comes up and divides into three branches as it reaches: the anterior cerebral artery, middle and posterior cerebral artery.
The two verebral arteries come up and unite to form the basilar artery and terminate into two posterior cerebral arteries
What are the two communicating arteries, what vessels do they connect?
- The anterior communicating artery connects the two anterior cerebral arteries together
- The Posterior communicating arteries connect the internal carotid system with the vertebrobasilar system by connecting the middle cerebral artery with the posterior cerebral artery.
What is the first branch given off the internal carotid artery in the brain?
What does it supply?
The first branch is the opthalmic artery that heads to the structures of the eye
- It supplies the eye and retina
- Top of the nasal cavity
- Frontal scalp
Describe the path of the anterior cerebral artery from its branching off the internal carotid artery
The arteries run anteriorly to the front of the brain on either side of the longitudinal fissure. It then arches up in front of the corpus callosum, hooks around and continues to run posteriorly forming a number of terminal branches
As it proceeds along this course, it gives of branches to the cortex
Based on its course what part of the brain does the anterior cerebral artery supply?
It supplies the:
- Medial part of the frontal lobe
- Medial part of the parietal lobe (to the level of the parieto-occipital sulcus)
These include the motor and sensory cortices of the lower limb (note the upper limb centres are more laterally situated)
What kind of pathology would be expected if a vascular event were to occur in the anterior cerebral artery?
- A contralateral spastic paralysis for the lower limb muscles (UMN lesion)
- Contralateral hemianesthesia of the lower part of the body as somatosensory neurons are affected
Describe the path of the middle cerebral artery
It runs outwards from the branching, travels between the parietal lobe and temporal lobe and comes out through the lateral fissure to the surface of the brain supplying the lateral surface of the brain including parts of the frontal, parietal, temporal and occipital bones and part of the insula cortex that it passes through
What territories of the brain does the middle cerebral artery supply?
It supplies the..
- Motor and sensory corticies for most of the body (except the lower limbs)
- Parietal-occipital association areas
- Language areas: Broca's and Wernike's on the dominant side
Describe the consequences of a vascular event occuring in the middle cerebral artery
- Spastic paralysis to the contralateral body (minus the lower limbs)
- Somatosensory deficits (hemianesthesia) in the contralateral body (minus the lower limbs)
- Problems associated with body and language deficits
Describe the path of the posterior cerebral artery as it is given off at the circle of wilis
It is given off as a termination of the basilar artery and it runs towards the occipital lobe giving off branches as it goes
What does the posterior cerebral artery supply?
- The medial and inferior surfaces of the temporal lobe
- The medial and posteroinferior occipital lobe
What are the consequences of a vascular event in the posterior cerebral artery?
- Visual impairment in the contralateral visual field (contralateral hemonamous hemianopia)
Clinical symptoms associated with occlusion of the posterior cerebral artery depend on the location of the occlusion
What artery is the most commonly occluded in a stroke?
The middle cerebral artery
What are the main perforating branches off the circle of willis? What do they supply?
They supply deep structures lying close to where they branch off
- Anterior perforating branches off the anterior communicating artery that supply optic chiams, anterior hypothalamus
- The Lenticulostriate arteries that supply the basal ganglia and the internal capsule
- Posterior perforating branches off the posterior communicating artery that supply the ventral midbrain, posterior hypothalamus and thalamus
- The pontine branches coming off the basilar arteries that supply the ventral pons
What structures does the lenticulostriate arteries supply?
The basal ganglia especially the lentiform nucleus and striatum
A vessel (other than the opthalmic artery) is given off before the branching into the anterior and middle cerebral artery. What is this vessel and what does it supply?
The Anterior Choroidal Artery
It comes off the internal carotid artery and passes posteriorly and wraps around the lateral ventricles supplying structures associated with the lateral hemispheres
It supplies structures in the deep, lateral hemispheres including the optic tract, lateral ventricles and hippocampus.
What structures off the vertebro-basilar system supply the brainstem and cerebellum?
- Posterior inferior cerebellar arteries
- Anterior inferior cerebellar arteries
- The Pontine arteries
- Superior cerebellar arteries
Describe how the cerebellar arteries arise
- The vertebral arteries give rise to the posterior inferior cerebellar arteries before they unite
- Just after they unite the basilar artery sends out branches of the anterior inferior cerebellar artery
- Just before the termination of the basilar artery into the posterior cerebral artery, it gives off the superior cerebellar artery
Where does the basilar artery terminate? What happens at this termination
It terminates at the junction betweeen the pons and the midbrain. Here it giives off the superior cerebellar arteries and becomes the posterior cerebral artery
What are the main arteries supplying the midbrain?
The posterior cerebral artery branches run the most of the midbrain all the way the posterior end. The superior cerebellar artery wraps around and supplies the posterior most part (the colliculi)
What are the main arteries that supply the pons?
The pontine branches off the basilar artery supply the majority of the pons
What are the main arteries that supply the medulla?
Three separate arteries supply the medulla:
- Anterior spinal artery supplies the just around the midline of the medulla
- Vertebral artery supplies the anterolateral aspects of the medulla
- The Posterior inferior cerebellar artery supplies the rest (the posterolateral)
What are the consequences of having a cerebrovascular accident associated with the anterior spinal artery?
- Lack of blood supply to pyramids (corticospinal tract) thus contralateral hemiperesis (UMN),
- Medial lemniscus also causes contralateral somatosensory deficit
- Include hypoglossal nucleus (ipsilateral flaccid, LMN paralysis and atrophy of tongue)
Describe how the anterior spinal artery is formed and its pathway down the spinal cord
It is formbed by 2 branches coming off the vertebral artery and uniting together
It runs in the mildline fissure of the spinal cord on the ventral surface
Radicular arteries branch off the anterior spinal artery
Describe how the posterior spinal arteries are formed and its pathway down the spinal cord
They are formed as 2 branches off each of the vertebral arteries
They run on the posteriolateral aspects of the spinal cord giving off radicular arteries as they go
What are the circumferential branches of the spinal crod?
Communication vessels between the anterior and posterior arteries. They are important for supplying the lateral tracts of the spinal cord