(From Dr. Glasser’s Lecture, 28 July 2000, by Brian Buschman)
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The brain is very sensitive to blood flow. If blood flow is occluded for only 10 seconds it goes unconsciousness. If it lasts for 20 second it results in electrical activity stopping. In only 5 min irreversible brain damage occurs.
The blood supply comes from both the vertebral arteries and the internal carotid arteries. The vertebral arteries first give off the PICA (posterior inferior cerebellar arteries) and then join to form the basilar artery on the anterior side of the spinal cord. The vertebral arteries also form the anterior spinal artery. The basilar artery gives the AICA (anterior inferior cerebellar artery) and the superior cerebellar artery. It then divides into the two posterior cerebral arteries which give off the posterior communicating arteries which form collaterals with the internal jugular arteries. One very important landmark is knowing that CN III comes out between the posterior cerebral and the superior cerebellar arteries.
The internal carotid comes up to the brain and gives the anterior cerebral and the middle cerebral arteries. The two anterior cerebral arteries have a small communication called the anterior communicating artery. The bulk of the blood goes to the middle cerebral which gives the anterior choroidal artery and the lenticulostriate arteries (supplies the internal capsule).
The anterior cerebral artery enters the longitudinal fissure to supply the internal parts of both hemispheres, especially in the frontal and parietal lobes.
Most of the middle cerebral artery goes to supply the lateral aspect of the brain including the frontal, parietal and temporal lobes. It gives the lenticulostriate artery which goes to supply the internal structures of the telencephalon and diencephalon including the entire internal capsule. It is the lenticulostriate arteries that make up the holes for the anterior perforated substance. Loss of the lenticulostriate arteries will cause complete loss of the internal capsule including the corticospinal, corticobulbar and the sensory tracts.
A loss of the MCA will cause a contralateral hemiparesis and sensory loss but will spare the lower body since that part is medial and supplied by the anterior cerebral artery. It it’s on the dominant side it can cause any type of aphasia.
The posterior cerebral artery goes to supply the occipital lobe and lower temporal lobes. The posterior perforated substance is from penetrating branches of the basilar arteries.
It is an abrupt blockage of the blood supply. Ischemic stroke can be caused by a thrombus (blood clot) or an embolus (anything else stuck in the artery).
If the occlusion is at the level of the circle of Willis collateral circulation is possible. If it is distal to the circle of Willie there is almost no collateral circulation.
TIAs are very short ischemic attacks that are cleared by natural mechanism and are usually short lived. There is usually a full recovery in 30min to one day.
When a small perforating artery or aneurysm ruptures it’s called a hemorrhagic stroke. It is usually associated with hypertension.
An aneurysm is a ballooning out in a blood vessel. It is most often near a bifurcation. It causes problem for two reasons:
1) The released blood compresses related structures.
2) The blood loss causes a drop in cerebral BP.
The lateral ventricle has an inferior horn, posterior hone, trigone, body and inferior horn. It connects through the interventricular foramen to the third ventricle. The third ventricle has a supraoptic, infundibular, pineal and sometimes suprapineal recesses. It is transversed by the interthalamic adhesion (massa intermedia).
The cerebral aqueduct then connects to the 4th ventricle. It’s floor is called the rhomboid fossa and it’s roof the anterior medullary velum. The fourth ventricle empties via the two foramen of Lusaka (lateral) and one foramen of Magendie (medial).
The spinal canal in the adult does not contain CSF therefore is not considered to be part of the ventricular system.
CSF is made by the choroid plexus. Most of the choroid plexus is in the lateral ventricles and is made of pia containing capillaries covered by ependymal cells that are attached to tela choroidea.
Functions of the CSF include:
1) Acting as a lymphatic system for the brain.
2) Protecting the brain from shock.
3) Controlling CNS excitability by regulation of extracellular ionic composition.
Normal CSH is clear, colorless, odorless and low in cells and protein. It is not an ultrafiltrate of plasma as it’s concentrations of Ca2+ and K+ are lower then that of plasma and it’s concentration so Mg2+ and Cl- are higher then those of plasma. The normal person has about 130mL of CSF with about 25ml in the ventricular system. Normal CSH pressure is ~70-180 mmH2O. The pressure in increased with a tumor, hemorrhage or hydrocephalus.
We make ~400-500ml of CSF/day. It is drained into the arachnoid villi which are visible in the lab as the arachnoid granulations and drain into the intradural sinuses. If blocked it will produce one of two types forms of hydrocephalus:
1) Non-communicating which is then the ventricular system does not communicate with the subdural space. This can come from a blockage of any of the holes in the ventricular system (IV foramen, cerebral aqueduct or foramen of Lusaka and Magendie). In such a patient their brain will be crushed from within.
2) In communicating hydrocephalus the ventricles communicate with the subdural space and the CSF builds up in the subdural space. It is due to something that blocks the arachnoid villi. In this case the patients brain is crushed from outside. If this happens in a child who’s calveria has not hardened yet the calveria will enlarge in response to the pressure so you will see a kid with a very big head.
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