Thursday, September 23, 2010

9.5 Brain Membranes - The Water On and In It/Brain Ventricles

Physician's Notebooks 9 - See Homepage -
5. Ventricles & Cerebrospinal Fluid and Coverings of Brain - Update 07 May 2019
Figure: The Ventricular System at the Brain’s Core
View of the system of ventricles at Brain’s core. Note the paired lateral ventricles. The front part of each ventricle (anterior horns toward your left) is the core of the frontal lobe and the rear part (posterior horns) is the core of the occipital lobe, while the middle length of the ventricles is the core of the parietal lobe. The inferior horns are the cores of the side, rabbit-ear-like temporal lobes.
An interventricular foramen (Not labeled in above figure) connects both lateral ventricles and the 3rd ventricle. The 3rd ventricle connects to the lower 4th ventricle (Shown below) via the cerebral aqueduct (Labeled). The lowest part of the 4th ventricle continues as a very narrow canal down into the spinal cord. Two openings in the fourth ventricle allow the cerebral fluid to flow over the surface of the brain and spinal cord as the cerebral spinal fluid (CSF).  When these openings become blocked as may happen before birth a condition known as hydrocephalus is like a sudden expansion inside the brain and must be relieved quickly. 
 The brain contains fluid-filled ventricles in its core. The paired ventricles form the cores of the 4 pairs of lobes of Brain's cerebrum.
   At the tent-like top of the 4th ventricle is an opening (and also paired openings on each side) that allows fluid to flow out between the brain crevices, and onto surface recesses of brain and also out to the space between the brain surface and the inner skull bone dura membrane, and also it flows down around the spinal canal where the fluid is called cerebrospinal fluid (CSF). This CSF forms a protective water jacket over the brain and spinal cord and is the fluid got from spinal tap.
The CSF circulation starts with its formation from filtration drippings from tiny arteries inside fronds of thin tissue along the roof of the brain's lateral ventricles, due to pressure of blood circulation on the thin tiny artery walls that allows only water and small molecules into the ventricle cavities. Fluid in the ventricles has an external pressure pushing it to fill the ventricles and the fluid circulates (also helped by gravity) down to the 4th ventricle. The CSF’s natural current causes it to be circulated back up over the spinal cord surface to the surface of the brain  where it gets reabsorbed back into the blood. The CSF circulation must be free to flow. In case of tumor or other swelling, or due to malformation in embryo, a block may occur in CSF circulation. When it happens before birth or before a fetus’s skull bones fuse, or in  babies before age 2, a hydrocephalus occurs and the head enlarges with progressive killing effect. After age 2, the head cannot enlarge and, in the absence of relief surgery, death comes rapidly with convulsions. The reconstructive surgery for hydrocephalus is peritoneal-ventricular shunt whereby an artificial tube-opening and circulation between the obstructed ventricles in brain and the peritoneal cavity in abdomen is created and hopefully functions permanently.
   The CSF gives nourishment to the brain and is useful to test as spinal tap for nervous system disease. Spinal tap is less frequent now because the MRI can give much the same information without a spinal tap's risks (internal brain herniation, infection, paralytic reaction and low-pressure headache). Today the indication to do a spinal tap is to diagnose meningitis or early subarachnoid hemorrhage.
   The normal CSF: Appearance clear & colorless, opening pressure 70-180 mm water, Red Blood Cells (RBC) 0, White Blood Cells (WBC) 0-5 lymphocytes per high-power field, protein <50 mg%, glucose 50-75 mg%, IgG (Globulin) index <0.77, oligoclonal bands negative, smear & culture negative for microbes. With brain hemorrhage the fluid will look red and show red blood cells microscopically, and high pressure. With bacterial infection the glucose in CSF goes low, and the germs may be seen or may grow on culture, and the WBC count goes high. With Multiple Sclerosis (MS) or late syphilis, the fluid's chemical and immunologic tests get positive. If the CSF pressure goes too low in erect position after spinal tap, it causes bad headache.
   Another use of CSF is for spinal anesthesia that allows major surgery on lower half of body without loss of consciousness.   
      (Use magnifying loupe for inspection)
The Figure shows a vertical slice down the midbrain with subject's head looking at you or away from you. (The slice plane is called coronal, ear to ear in the mid-plane between brow and back of head) The rectangle outline at the very top of the head is enlarged in the lower figure to show the coverings of the brain: from without inward.
The brain and spinal cord membranes are as follows: 
The directly brain covering, closely-adherent-to-brain-surface pia membrane, continuing into the crevices. 
 Next, outwardly, the arachnoid, a transparent, web-like membrane, which is not attached to the pia except by weblike strands and so there is space between pia and arachnoid (Subarachnoid space) where the CSF circulates over the pia-covered surface of brain and into the crevices and which supports blood vessels, and when a blood vessel ruptures, usually from high blood pressure or ruptured aneurysm, it causes subarachnoid hemorrhage that makes spinal taps bloody and causes brain compression. 
The outermost membrane is the dura, closely attached to the overlying bone. It is only loosely applied to the underlying arachnoid, allowing a potential space (Subdural space). Blood vessels run just beneath the skull in the dura and when one is broken due to a bang on head, it causes subdural hemorrhage and brain compression but may not make the spinal tap bloody. (It is a common complication in aged person or someone taking a blood thinning anticoagulant from even mild head bang) 
End note: Normally the blood and its red and white cells are blocked from entering the nervous system by a blood-brain barrier which is effected by the brain membranes that enclose neural tissue having super-small channels that filter out the blood. But in many disease states this barrier fails so even small amounts of blood cells detected in neural tissue are a sign of disease. Furthermore because the immune-functions of the white blood cells do not normally get into the brain and spinal cord; special glial cells, the microglia, serve the defensive protective, phagocytic (cleaning the debris of dead cells) functions of white blood cells.
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