The Ventricular System and Cerebrospinal Fluid

Rate of Production of CSF

1. 500-750 mL/day 2. Therefore, 4 or 5 volume turnover per day.

Hydrocephalus (water on the brain)

1. Abnormal increase in the volume of cerebrospinal fluid. 2. Types: Communicating hydrocephalus and noncommunicating hydrocephalus

Absorption of the CSF

1. Absorption of CSF occurs via the arachnoid villi which project into the dural venous sinuses especially the superior sagittal sinus. 2. Arachnoid villi are usually clumped to form elevations called arachnoid granulations. 3. Each arachnoid villus is a diverticulum of the subarachnoid space that pierces the dura mater. 4. The arachnoid villus is covered by a thin cellular layer which is in contact with the endothelium of the venous sinus. 5. The arachnoid granulation increase in number and size with age. 6. Note: CSF pressure is usually greater than the venous pressure. Therefore, channels are open allowing outward flow of CSF. 7. Note: If CSF pressure falls below venous pressure, the channels close, preventing backflow of blood into the subarachnoid space.

Circulation of CSF

1. Begins with its secretion from the choroid plexuses of the ventricles. 2. Fluid passes from the lateral ventricles into the third ventricle via the interventricular foramena. 3. Fluid passes from the third ventricle into the fourth ventricle via the cerebral aqueduct. 4. The fluid then enters the subarachnoid space via the media aperture and the lateral apertures of the fourth ventricle. 5. The fluid in the subarachnoid space circulates through the cisterns (enlargements of the subarachnoid space) around the brain and the spinal cord.

Cerebrospinal Fluid (CSF) COMPOSITION

1. Clear, colorless fluid, approximately 150 mL. 2. Glucose content approximately half the amount found in blood. 3. Only trace amounts of protein as compared with blood. 4. Excess of magnesium and chloride ions as compared with blood. 5. Reduced potassium and calcium ions compared with blood. 6. Under normal conditions, almost cell free with a few leukocytes per microliter.

CSF Formation

1. Produced mainly in the choroid plexuses of the ventricles. 2. CSF is formed by filtration of blood through fenestrated endothelium of the capillaries by active transport of substances across the choroid epithelium into the ventricle. 3. The choroid epithelium contains well developed tight junction preventing diffusion of substances across it in an uncontrolled manner. 4. The tight junctions of the choroid epithelial cells serve as a blood-CSF barrier, preventing the passive diffusion of most serum constituents into the ventricles. 5. The epithelial cells take up and actively secrete the required electrolytes and other CSF constituents into the ventricles. 6. This explains the differences in the composition between CSF and blood serum.

CSF Functions

1. The brain and spinal cord normally "float" in the CSF because the specific gravities of the central nervous system structures are approximately the same. This buoyant effect results in a reduction of traction exerted on nerves and blood vessels connected with the CNS. 2. Provides a cushioning effect on the CNS and dampens the effects of trauma. 3. Serves as a vehicle for the removal of metabolites from the CNS. 4. Provides a stabile ionic environment for the CNS.

Choroid Plexus of the Lateral Ventricle (Structure)

1. The choroid plexus projects into the ventricle on its medial aspect. 2. Basic Structure: a. An invagination of the pa mater projects into the ventricle. b. The invagination is covered by a single layer of ependymal cells which are attached to each other by well developed tight (occluding, zonula occludens) junctions. c. Here, the ependyma layer is specialized secretory epithelium called choroid epithelium. d. Fenestrated capillaries derived from the choroidal branches of the internal carotid and vertebral arteries indent the invagination.

Communicating Hydrocephalus

Blockage of movement of CSF into the dural venous sinuses a. may occur due to scarring of the arachnoid villi from meningitis b. may occur as the result of tumors that collapse the villi

Noncommunicating Hydrocephalus

Blockage somewhere along the ventricular system so that CSF cannot enter the subarachnoid space (e.g. blockage of the cerebral aqueduct, blockage of the foramina of the fourth ventricle. a. Ventricular tumors may result in the blockage.

The Ventricular System: GENERAL

There are four ventricles: 1. Lateral Ventricles (2) 2. Third Ventricle 3. Fourth Ventricle The ventricles are derived from the primitive embryonic cavity of the neural tube.

Third Ventricle

a. Cavity of the diencephalon b. Communicates anteriorly with the lateral ventricles via the interventricular foramina (of Monroe) and posteriorly with the fourth ventricle via the cerebral aqueduct (of Sylvius) c. Choroid plexuses of the third ventricle are located in the roof and have the same structure as those of the lateral ventricles.

Cerebral Aqueduct of Sylvius

a. The cerebral aqueduct is the cavity of the midbrain and connects the third and the fourth ventricles. b. It is surrounded by a layer of gray matter called the central or periaqueductal gray matter. c. Lacks a choroid plexus.

The Central Canal

a. The fourth ventricle is continuous with the central canal of the spinal cord which is lined with ependyma like the ventricles. b. The central canal contains cerebrospinal fluid and is closed at its lower end.

Fourth Ventricle

a. The fourth ventricle is the cavity of the pons and cephalic half of the medulla oblongata. b. It communicates with the subarachnoid space via two lateral apertures of the lateral recesses called the foramena of Luschka and a median aperture called the foramen of Magendie. c. The choroid plexus of the fourth ventricle is suspended from the roof

Lateral Ventricles

a. roughly C-shaped cavity, present in each hemisphere of the brain b. Major parts 1. Body (central Part)--occupies the parietal lobe 2. Frontal (anterior) horn--occupies the frontal lobe 3. Occipital (posterior) horn--occupies the occipital lobe 4. Temporal (inferior) horn--occupies the temporal lobe c. The lateral ventricles communicate with the third ventricle via the interventricular foramena (of Monroe) which are located immediately posterior to the anterior column of the fornix and anterior to the anterior end of the thalamus

The Subarachnoid Space

a. the space between the arachnoid membrane and the pia mater. b. filled with CSF and contains the large blood vessels of the brain. c. Transversed by a network of fine trabeculae giving it a cobwebby appearance. d. The subarachnoid space completely surrounds the brain and spinal cord.