Porth's Essentials of Pathophysiology, 4e

845

Organization and Control of Neural Function

C h a p t e r 3 4

A

Arachnoid villi

Superior sagittal sinus

Subarachnoid space

Choroidal plexus third ventricle

Lateral ventricle

Cerebellum Cerebral aqueduct

Pons

Interventricular foramen

B

Arteriole

Capillary

Vein

Lateral ventricle

Choroidal plexus fourth ventricle Foramen of Magendie

Interventricular foramen

CSF

Third ventricle

Cerebral aqueduct

Fourth ventricle

Choroid plexus epithelium

Lateral and median foraminae

Arterial blood

Subarachnoid space

Heart

Arachnoid villi

Venous blood

FIGURE 34-20. (A) The flow of cerebrospinal fluid (CSF) from the time of its formation from blood in the choroid plexuses until its return to the blood in the superior sagittal sinus. Plexuses in the lateral ventricles are not illustrated. (B) Cerebrospinal fluid is a blood filtrate produced by the choroid plexus epithelium that is found in each brain ventricle.The flow of CSF from the lateral ventricles flows through the interventricular foramen (Monro) into the third ventricle. From the third ventricle, CSF is conveyed to the fourth ventricle via the cerebral aqueduct (Sylvius).Three openings, a midline foramen of Magendie and two lateral foramina (Luschka), pass the CSF into the subarachnoid space, where it is returned to the venous circulation through the arachnoid villi.

Blood–Brain and Cerebrospinal Fluid–Brain Barriers Maintenance of a chemically stable environment is essential to the function of the brain. In most regions of the body, extracellular fluid undergoes small fluctu- ations in pH and concentrations of hormones, amino acids, and potassium ions during routine daily activi- ties such as eating and exercising. If the brain were to undergo these fluctuations, the result would be uncon- trolled neural activity because some substances, such as amino acids, act as neurotransmitters, and ions such as potassium influence the threshold for neural firing. Two barriers, the blood–brain barrier and the CSF-brain barrier, provide the means for maintaining the stable chemical environment of the brain. Only water, carbon

the subarachnoid space. Two of these, the foramina of Luschka, are located at the lateral corners of the fourth ventricle. The third, the median foramen of Magendie, is in the midline at the caudal end of the fourth ventricle (see Fig. 34-20A). Approximately 30% of the CSF passes down into the subarachnoid space that surrounds the spi- nal cord, mainly on its dorsal surface, and moves back up to the cranial cavity along its ventral surface. Reabsorption of CSF into the vascular system occurs along the sides of the superior sagittal sinus in the anterior and middle fossa. Here, the waterproof arachnoid mater has protuberances, the arachnoid villi , which penetrates the inner dura and venous walls of the superior sagittal sinus (see Fig. 34-20A). The arachnoid villi function as one-way valves, permitting CSF outflow into the blood but not allowing blood to pass into the arachnoid spaces.