Fig. 4.9 ). The lateral ventricles are situated within the two cerebral hemispheres. They each connect with the third ventricle through one of the inter-ventricular foramina (of Monro). The third ventricle lies in the midline between the diencephalon on the two sides. The cerebral aqueduct (of Sylvius) traverses the midbrain and connects the third ventricle with the fourth ventricle. The fourth ventricle is a space defined by the pons and medulla below and the cerebellum above. The central canal of the spinal cord continues caudally from the fourth ventricle, although in adult humans the canal is not fully patent and continues to close with age.
CSF escapes from the ventricular system through three apertures or foramina (a medial foramen of Magendie and two lateral foramina of Luschka) located in the roof of the fourth ventricle. After leaving the ventricular system, CSF circulates through the subarachnoid space that surrounds the brain and spinal cord. Regions where these spaces are expanded are called subarachnoid cisterns. An example is the lumbar cistern, which surrounds the lumbar and sacral spinal roots below the level of termination of the spinal cord. The lumbar cistern is the target for lumbar puncture, a clinical procedure to sample CSF. A large part of CSF is removed by bulk flow through the valvular arachnoid granulations into the dural venous sinuses in the cranium.
Because the extracellular fluid within the CNS communicates with the CSF, the composition of the CSF is a useful indicator of the composition of the extracellular environment of neurons in the brain and spinal cord. The main constituents of CSF in the lumbar cistern are listed in
Table 4.2.
For comparison the concentrations of the same constituents in blood are also given. CSF has a lower concentration of K+ , glucose, and protein but a greater concentration of Na+ and Cl− than blood does. Furthermore, CSF contains practically no blood cells. The increased concentration of Na+ and Cl− enables CSF to be isotonic to blood.
The pressure in the CSF column is about 120 to 180 mm H2O when a person is recumbent. The rate at which CSF is formed is relatively independent of the pressure in the
Na+ (mEq/L)148136–145K+ (mEq/L)2.93.5–5Cl− (mEq/L)120–130100–106Glucose(mg/dL)50–7570–100Protein(mg/dL)15–456.8× 103pH7.3
FromWillisWD,GrossmanRG.Medical Neurobiology. 3rded.StLouis:Mosby;1981.
ventricles and subarachnoid space, as well as systemic blood pressure. However, the absorption rate of CSF is a direct function of CSF pressure.
ObstructionofthecirculationofCSFleadstoincreasedCSFpressureandhydrocephalus,anabnormalaccumulationoffluidinthecranium.Inhydrocephalustheventriclesbecomedistended,andiftheincreaseinpressureissustained,brainsubstanceislost.Whentheobstructioniswithintheventricularsystemorintheforaminaofthefourthventricle,theconditioniscalledanoncommunicating hydrocephalus. Iftheobstructionisinthesubarachnoidspaceorthearachnoidvilli,itisknownasacommunicating hydrocephalus.
The Blood-Brain Barrier