Meninges The brain and spinal cord are surrounded by three connective tissue
membranes, the meninges (Fig. 1).
The pia mater and arachnoid mater are together called the leptomeninges. In the subarachnoid space that lies between them run superficial cerebral blood vessels. These are invested by a leptomeningeal coat and suspended in the space
Key Notes
The brain and spinal cord are invested by three connective tissue layers, the meninges. Directly covering the brain is the pia mater, above which is the arachnoid mater. Between these layers lies the subarachnoid space, which is filled with cerebrospinal fluid (CSF) and through which run blood vessels, branches of which enter the brain. Passive exchange of water and solutes across the pia mater keeps brain extracellular fluid and CSF in equilibrium. The tough outer layer is the dura mater, which contains venous sinuses. Projections of the arachnoid mater herniate into the venous sinuses. Here, tiny one-way valves allow the bulk flow of CSF from subarachnoid space into the venous circulation. The potential space between the arachnoid mater and the dura mater is the subdural space. Traumatic rupture of the veins passing through this space from brain to venous sinuses causes subdural hemorrhage. Between the dura and the cranial bones is the extradural space through which run major arteries. Traumatic rupture of these results in extradural hemorrhage.
CSF is actively secreted by the choroid plexuses located in the ventricles. The direction of CSF flow is from lateral to 3rd to 4th ventricles, from where it enters the subarachnoid space. Finally it drains into the venous sinuses. Obstruction to the flow of CSF causes hydrocephalus.
About 500 cm3of CSF is secreted per day into a volume of 100–150 cm3.
Choroid plexus epithelium contains a variety of active transport mechanisms. This results in secretion into the CSF of sodium, chloride, and bicarbonate but resorption of potassium, glucose, urea, and a number of neurotransmitter metabolites. The protein concentration of CSF is very much lower than that of blood plasma.
The CSF acts as a sink for metabolites that eventually are dumped into the blood via arachnoid villi or choroid plexuses. Mechanical functions of CSF and meninges are to reduce the weight of the brain in the skull, to resist changes in intracranial pressure due to altered brain blood flow and to cushion the brain during violent movements of the head.
Related topic Organization of the central nervous system (A5)
Meninges CSF and meningeal functions Cerebrospinal fluid circulation Cerebrospinal fluid secretion
by trabeculae. The subarachnoid space is filled with cerebrospinal fluid. Branches of the subarachnoid vessels penetrate the brain, becoming surrounded by a cuff of pia mater that extends as far as the capillaries. The perivascular (Virchow–Robin) space between the vessel wall and the pia mater is continuous with the subarachnoid space. Here passive exchange of water and solutes across the pia mater keeps the CSF in equilibrium with brain extracellular fluid. At the cerebral capillaries the pia mater is lost and the single layer of capillary endothe- lial cells, with their basement membrane, are covered by glial cells. Expanded regions of the subarachnoid space are cisterns. The lumbar cistern is the target for sampling CSF (lumbar puncture) since there is no risk of damage to the cord.
The dura mater is a thick, tough, outer layer with venous sinuses running through it. Small herniations of the arachnoid mater called arachnoid villi (arachnoid granulations) protrude through the dura into the venous sinuses. Here bulk flow of CSF into blood occurs via mesothelial tubes in the arachnoid villi that act as valves, closing when the pressure in the venous sinus exceeds that of subarachnoid space to prevent reflux of blood into the CSF.
The subdural space is a potential space between the dura mater and the arachnoid mater. It is traversed by cerebral veins entering the venous sinuses in the dura. Traumatic rupture of these vessels as they pass through the space causes subdural hemorrhage, which may present clinical problems at any time from the moment of injury to months later. Trauma which shears major vessels going from the dura mater into the cranial bone causes bleeding in the extradural spacethat opens up between meninges and skull. Extradural hemor- rhageis a life-threatening surgical emergency because it causes brain compres- sion. In the vertebral canal the dura mater forms a loose sheath leaving an epidural spacebetween it and the canal wall. Injection of local anesthetics into this space produces epidural nerve block.
CSF is actively secreted by choroid plexuses situated in the lateral, third and fourth ventricles (Fig. 2). Flow of CSF is from the lateral ventricles through the foramen of Munro into the third ventricle, and then through the aqueduct of Sylviusinto the fourth ventricle. From here it drains via three orifices, a medial
foramen of Magendie and two lateral foramina of Lushka, to enter the
subarachnoid space. Here it equilibrates with extracellular fluid in the perivas- cular spaces. Finally it is dumped into the venous sinuses via the arachnoid villi. Cerebrospinal
fluid circulation
A7 – Meninges and cerebrospinal fluid 27
Cranial bone Venous sinus Arachnoid villus Dura mater Arachnoid mater Cerebral blood vessel Pia mater Cistern Brain Sulcus Perivascular (Virchow–Robin) space Extradural space Subdural space Subarachnoid space Subpial space
Obstruction to the flow of CSF causes hydrocephalus, an accumulation of fluid in the cranium. This may increase CSF pressure, distending the ventricles and inflicting damage to the surrounding neural tissue. An obstruction of the ventricular system is non-communicating hydrocephalus. It is the result of congenital malformation, scarring or tumors. In communicating hydrocephalus there is a failure of CSF flow from the arachnoid villi. This may happen if the concentration of protein in the CSF gets abnormally high, as with some spinal cord tumors, subarachnoid hemorrhage, or meningitis.
CSF secretion Each choroid plexus consists of a cuboidal epithelium derived from the
ependyma (the lining of the ventricles and spinal cord central canal), covering a core of highly vascular pia mater. In adult humans CSF is secreted at about 500 cm3day–1into a steady state volume of 100–150 cm3. Of this, about 30 cm3is in
the ventricles and the rest in the subarachnoid space. Cerebrospinal fluid is turned over about every 5–7 hours.
The choroid plexus secretes some substances and absorbs others specifically, most by active transport mechanisms. In this way it acts as a selective interface between blood and CSF, the blood–CSF barrier. The result is that by compar- ison with blood plasma CSF has somewhat higher Na+, Cl–, and HCO
3–concen-
trations but lower K+, urea, glucose and amino acid concentrations. Although
the protein concentration of CSF is about 1000-fold lower than blood plasma, its higher ionic concentration gives the two fluids the same osmolality.
Some of the mechanisms involved in ion transport across the blood–CSF barrier are shown in Fig. 3. Na+, K+-ATPase on the apical border of the epithelial
cell pumps sodium into the CSF. This generates a sodium gradient that drives two secondary active transport mechanisms bringing Na+across the basolateral
border; Na+–H+exchange and a Na+–Cl–symport. The Cl–influx in turn drives a
Cl–_HCO
3–antiport. Bicarbonate brought into the cell in this way is added to that
Dura mater Subarachnoid space
Arachnoid villus Sagittal sinus Choroid plexus Transverse sinus IV ventricle Choroid plexus Central canal Foramen of Magendie (and Luschka) Aqueduct of Sylvius III ventricle Foramen of Munro Lateral ventricle
formed intracellularly by hydration of CO2, a reaction greatly accelerated by the
high levels of carbonic anhydrase present in the choroid plexus. The bicar- bonate diffuses via an apical anion transporter into the CSF.
The ability of the choroid plexus to absorb materials from the CSF means that it acts as an excretory organ for the brain. It scavenges choline, dopamine and serotonin metabolites, urea, creatinine and K+, dumping them into the blood.
The functions of the CSF are metabolic and mechanical. By equilibrating with brain extracellular fluid unwanted metabolites are removed to the blood, either via arachnoid villi or choroid plexuses. There are three mechanical effects: 1. Because the subarachnoid space is a fluid-filled compartment in which the
brain floats, the effective weight of the brain is reduced from about 1350 g to about 50 g.
2. Adjustments to CSF and meninges prevent changes in intracranial pressure due to alterations in cerebral blood flow. When blood flow increases, CSF is squeezed from ventricles into the subarachnoid space around the spinal cord. Here the dura mater is more elastic and stretches to accommodate the rise in volume. Longer-term increases in intracranial pressure can be offset by a rise in CSF flow into the venous sinuses through the arachnoid villi.
3. The meninges support the brain and the CSF reduces the force with which the brain impacts the inside of the cranium when the head moves.
CSF and meningeal functions
A7 – Meninges and cerebrospinal fluid 29
Basolateral border Secondary active transport Facilitated diffusion Blood Choroid plexus epithelial cell Facilitated diffusion CSF Apical border Primary active transport H 2CO3 CO2 + H2O HCO3 – + H+ Na+–K+ATPase Na+ 3 Na+ CI– K+ 2 K+ Na+ H+ CI– CI– HCO−3 HCO−3