The cranial meninges
The most complex and coordinated system which is basically the control point of our entire body is the nervous system. It has two parts; the central nervous system and the peripheral nervous system. The brain and spinal cord are the components of the CNS and they are both covered by the meninges. There are three layers of meninges; from outside in,
- Dura mater
- Arachnoid mater
- Pia mater.
Dura is the thickest of these coverings. It has two layers; endosteal layer and meningeal layer. The outter endosteal layer is relatively thin and is basically the periosteum of the skull bones. So, around the foramens of the skull it just covers the margins of the foramen and slide in to the other surface of the bone to be continuous with the periosteum of that surface. (Eg: the pericranium) At the regions of cranial surtures, this layer is continuous with the surtural ligaments. It doesnt continue into the spinal region. So the spinal meninges are devoid of endosteal dura mater.
The inner meningeal layer is the dura mater proper. Compared to the outter layer, it is dense and strong fibrous membrane. At the foramens it provides tubular sheaths for the nerves which ultimately fuses with the epineurium. It invests the spinal region through the foramen magnum.
Both the dural layers are adherent to each others throughout the surface except at the points where they split to enclose the dural venous sinuses. Between these layers lie some blood vessels. (Eg: middle meningeal vessels) As the endosteal layer is very thin, these thicker vessels appear to be extradural and they make impressions of the inner surface of cranial bones. Rupture of these vessels cause extra dural hemorrhage.
The dura mater is thrown into several folds which separates the cranial cavity into freely communicable spaces. These folds limit the rotary movement of the brain during the acceleration and deceleration caused by head movement. This is a mechanism to prevent concussion (temporary unconsciousness or confusion). There are 4 such folds,
- falx cerebi
- falx cerebelli
- tentorium cerebelli
- diaphragma sellae
The second or middle layer of the meninges is the arachnoid mater. It is a thin impermeable delicate membrane. Lined by flat mesothelial cells in the outter and inner surfaces. Situated between the subdural and subarachnoid spaces. Connected with the pia mater by thin fibrous tissue strands that run across the subarachnoid space. This space is filled with the cerebro spinal fluid. The arachnoid mater is widely separated from the pia at some regions to form the subarachnoid cisternae. There are 5 such cisterns.
(All are freely communicable)
- superior cistern
- interpeduncular cistern
- cerebello-medullary cistern
- cerebello-pontine cistern
- ambient cistern
The arachnoid mater plays a major role in the CSF circulation. It does so by projecting into the dural venous sinuses as the arachnoid villi. These villi aggregate to form the arachnoid granulations. Through these the CSF diffuse into blood circulation and so these are very important in maintaining the CSF homeostasis.
The superior sagittal sinus has the most amounts of arachnoid villi.
The innermost layer is named the pia mater. Its a well vascularized delicate layer. it invests all of the surface of brain very closely. So it lines even the deepest of the sulci. Also extends with the cranial nerves to be fused with the epineurium. The pia mater is a part of choroid plexus. So it helps in the formation/ secretion of the CSF into the ventricles. This plexus is formed by the fusion of pia mater, ependymal cell layer and the endothelium of the blood vessels.
The pia and arachnoid mater are collectively known as the "lepto meninges". This term explains the delicateness of the two layers.
The cranial cavity has been divided into spaces according to their relation with the meningeal layers.
- The epidural space - above dura mater. Potential space in cranium. Anatomical space in spinal region.
- The subdural space - between dura and arachnoid. A potential space. Filled by a very thin film of space.
- The subarachnoid space - between arachnoid and pia maters. It is an anatomical space which contains the cerebro spinal fluid (CSF). All the cerebral arteries, veins and the cranial nerves lie in this space.
Hemorrhages can occure in all of these 3 spaces. The causes, complications, symptoms and management methods differ with the type of hemorrhage.