The brain in the skull and the spinal cord in the vertebral column are surrounded by three protective membranes called meninges. These three protective layers are; 1) A tough, outer layer called the dura mater. 2) A delicate, middle layer called the arachnoid mater. 3) An inner layer firmly attached to the surface of the brain called the pia mater.
Let’s first of all discuss the protective covering meninges of the brain.
The dura mater of the brain is consists of two layers: the endosteal layer and the meningeal layer. Both these layers are closely united except along at certain lines, where these separate to form venous sinuse
Endosteal layer of Dura
- The endosteal layer is outer periosteum covering the inner surface of the skull bones.
- It does not become continuous with the dura mater of the spinal cord at foramen magnum.
- It is continuous with the sutural ligaments.
- It is most strongly adherents to the bones over the base of the skull.
Meningeal layer of Dura
- The meningeal layer is proper the dura mater and
- It is a dense, strong fibrous, and membranous covering of the brain.
- It is continuous through the foramen magnum with the dura mater of the spinal cord.
- It provides tubular sheaths for the cranial nerves as the latter pass through the foramina in the skull and then the sheaths fuse with the epineurium of the nerves.
- It has four septa or dural partition
Dural partitions of Meninges
The meningeal layer sends inward four septa, which divide the cranial cavity into freely communicating spaces that lodge the subdivisions of the brain. The dural partitions project into the cranial cavity and partially subdivide the cranial cavity. The function of these septa is to restrict the displacement of the brain associated with acceleration and deceleration, when the head is moved. They include the falx cerebri, tentorium cerebelli, falx cerebelli, and the diaphragm sellae.
The falx cerebri is a crescent-shaped or sickle shaped downward projection of meningeal dura mater that lies in the midline between the two cerebral hemispheres. It is attached anteriorly to the crista galli of ethmoid bone and frontal crest of the frontal bone and posteriorly with the tentorium cerebelli.
- The superior sagittal sinus runs in its upper fixed margin.
- The inferior sagittal sinus runs in its lower concave free margin.
- The straight sinus runs along its attachment to the tentorium cerebella.
The tentorium cerebelli is a horizontal crescent shaped projection of the meningeal dura mater that covers and separates the cerebellum in the posterior cranial fossa from the cerebral hemispheres. It is attached posteriorly to the occipital bone along the grooves. Laterally, it is attached to the superior border of the petrous part of the temporal bone. The anterior and medial borders of the tentorium cerebella are free which form an oval shape opening in the midline called the tentorial notch, through which the midbrain passes.
The falx cerebelli, a small, sickle-shaped fold of dura mater attached to the internal occipital crest, projects forward between the two cerebellar hemispheres. Its posterior fixed margin contains the occipital sinus.
The diaphragma sellae is a small, circular fold of dura mater that forms the roof for the sella turcica. There is small opening in its center of the diaphragma sellae through which passes the infundibulum, connecting the pituitary gland with the base of the brain, and any accompanying blood vessels.
Arterial supply of the dura
The arterial supply to the dura mater travels in the outer periosteal layer of the dura and consists of:
- Anterior meningeal arteries: These are branches of the ethmoidal arteries in anterior cranial fosse
- Middle and accessory meningeal arteries: These are branches of maxillary artery in the middle cranial fossa and
- Posterior meningeal artery: These are branches from ascending pharyngeal atery and other meningeal branches in the posterior cranial fossa.
Dural Nerve Supply
Branches of the trigeminal, vagus, and the first three cervical spinal nerves and branches from the sympathetic trunk pass to the dura.
- The arachnoid mater is a delicate, impermeable membranous covering of brain the brain.
- It lies between the pia mater internally and the dura mater externally.
- The arachnoid mater is separated from the dura by a potential space called the subdural space, filled by a film of fluid.
- It is separated from the pia mater by the subarachnoid space, which is filled with cerebrospinal fluid.
- The outer and inner surfaces of the arachnoid mater are covered with flattened mesothelial cells.
Arachnoid villi of Meninges and CSF:
In certain areas of the brain, arachnoid projects into the venous sinuses to form arachnoid villi. Arachnoid villi serve as sites where the cerebrospinal fluid (CSF) diffuses into the bloodstream. The arachnoid villi are most numerous along the superior sagittal sinus. Aggregations of arachnoid villi are referred to as arachnoid granulation.
Cerebrospinal fluid circulation in Meninges
The cerebrospinal fluid is produced by the choroid plexuses within ventricles of the brain. CSF escapes from the ventricular system of the brain through the three foramina in the roof of the fourth ventricle and so enters the subarachnoid space. It now circulates both upward over the surfaces of the cerebral hemispheres and downward around the spinal cord. The spinal subarachnoid space reaches down as far as the second sacral vertebra. At last the fluid enters the bloodstream by passing into the arachnoid villi and diffusing through their walls.
Function of Cerebrospinal fluid
- Removing waste products which are associated with neuronal activity.
- The cerebrospinal fluid provides a fluid medium in which the brain floats.
- CSF effectively protects the brain from trauma.
- Cerebrospinal fluid is believed to play a role in hormonal transport.
- The pia mater is a vascular membrane of meninges covered by flattened mesothelial cells.
- Pia mater closely invests the brain, covering the gyri and descending into the deepest sulci.
- It extends outover the cranial nerves and fuses with their epineurium.
- The pia mater forms the tela choroidea of the roof ventricles of the brain which fuses with the ependyma to form the choroid plexuses.
The archnoid and pia mater sometimes together called leptomeninges. During acute meningococalmeningitis there is exudates within leptomeninges, as the arcahonoid is connected with pia mater in strands hence look structurally look similar called arachnoid-pia or leptomeninges.
Meninges of Spinal cord
Let’s discuss in short the meninges of spinal cord.
- The dura mater is a dense, strong, fibrous membrane that encloses the spinal cord and the cauda equina. It is continuous above with foramen magnum and ends on the filum terminale at the level of the lower border of the second sacral vertebra.
- The arachnoid mater is continuous above through the foramen magnum with the arachnoid covering the brain and ends on the filum terminale at the level of the lower border of the second sacral vertebra. The arachnoid mater continues along the spinal nerve roots, forming small lateral extensions of the subarachnoid space.
- The pia mater is a vascular membrane that closely covers the spinal cord. The piamater is thickened on either side between the nerve roots to form the ligamentum denticulatum, which passes laterally to adhere with arachnoid and dura. By this means the spinal cord is suspended in the middle of the dural sheath. The pia mater extends along each nerve root and becomes continuous with the connective tissue surrounding each spinal nerve.
Function of Meninges
- The meninges of the brain and spinal cord form three concentric protective membranous coverings. The outermost covering, the dura mater, by virtue of its toughness, serves to protect the underlying nervous tissue. The dura protects the cranial nerves by forming a sheath that covers each cranial nerve for a short distance as it passes through foramina in the skull. The dura mater also provides each spinal nerve root with a protective sheath.
- The falx cerebri, which is a vertical sheet of dura between the cerebral hemispheres and the tentorium cerebelli, which is a horizontal sheet that projects forward between the cerebrum and cerebellum. These dural sheats serve to limit excessive movements of the brain within the skull.
- The arachnoid mater is a much thinner impermeable membrane that loosely covers the brain. The interval between the arachnoid and pia mater, the subarachnoid space, is filled with cerebrospinal fluid. The cerebrospinal fluid gives buoyancy to the brain and protects the nervous tissue from mechanical forces applied to the skull. Removes waste material and transport hormones.
- The pia mater is a vascular membrane that closely invests and supports the brain and spinal cord. . The piamater is thickened on either side between the nerve roots to form the ligamentum denticulatum, which passes laterally to adhere with arachnoid and dura. By this means the spinal cord is suspended in the middle of the dural sheath.
Clinical significance of Meninges
Epidural hemorrhage results from injuries or trauma to the meningeal arteries or veins. The most common artery to be damaged is the middle meningeal artery. During the bleed intracranial pressure rises, and the enlarging blood clot put local pressure on the underlying motor area in brain and produce neurological symptoms. Blood also passes laterally through the fracture line to form a soft swelling under the temporalis muscle.
Subdural hemorrhage results from tearing of the superior cerebral veins at their point of entrance into the superior sagittal sinus. The cause of subdural hemorrhage is usually a blow on the front or the back of the head, causing excessive anteroposterior displacement of the brain within the skull.
Subarachnoid hemorrhage occurs in subarachnoid space results from arterial aneurysms in brain arteries usually from trauma to the brain or rupture of the aneurysm due to high blood pressure or bleeding disorder.
The dura mater of brain receives its sensory nerve supply from the trigeminal and the first three cervical nerves. Dura mater above the tentorium is innervated by the trigeminal nerve so the headache is referred to the forehead and face. The dura below the tentorium is innervated by the cervical nerves hence headache is referred to the back of the head and neck.
Meningitis of Meninges
Meningitis is infection of the leptomeninges (the leptomeninges are a combination of the arachnoid mater and the pia mater). Infection of the meninges typically occurs via a bloodborne route, though in some cases it may be by direct spread (e.g., trauma to the brain or spinal cord) or from the nasal cavities through the cribriform plate in the ethmoid bone.
Meningioma is the tumor of meninges. it is also known as meningeal tumor which is typically a slow growing tumor. The symptoms og meningioma are depend upon the location of tumoras result of tumor pressure on the nearby tissue. It has 3 grades I,II & III. Treatment may be observation, surgery, radiation therapy depends upon tumor.