Meninges of the brain and spinal cord
The meninges are the three membranes that envelop the brain and spinal cord and separate them from the walls of their bony cases (skull and vertebral column). Based on their location, meninges are referred to as the cranial meninges which envelop the brain, and spinal meninges which envelop the spinal cord. However, the cranial and spinal meninges are continuous with each other and consist of the same three meningeal layers. From superficial to deep the meninges are the:
- Dura mater, also known as the pachymeninx
- Arachnoid mater
- Pia mater
These layers bound three clinically important potential spaces: the epidural, subdural, and subarachnoid spaces. The function of the meninges is to protect the brain and spinal cord from mechanical trauma, to support the blood vessels and to form a continuous cavity through which the cerebrospinal fluid (CSF) passes. Specifically, the CSF passes between the inner two meningeal layers (arachnoid and pia) which are together called the leptomeninges.
This article will discuss the anatomy and function of the cranial and spinal meninges.
|Definition||Three membranous layers that envelop the brain and the spinal cord|
|Meninges and meningeal spaces||Meninges: Dura mater, arachnoid mater, pia mater
Meningeal spaces: Epidural space, subdural space, subarachnoid space
|Function||Mechanical protection of brain and spinal cord, support of cerebral and spinal blood vessels, passage of the cerebrospinal fluid (CSF)|
- Dura mater
- Arachnoid mater
- Pia mater
- Spinal meninges
- Meningeal spaces
- Clinical relations
The cranial dura mater is the outermost meningeal layer, consisting of dense irregular connective tissue. It is composed of two layers;
- The superficial layer is the periosteal cranial dura It overlies the inner surface of diploë, acting like the periosteal layer of the cranium.
- The meningeal cranial dura, which lies superficial to the arachnoid mater.
The two dural layers are firmly attached to each other, except in places where they separate to enclose the dural venous sinuses. In these places, the meningeal layer projects inward, towards the cerebral tissue, forming the fibrous septa that partially separate the cranial cavity. The fibrous septa within the cranium are the:
- Falx cerebri, which is the largest of fibrous septa. It extends across the midline on the inner surface of the calvaria, from crista galli to the internal occipital protuberance. It separates the left and right cerebral hemispheres and houses the superior sagittal and inferior sagittal sinuses. Posteriorly, the falx blends with tentorium cerebelli.
- Tentorium cerebelli, which spans in a transverse plane from the inner surface of the occipital bone. It separates the cerebrum from the cerebellum and contains the transverse, straight and superior petrosal sinuses. The tentorium divides the intracranial space into supratentorial and infratentorial compartments that contain the forebrain and hindbrain, respectively.
- Falx cerebelli, which projects from the midline of the occipital bone. It separates the hemispheres of the cerebellum and houses the occipital sinus.
- Diaphragma sellae, which is a flat membrane that surrounds the pituitary stalk and forms a roof over the hypophyseal fossa. It contains the anterior and posterior intercavernous sinuses.
The meningeal dura mater overlies the trigeminal ganglion, enclosing it in a compartment known as the trigeminal cave (Meckel’s cave).
Struggling to understand the dural septa? Learn the parts of the brain with our diagrams and quizzes!
The cranial arachnoid mater is a spiderweb-like meningeal layer, interposed between the dura and pia. The potential space between the arachnoid and dura is called the subdural space and according to some authors, it contains a very thin layer of fluid. The space between the arachnoid and pia is called the subarachnoid space and it is filled with the cerebrospinal fluid (CSF). Additionally, all cerebral arteries and veins are located in this space.
The outer surface of the arachnoid attaches to the dura mater forming a barrier that prevents the leakage of CSF into the subdural space. At the sites where dura forms the venous sinuses, the arachnoid shows mushroom-like protrusions called the arachnoid granulations. The inner surface of arachnoid shows thin fibrous projections called the arachnoid trabeculae that traverse the subarachnoid space and attach to the outer surface of the pia mater. Due to their embryological and cellular similarities the pia mater and arachnoid together are referred to as the leptomeninges.
The arachnoid granulations (Pacchionian bodies) are protrusions of the arachnoid mater that pierce the meningeal dura and protrude into the lumina of the dural venous sinuses. The core of each arachnoid granulation is continuous with the subarachnoid space, therefore, containing the cerebrospinal fluid.
The CSF diffuses through the lining of the arachnoid granulations into the dural venous sinuses. Therefore, the function of the arachnoid granulations is to enable the continuous drainage of the cerebrospinal fluid from the subarachnoid into the vascular system. It is important that the CSF drainage is held in balance with the production of new CSF from the choroid plexus, warranting a constant amount of the CSF in the brain (normally around 150 milliliters). Since the skull is a rigid case, any increase in the amount of CSF in the brain increases the intracranial pressure and can cause various neurological disorders (e.g. hydrocephalus)
The arachnoid mater does not follow precisely the contours of the brain. Moreover, in certain sites, it completely separates from the pia mater, thus forming the expansions of the subarachnoid space, called the subarachnoid cisterns. There are 10 major subarachnoid cisterns of the brain that are continuous with each other through the general subarachnoid space. These include the:
- Cisterna magna (cerebellomedullary cistern)
- Pontine cistern
- Chiasmatic cistern
- Quadrigeminal cistern
- Interpeduncular cistern
- Ambient cistern
- Crural and carotid cisterns
- Cistern of lateral cerebral fossa (Sylvian cistern)
- Cerebellopontine cistern
- Cistern of lamina terminalis
Learn more about the subarachnoid cisterns with our video tutorials, quizzes, articles, and labelled diagrams.
The cranial pia mater is a highly vascular membrane that closely follows the contours of the brain. It doesn’t lie on the surface of the brain directly but rather is separated from it by a thin space called the subpial space formed by the end-feet of the astrocytes (glia limitans). Many superficial blood vessels of the brain are related to the pia mater. However, given that it is a very thin membrane, these blood vessels are partially embedded within the thickness of the pia, while partially are suspended by the arachnoid trabeculae.
The function of the pia mater is to physically separate the neural tissue from the blood vessels in the subarachnoid space, adding to the efficacy of the blood-brain barrier. Furthermore, it contributes to the degradation of the neurotransmitters, preventing their prolonged action on the nervous tissue.
Spinal dura mater
The dura mater of the spinal cord differs from that of the brain by having only one layer; the meningeal layer. The periosteal layer is missing because the vertebral canal, unlike the skull, has its own, true periosteum. The spinal dura mater attaches to the tectorial membrane and posterior longitudinal ligament superiorly. Inferiorly, it extends up to S2 vertebral level, thus extending below the spinal cord termination (L1/L2).
The space between the spinal dura mater and the periosteum of the vertebral column is called the epidural space. It is filled with loose connective and adipose tissues, and traversed by the anterior and posterior internal vertebral venous plexuses.
Spinal arachnoid mater
The arachnoid mater of the spinal cord is continuous with that of the brain. It lies close and beneath the spinal dura, with a narrow subdural space existing between them. Deep to the arachnoid is the spinal pia mater. Between arachnoid and pia, there is the spinal subarachnoid space. This space expands at the level of the conus medullaris of the spinal cord, forming the lumbar cistern.
The lumbar cistern extends from L1-S2 and it contains the dorsal and ventral rootlets of L2-Co spinal nerves (cauda equina). It is clinically significant as it is the site of lumbar puncture (extraction of CSF for biochemical, microbiological and cytological analyses or application of certain medicine).
Spinal pia mater
The spinal pia mater continues onto the cranial pia at the level of the foramen magnum. It closely envelops the spinal cord, containing a vascular plexus for the spinal cord tissue. From the apex of the conus medullaris, the pia mater gives off a fibrous projection called the filum terminale. The filum terminale extends around 20 centimeters downwards and attaches to the periosteum of the first coccygeal vertebra.
Starting from the level of the foramen magnum to the level of vertebra T12, the spinal pia shows 21 pairs of ligamentous lateral projections that pass through the arachnoid and attach to the spinal dura mater. These projections are called the denticulate ligaments. Each pair of denticulate ligaments is located halfway between the successive pairs of the spinal nerves. The function of denticulate ligaments is to position and hold the spinal cord in place.
The meningeal spaces are the spaces between the meningeal layers. There are three clinically significant meningeal spaces; epidural, subdural, and subarachnoid. We have described the anatomy of each of the spaces in the text above, however, we’d like to recap the most important facts and sum them up in the following paragraphs.
“Epi” is a prefix indicating that something is “above”. Thus, it should come easy to remember that the cranial epidural space is a potential space between the superficial layer of dura mater and the calvarium. On the other hand, the spinal epidural space is located between the spinal dura mater and the tissues that line the vertebral canal.
The spinal epidural space is a site of applying the local epidural anesthesia. The procedure may be performed at any vertebral level, and the choice depends on the body region that is desired to be anesthetized for an upcoming surgical/obstetric procedure. The applied anesthetics (e.g. lidocaine) anesthetize the local spinal nerve rootlets resulting in analgesia (pain relief).
“Sub” is a prefix that tells us that something is “below”. So, the subdural space is a potential space between the dura mater and the underlying arachnoid mater. The spinal subdural space is continuous with the cranial subdural space. They’re both very narrow and likely contain a thin film of fluid.
The subarachnoid space is a space between the arachnoid and pia mater. The subarachnoid space contains cerebrospinal fluid (CSF) and major blood vessels and provides expansions known as cisterns. The subarachnoid spaces of the cranium and vertebral column are continuous with each other, creating a closed route for the CSF circulation. Let’s recap the route of the cerebrospinal fluid in order to understand the continuity of the subarachnoid space;
- The CSF is formed by the cells of the choroid plexus within the walls of the brain ventricles. The fluid passes from the lateral to the third ventricle, and then to the fourth ventricle.
- From the fourth ventricle, the CSF passes into the central canal of the spinal cord and into the interpeduncular and quadrigeminal subarachnoid cisterns.
- The CSF then reaches the subarachnoid space of the brain and spinal cord, circulating through them.
- Finally, the CSF is reabsorbed into the dural venous sinuses by diffusing through the subarachnoid granulations in the cranial subarachnoid space.
Epidural hemorrhage is a rare condition caused by the rupture of one of the meningeal vessels. Usually, the source of bleeding is the middle meningeal artery, or rarely, one of the dural venous sinuses. Epidural bleeding usually occurs due to a fracture of the parieto-temporal region of the skull which often causes a rupture of the aforementioned blood vessels. The hemorrhage separates the periosteal dura mater from the skull, forming the epidural hematoma between the calvaria and dura mater.
The epidural bleeding usually affects the younger population, since with age, the periosteal dura gets more firmly attached to the diploë. The epidural hematoma is diagnosed with CT, on which it is seen as a biconvex hyperdense formation. The biconvex form is usually caused by a firm attachment of the dura to the cranial sutures, limiting the bleeding. If treated surgically and on time, the individual usually recovers fully within a couple of days.
Subdural hemorrhage refers to the process of collecting the blood within the subdural space, between the dura and arachnoid mater. It is usually caused by a traumatic injury to the head and rupture of the bridging veins that connect the superficial veins of the brain with the dural venous sinuses. The subdural hematoma is usually located in the convexities of the skull. They are usually diagnosed by head CT, in which they are seen as high-density crescent shaped formations. Subdural hematoma are divided into acute and chronic, depending on the timeframe of their formation.
- Acute subdural hematoma is mostly caused by a strong force that rips the bridging veins, has progressive growth and is often associated with underlying brain injury. If it’s compressing the brain it is surgically treated by evacuation. Small asymptomatic cases, however, may be managed expectantly.
- Chronic subdural hematoma can result from even an insignificant or weak trauma of the head. It usually occurs in the elderly. Other risk factors are alcohol abuse, seizures, CSF shunts, coagulopathies and patients at risk for falls (e.g. hemiparesis from a stroke). The patients frequently don’t even remember any trauma that might have caused the subdural bleeding, as it usually happens during falls which they don’t perceive as significant.The chronic subdural bleeding and the formation of the hematoma is quite slow. The symptoms appear only once the hematoma reaches a critical size so it starts pressing the underlying brain tissue. The chronic subdural bleeding is diagnosed usually with a CT, on which it is seen as a crescent-shaped formation with a fibrous capsule. The treatment is the same as for the acute subdural hematoma, i.e. neurosurgical procedure.
Subarachnoid hemorrhage (SAH) is the most common post-traumatic bleeding. Note that it can also be non-traumatic, related to the rupture of an intracranial aneurysm or arterial malformation.
The key symptom of this condition is a sudden onset of a headache which patients describe as the worst they’ve ever had in their life. Other symptoms include deterioration of consciousness (confusion, stupor or even coma) and positive meningeal signs. It is diagnosed with Angio-CT, classical angiography, or lumbar puncture. The treatment is oriented towards three key principles;
- Support of vital functions - breathing and heart rate.
- Treating the cause of bleeding - neurosurgical closing of the ruptured blood vessel.
- Avoiding vasospasm due to the degradation of the blood and reflexive vasoconstriction.