Being the most vital organ in the human body, our brain has special protective mechanisms with a unique and common function – to prevent injuries to its structures.
The skull protects the central nervous system structures from injury, but the bony structure of the skull and the vertebral column are not the only protective formation of the cerebral cortex. Namely, the brain and the spinal cord are additionally protected by the cerebrospinal fluid, the blood-brain barrier, and the meninges.
Located between the brain and the skull, as well as between the spinal cord and the spinal vertebrae, the brain meninges are three-layered tissue envelopes that have a protective, supportive, and metabolic role. The capsule-like thick membranes are labeled as dura mater (closest to the bone), arachnoid mater (around the brain), and pia mater (closely attached to the brain and the surface of the spinal cord).
What Is the Main Function of the Meninges?
The meninges protect the structures of the central nervous system by housing a fluid-filled space that serves as a cushion against any external trauma or injury. They also function as a framework of blood vessels, carrying nutrition and oxygen to all parts of the brain.
What Is the Anatomy and Structure of the Meninges?
The meninges are composed of loose and dense connective tissues, blood vessels, and are also lined with flattened cells. These meningeal cells participate in the control of the development of the cerebellum. They trigger the branching of neurons and activate the radial glial cells by supporting various cellular interactions.
The meninges of the brain and the spinal cord are comprised of:
- Dura mater
- Arachnoid mater
- Pia mater
Dura Mater
The dura mater (pachymeninx) is the strongest protective meninx of the brain that’s composed of elongated fibroblasts and large amounts of collagen fibrils. It houses venous sinuses that carry the blood from the brain back to the heart.What Are the Layers of the Dura Mater?
This meninx of the brain and the spinal cord is composed of 2 layers:
- Endosteal layer (which covers the inner surface of the skull bones)
- Meningeal layer (which covers the brain and forms folds)
What Are the Major Folds of the Dura Mater?
The dura mater can be divided into 4 areas that support the brain, according to the folds it creates:
- Falx cerebri – It’s the largest dural infolding, located in the longitudinal cerebral fissure that lies between the right and left cerebral hemispheres.
- Tentorium cerebelli – The tentorium cerebelli separates the cerebrum from the cerebellum and brain stem. It provides protection against the pressure caused by the heavier upper part of the brain.
- Falx cerebelli – It lies in the middle portion of the brain, located between the two cerebral hemispheres. The falx cerebelli contains three sinuses: an upper sagittal sinus in the upper margin, a lower sagittal sinus in the back part of lower free margin, and a straight sinus in the lower margin attached to the tentorium cerebelli.
- Diaphragma cellae – This two-layer sheet of dura mater with a circular hole allows the vertical passage of the pituitary stalk. The diaphragma cellae fold of dura matter makes up the roof of the sella turcica.
In addition, the dura mater divides the cranial cavity into different areas and aids the functions of different parts of the brain.
What Are the Venous Sinuses of the Dura Mater?
The spaces between the two layers of the dura mater created by the dural foldings are called the dural venous sinuses. Their main function is to collect the blood from the veins located on the surface of the cerebral cortex.
The group of dural venous sinuses comprises of the following structures:
- Superior sagittal sinus – Located in the superior part of the falx cerebri, this sinus drains the anterior corpus callosum, medial hemispheres, and the cingulate gyrus.
- Inferior sagittal sinus – Also labeled as the longitudinal inferior sinus, the inferior sagittal sinus runs along the bottom of the falx cerebri and drains the medial cortical structures of the brain.
- Straight sinus – This dural venous sinus is located at the merge point of the inferior sagittal sinus with the great cerebral vein.
- Transverse sinuses – The transverse (or lateral sinuses) sinuses extend from the petrous temporal bone, and morph with the sigmoid sinuses once they reach the temporal bone.
- Sigmoid sinuses – They make an S-shaped turn and pass through the jugular foramen. From this point on, they are labeled as internal jugular veins.
- Occipital sinus – The occipital sinus is an unpaired venous sinus, and stretches along the inner surface of the occipital bone. It drains the foramen magnum and the internal vertebral venous plexuses.
- Cavernous sinuses – Located on each side of the sella turcica of the sphenoid bone, the cavernous sinuses receive blood from the inferior ophthalmic veins, superior ophthalmic vein, superficial middle cerebral vein, basilar plexus, inferior cerebral vein, and the sphenoparietal sinus. They drain into the superior and inferior petrosal sinuses.
- Superior petrosal sinuses – These two paired dural venous sinuses arise from the sinus cavernosus and drain into the transverse sinus.
- Inferior petrosal sinuses – Running from the back part of the cavernous sinuses, the inferior petrosal sinuses are two small sinuses that drain into the internal jugular veins.
Arachnoid Mater
The arachnoid mater (leptomeninx) refers to the middle, vascular web-like layer of the brain’s protective structures, attached to the inner surface of the dura mater. It envelops both the brain and the spinal cord.
The Arachnoid Cisternae
In some areas, the arachnoid mater forms subarachnoid cisternae, that are filled with cerebrospinal fluid and freely communicate with each other.
There are two important subarachnoid cisternae:
- Cisterna Cerebellomedullaris (located between the lower surface of the cerebellum and the fourth ventricle)
- Cisterna interpeduncular (located between the cerebellar peduncles)
The Arachnoid Granulations
The arachnoid granulations, also known as Pacchionian bodies, are branches of the arachnoid mater that protrude the meningeal dura, reaching to the lumina of the dural venous sinuses. Since the core of the arachnoid granulations is continuous with the subarachnoid space, they are also filled with the cerebrospinal fluid.
The arachnoid granulations are mainly in charge of allowing continuous drainage of the cerebrospinal fluid from the subarachnoid system into the vascular system.
Pia Mater
The pia mater (leptomeninx) is the deepest layer of the meninges that is loosely wrapped over the brain and spinal cord. It’s separated from these structures by a thin space referred to as the subpial space.
This meninx is composed of a large number of blood vessels that supply the nervous tissue of these two cerebral cortex structures with oxygen and nutrients. Even though it’s as vascular in structure as the arachnoid mater, the pia mater forms a membrane around the brain and spinal cord instead of a cobweb-like structure.
The pia mater functions as a barrier that prevents external traumas and injuries to the brain, and also supports the production of cerebrospinal fluid in the brain. Moreover, it physically separates the neural tissue from the blood vessels in the subarachnoid space.
Meningeal Spaces
The spaces among the meninges can be subdivided into:
- Epidural Space of the Meninges
- Subdural Space of the Meninges
- Subarachnoid Space of the Meninges
Epidural Space of the Meninges
The potential anatomic space (meaning: its height is zero) between the dura mater layer is labeled as the epidural space of the meninges. It’s comprised of lymphatics, spinal nerve roots, loose fatty tissue, small arteries, as well as a network of internal vertebral venous plexuses.
The epidural space can be divided into a cranial epidural space and a spinal epidural space. It is composed of spinal arteries, fat, veins, and lymphatics. Its veins are a part of the large internal vertebral venous plexus, which is connected to the occipital, sigmoid, and basilar sinuses located above the cranium.
The spinal epidural space is the location where local epidural anesthesia is applied for a surgical/obstetric procedure. The applied anesthetics anesthetize the local spinal nerve roots, which results in pain relief.
Subdural Space of the Meninges
The arachnoid villus protrudes from the arachnoid mater, allowing the cerebrospinal fluid to exit the brain and flow into the upper sagittal sinus, which is filled with blood. This fluid-filled space is called subdural space.
Unlike the potential epidural space, the subdural space is a real space that follows the contours of the brain.
Subarachnoid Space of the Meninges
The void space between the arachnoid and pia meninges is called a subarachnoid space. It is filled with the cerebrospinal fluid and contains all the cerebral arteries and veins.
The cerebrospinal fluid protects and nourishes the brain and spinal cord while it flows around them. It carries the proteins and glucose that provide energy for the proper function of the brain cells. It also contains lymphocytes that guard the brain structures against infection.
What Are the Subarachnoid Cisterns of the Meninges?
The subarachnoid meningeal space includes several cisterns that function as reservoirs, i.e. areas where the space between the pia and arachnoid mater is widened due to an accumulation of cerebrospinal fluid.
There are 10 major subarachnoid cisterns of the brain:
- 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
What Happens If an Injury Affects the Meninges?
Owing to the fact that a large number of blood vessels travel through the meninges, many of the medical problems and conditions are of vascular nature. The most common ones are:
- Subarachnoid Hemorrhage
- Subdural Hematoma
- Epidural Hematoma
- Meningitis
- Meningiomas
- Meningeal Carcinomatosis (Tumors)
Fun Facts
Did you know?
- Our deepest brain-protective layer is the pia mater which is a very delicate connective tissue. Its Latin translation is “soft mother”, while the translation of dura mater means “durable mother.”
- The brain is protected from injuries and external traumas by 4 structures. Those are the skull, the meninges, the cerebrospinal fluid, and the blood-brain barrier.
- The singular form of meninges is meninx. This term is derived from the Ancient Greek word μῆνιγξ, meaning “a membrane”.
- The dura mater is the most superficial meninx of all meningeal barriers of the brain.
- Without the support of the tentorium cerebelli or the falx cerebri, the brain would lose its firm structure and sag.
- The Latin meaning of the tentorium cerebelli is “a tent of the cerebellum”.
- The “falx” in falx cerebelli stands for “a structure shaped like a sickle”.
- The sella turcica is a midline structural niche in the sphenoid bone that contains the pituitary gland.
- The trigeminal cave (also known as the Meckel’s cave) is a pouch created between the dura mater and the trigeminal ganglion that is filled with cerebrospinal fluid. It serves as a passage of the largest cranial nerve, the trigeminal cranial nerve 5 (CN V).
- The pia mater and arachnoid together are labeled as the leptomeninges.