Meninges of the brain and spinal cord

The meninges are three membranes that surround the brain and spinal cord and separate them from the skull and spinal walls. The meninges are called the cranial meninges that surround the brain and the spinal meninges that surround the spinal cord, depending on their location. However, the cranial and spinal meninges are continuous with each other and consist of the same three meninges. Superficial to deep meninges are:

Dura mater, also known as Pachymeninx

arachnoid mater

pia mater

These layers delimit three clinically important potential areas: the epidural, subdural, and subarachnoid spaces. The function of the meninges is to protect the brain and spinal cord from mechanical trauma, to support blood vessels, and to create a continuous space through which cerebrospinal fluid (CSF) passes. Specifically, CSF passes between the inner two meningeal layers (arachnoid and pia), which together are called the leptomeninges.

dura mater

The dura mater is the outermost meningeal layer composed of dense irregular connective tissue. It consists of two layers;

The superficial layer is the periosteal cranial dura. It covers the inner layer of the cranial dome bones, acting as the periosteal layer of the skull.

Meningeal cranial dura extending superficially to the arachnoid mate.

The two dural layers are tightly bound together, except where they separate to close the dural venous sinuses. In these places, the meningeal layer extends inward into the brain tissue, forming fibrous septa that partially separate the cranial cavity. Fibrous septums within the skull are:

Falx cerebri, the largest of the fibrous septa. It extends along the midline on the inner surface of the calvaria, from the crista galli to the inner occipital protuberance. It separates the left and right cerebral hemispheres and houses the superior sagittal and inferior sagittal sinuses. Behind, the falx blends with the tentorium cerebelli.

Tentorium cerebelli, which extends in a transverse plane from the inner surface of the occipital bone. It separates the cerebrum from the cerebellum and includes the transverse, straight, and superior petrosal sinuses. The tentorium divides the intracranial cavity into supratentorial and infratentorial sections, which contain the forebrain and hindbrain, respectively.

Falx cerebelli protruding from the midline of the occipital bone. It separates the cerebellum hemispheres and houses the occipital sinus.

The diaphragm sella is a flat membrane that surrounds the pituitary stalk and forms a roof over the pituitary fossa. It includes the anterior and posterior intercavernous sinuses.

The meningeal dura mater lies above the trigeminal ganglion and surrounds it by a chamber known as the trigeminal cave (Meckel's cave).

arachnoid mater

The cranial arachnoid mater is a spider web-like meningeal layer located between the dura and the pia. The potential space between the arachnoid and dura is called the subdural space and contains a very thin layer of fluid, according to some authors. The space between the arachnoid and the pia is called the subarachnoid space and is filled with cerebrospinal fluid (CSF). In addition, all cerebral arteries and veins are located in this space.

The outer surface of the arachnoid adheres to the dura mater and forms a barrier that prevents CSF from leaking into the subdural space. In the areas where the dura forms the venous sinuses, the arachnoid shows mushroom-like projections called arachnoid granulations. The inner surface of the arachnoid shows fine fibrous projections called arachnoid trabeculae that cross the subarachnoid space and attach to the outer surface of the pia mater. Because of their embryological and cellular similarities, the pia mater and arachnoid are collectively referred to as the leptomeninges.

Arachnoid granulations (Pacchionian bodies) are protrusions of the arachnoid mater that pierce the meningeal dura and protrude into the lumen of the dural venous sinuses. The nucleus of each arachnoid granulation is continuous with the subarachnoid space, therefore it contains the cerebrospinal fluid.

CSF diffuses from the lining of the arachnoid granulations into the dural venous sinuses. Therefore, the function of arachnoid granulations is to provide continuous drainage of cerebrospinal fluid from the subarachnoid to the vascular system. It is important that CSF drainage is kept in balance with the production of new CSF from the choroid plexus, which ensures a constant amount of CSF (normally about 150 milliliters) in the brain. Since the skull is a rigid state, any increase in the amount of CSF in the brain increases intracranial pressure and can cause various neurological disorders (eg hydrocephalus).