Meninges Anatomy

Meninges: Anatomy and Clinical Relevance

The meninges refer to the membranous coverings of the brain and spinal cord. There are three layers of meninges, known as the dura mater, arachnoid mater and pia mater. These coverings have two major functions- to provide a supportive framework for the cerebral and cranial vasculature, and to act together with cerebrospinal fluid to protect the CNS from mechanical damage.

The dura mater is the outermost layer of the meninges and is located directly underneath the bones of the skull and vertebral column. It is composed of two layered sheets of connective tissue. The outer periosteal layer lines the inner surface of the bones of the cranium, while the middle meningeal layer lies deep to the periosteal layer and is continuous with the dura mater of the spinal cord. The dural venous sinuses, responsible for the venous drainage of the cranium, are located between the two layers of dura mater and empty into the internal jugular veins. The dura mater receives its own vascular supply - primarily from the middle meningeal artery and vein, and is innervated by the trigeminal nerve (V1, V2 and V3).

The arachnoid mater is the middle layer of the meninges, lying directly underneath the dura mater. It consists of layers of connective tissue, is avascular, and does not receive any innervation. Underneath the arachnoid is a space known as the sub-arachnoid space, which contains cerebrospinal fluid that acts to cushion the brain. Small projections of arachnoid mater into the dura, known as arachnoid granulations, allow cerebrospinal fluid to re-enter the circulation via the dural venous sinuses.

The pia mater is located underneath the sub-arachnoid space. It is very thin and tightly adhered to the surface of the brain and spinal cord. It is the only covering that follows the contours of the brain (the gyri and fissures), and is highly vascularised, with blood vessels perforating through the membrane to supply the underlying neural tissue.

Clinical Relevance: Meningitis

Meningitis refers to inflammation of the meninges. It is usually caused by pathogens, but can be drug induced. Bacteria are the most common infective cause, with the most common organisms being Neisseria meningitidis and Streptococcus pneumoniae.

The immune response to the infection causes cerebral oedema, consequently raising intra-cranial pressure. This has two main effects:

• Part of the brain can be forced out of the cranial cavity, a condition known as cranial herniation.
• In combination with systemic hypotension, raised intracranial pressure reduces cerebral perfusion.

Both of these complications can rapidly result in death.