The nerves of the spinal cord form a tube that begins at the brain and extends the entire length of the vertebral column. This cord and the brain comprise the central nervous system, a complex and delicate structure that requires many layers of protection. The meninges help provide this protection, in the form of a set of membranes that surround and support the spinal cord and brain. They are composed of three membranes, called the pia mater, the arachnoid mater and the dura mater.
Each layer of the meninges is a continuous layer, covering the entire brain and spinal cord. The pia mater is the innermost membrane and is the most delicate of the three layers. It also is the most closely fitting, running smoothly over the spinal cord and brain, following the contours of their surfaces. Within the pia mater is a network of capillaries that supply blood to the brain and spinal cord.
Covering the pia mater is the middle layer, called the arachnoid mater. This membrane layer covers the pia mater very loosely. Between the pia mater and the arachnoid mater, a layer of cerebral spinal fluid helps to further protect the brain and spinal cord by acting as a fluid cushion and shock absorber. This layer of spinal fluid is the subarachnoid space.
The outermost membrane, the dura mater, is the densest and thickest of the three, and it is the most durable of the layers. It contains a network of blood vessels that help supply blood to the other two layers of the meninges. This membrane actually is made up of two sub-layers: the periosteal layer and the inner meningeal layer. In addition to providing protection for the nervous system and blood for the inner membrane layers, the dura mater also surrounds and protects the dural sinuses. These are large channel-like structures that direct blood to veins, which return it to the heart.
Although the primary function of the meninges is to protect the central nervous system, these membranes themselves are vulnerable to certain types of injury. Head injuries can result in bleeding of the vessels of the membranous layers, potentially causing blood to pool between layers or over the surface of the brain. If sufficient blood builds up around the brain, it can cause brain damage because of increased intracranial pressure.
The meninges also are vulnerable to an infection called meningitis. This is a bacterial or viral infection that causes inflammation of the membranes. In most cases, bacterial infections are more dangerous than viral infections, because the latter tend to last only a few days. Bacterial infections, however, can persist much longer, potentially causing seizures and other life-threatening symptoms. Antibiotics can effectively treat the infection, but temporary or permanent side effects such as epilepsy, deafness and cognitive disability can result.