Meninges (singular is meninx) is the collective term for the three membranes covering the brain and spinal cord. The meninges are composed of the dura mater (outer), the arachnoid (middle), and the pia mater (inner). In common usage, the membranes are often referred to as simply the dura, pia, and arachnoid.
Description
Dura is the Latin word for hard, while pia in Latin means soft. The dura mater was so-named because of its tough, fibrous consistency. The pia mater is thinner and more delicate than the dura mater, and is in direct contact with the neural tissue of the brain and spinal cord. Along with the arachnoid layer and the cerebrospinal fluid (CSF), the dura and pia membranes help cushion, protect, and nourish the brain and spinal cord.
Mater is Latin for mother, and thus refers to the membranes' protective and nourishing functions. Each of the meninges can also be classified as to the portion that covers the brain (e.g., dura mater cerebri or dura mater encephali), or that portion lining the spinal cord (e.g., pia mater spinalis). Arachnoid means "spidery," referring to the membrane's webbed appearance and consistency. The space between the arachnoid membrane and pia mater contains many fibrous filaments and blood vessels that attach the two layers.
Anatomy
The outer surface of the dura adheres to the skull, while the inner surface is loosely connected to the arachnoid layer. The exception is the spinal canal, where there is normally a thin layer of fat and a network of blood vessels between the dura and the bony portion of the vertebrae. There is normally no space between the dura and skull on one side, and the dura and arachnoid on the other. However, these are sometimes called "potential" spaces because abnormal conditions may create "actual" spaces there. Anything in the space between the dura and skull is called epidural (above the dura), while the space between the dura and arachnoid is considered subdural (below the dura).
There is normally an actual space between the arachnoid layer and the pia mater known as the subarachnoid space. As noted, it contains many fibrous filaments, known as trabeculae (little beams), joining and stabilizing the two layers. The importance of the subarachnoid space is that it contains the circulating CSF. It is this layer of fluid that helps to cushion the brain and protect it from sudden movements and impacts to the skull.
The pia mater has the appearance of a thin mesh, with a network of tiny blood vessels interlacing it. It is always in contact with the neural tissue of the brain and spinal cord, much like a skin. It follows all of the grooves, folds, and fissures of the brain's various lobes and prominences.
All of the meninges are composed of connective tissue, which is made up of relatively few cells, with an abundance of structural and supportive proteins.
Function
Given the singular importance of the central nervous system (CNS) to both basic and higher-level functions of the body, it is not surprising that a system evolved to help protect it. Thicker skull bones would certainly afford more protection against skull fracture and open head injury, but would come at the cost of greater weight for the spine to bear. If the head is struck, or strikes some other object, even unbreakable skull bones would not protect the brain from the injury that results as brain tissue impacts the inside of the skull (concussion). The layer of CSF that circulates in the subarachnoid space helps to lower this risk, although it cannot eliminate it. Wearing a sports helmet composed of a hard, plastic outer shell with firm padding inside simply mimics and augments the safety mechanism already present in the skull and outer lining of the brain.
The dura mater is the tough, but flexible, second line of defense for the brain after the skull. The flexibility of the dura is important in that most skull fractures, other than those involving severe penetrating injuries, will not result in loss of CSF through the injury site which, before the days of antibiotics and emergency medicine, would pose a serious risk for infection and death.
The arachnoid membrane provides a stable substrate and space through which the CSF can circulate, and also provides specialized tissue necessary for absorption of the CSF back into the bloodstream. The arachnoid trabeculae help to anchor the surrounding membranes and keep the subarachnoid space at a constant depth.
While the CSF is normally sterile and mostly inert—containing glucose, proteins, electrolytes (necessary minerals), and very few cells—the brain and spinal neurons nonetheless need some protection from direct contact with the fluid, which is provided by the pia mater. As blood vessels pass through the dura mater and then the subarachnoid space, they pierce the pia mater as they enter the CNS. The membrane follows the blood vessel down and becomes the external portion of the blood vessel wall.
CSF Production and Circulation
In a sense, the CSF can be thought of as a fourth layer of the meninges. The fluid is produced in, circulates through, and is reabsorbed by the meningeal layers, thus creating a self-contained system. The volume of fluid in adults is normally 100–150 ml. About 500 ml of new fluid is produced and reabsorbed each day, which means the CSF is "turned over" three times in 24 hours. It is important for the body to maintain CSF volume within the normal range, since there is limited space within the skull and spinal column. It is also important for the fluid to remain at a constant pressure. Increased fluid pressure typically leads to compression of the surrounding neural tissue, which then leads to increased fluid volume. Since the bones of the skull are not fused in a developing fetus or newborn infant, increased fluid pressure in the brain may cause the head to grow to an abnormally large size (see Hydrocephalus), called macrocephaly. The skull bones are fused after about 2 years of age, so increased fluid pressure and volume after that point will most likely result in compression of, and damage to, neural tissue.
The CSF is produced by a layer of densely packed capillaries and supporting cells known as the choroid plexus. It lines the upper portion of the lateral (cerebral), third, and fourth ventricles. Once produced, the CSF flows down through the fourth ventricle, and then through openings at the base of the brain and around the brain stem. Some of the fluid circulates down through the subarachnoid space encircling the length of the spinal cord, while the remainder flows up to the subarachnoid space around the brain.
Most of the fluid is absorbed back into the bloodstream through vessels lining branched projections from the arachnoid membrane called arachnoid villi, or granulations. These arachnoid granulations extend into the dura, primarily at points where large blood veins lie within the dural membrane itself. These veins traveling through the dura that drain blood and absorbed CSF from the brain are collectively known as the venous sinuses of the dura mater. The remainder of the CSF is absorbed through small lymph sacs scattered around the CNS known as perineural lymphatics.
Causes and symptoms
Infection/inflammation of the meninges is covered elsewhere (see Meningitis). Other abnormalities of the meninges typically involve situations in which a fluid occupies and expands the epidural, subdural, or subarachnoid spaces. For instance, blood accumulation that separates the dura from the inner side of the skull is known as an epidural hematoma (blood swelling). The same process occurrence between the dura and arachnoid layers is a subdural hematoma. Both of these conditions are most frequently caused by head trauma, but may also result from a bleeding disorder or defect in a cranial blood vessel (aneurysm).
A hemorrhage between the arachnoid membrane and the pia mater is called a subarachnoid bleed, and is usually caused by the rupture of a congenital aneurysm, hypertension, or trauma. Unlike conditions affecting the epidural and subdural spaces, a bleed into the subarachnoid space is less likely to affect its volume and increase pressure. A subarachnoid CSF infection (abscess), however, may cause increased pressure.
Meningitis may also cause bleeding into the subdural or epidural spaces, but more often results in the accumulation of fluid and pus, which are consequences of the body's response to the infection.
BOOKS
DeMyer, William. Neuroanatomy, 2nd ed. Baltimore: Williams & Wilkins, 1998.
Walker, Pam and Elaine Wood. The Brain and Nervous System. Farmington Hills: Lucent Books, 2003.
Weiner, William J. and Christopher G. Goetz, eds. Neurology for the Non-Neurologist, 4th ed. Philadelphia: Lippincott Williams & Wilkins, 1999.
