The meninges is a layered unit of membranous connective tissue that covers the brain and spinal cord. These coverings encase central nervous system structures so that they are not in direct contact with the bones of the spinal column or skull. The meninges are composed of three membrane layers known as the dura mater, arachnoid mater, and pia mater. Each layer of the meninges serves a vital role in the proper maintenance and function of the central nervous system.
The meninges functions primarily to protect and support the central nervous system (CNS). It connects the brain and spinal cord to the skull and spinal canal. The meninges forms a protective barrier that safeguards the sensitive organs of the CNS against trauma. It also contains an ample supply of blood vessels that deliver blood to CNS tissue. Another important function of the meninges is that it produces cerebrospinal fluid. This clear fluid fills the cavities of the cerebral ventricles and surrounds the brain and spinal cord. Cerebrospinal fluid protects and nourishes CNS tissue by acting as a shock absorber, by circulating nutrients, and by getting rid of waste products.
- Dura Mater: This outer layer connects the meninges to the skull and vertebral column. It is composed of tough, fibrous connective tissue. Dura mater that surrounds the brain consists of two layers. The outer layer is called the periosteal layer and the inner layer is the meningeal layer. The outer periosteal layer firmly connects the dura mater to the skull and covers the meningeal layer. The meningeal layer is considered the actual dura mater. Located between these two layers are channels called dural venous sinuses. These veins drain blood from the brain to the internal jugular veins, where it is returned to the heart. The meningeal layer also forms dural folds that divide the cranial cavity into different compartments, which support and house various subdivisions of the brain. Cranial dura mater forms tubular sheaths that cover cranial nerves within the skull. The dura mater of the spinal column is composed of the meningeal layer and does not contain a periosteal layer.
- Arachnoid Mater: This middle layer of the meninges connects the dura mater and pia mater. The arachnoid membrane loosely covers the brain and spinal cord and gets its name from its web-like appearance. The arachnoid mater is connected to the pia mater through tiny fibrous extensions that span the subarachnoid space between the two layers. The subarachnoid space provides a route for the passage of blood vessels and nerves through the brain and collects cerebrospinal fluid that flows from the fourth ventricle. Membrane projections from the arachnoid mater called arachnoid granulations extend from the subarachnoid space into the dura mater. Arachnoid granulations remove cerebrospinal fluid from the subarachnoid space and send it to the dural venous sinuses, where it is reabsorbed into the venous system.
- Pia Mater: This thin inner layer of the meninges is in direct contact with and closely covers the cerebral cortex and spinal cord. The pia mater has a rich supply of blood vessels, which provide nutrients to nervous tissue. This layer also contains the choroid plexus, a network of capillaries and ependyma (specialized ciliated epithelial tissue) that produce cerebrospinal fluid. The choroid plexus is located within the cerebral ventricles. Pia mater covering the spinal cord is composed of two layers, an outer layer consisting of collagen fibers and an inner layer that encases the entire spinal cord. Spinal pia mater is thicker and less vascular than pia mater that covers the brain.
Problems Related to the MeningesThis brain scan shows a meningioma, a tumor that develops in the meninges. The large, yellow and red mass is the meningioma. Science Photo Library - MEHAU KULYK/Brand X Pictures/Getty Images
Due to its protective function in the central nervous system, problems that involve the meninges can result in serious conditions.
Meningitis is a dangerous condition that causes inflammation of the meninges. Meningitis is typically precipitated by an infection of the cerebrospinal fluid. Pathogens such as bacteria, viruses, and fungi can induce meningeal inflammation. Meningitis may result in brain damage, seizures, and can be fatal if not treated.
Damage to blood vessels in the brain can cause blood to collect in brain cavities and brain tissue forming a hematoma. Hematomas in the brain cause inflammation and swelling that can damage brain tissue. Two common types of hematomas that involve the meninges are epidural hematomas and subdural hematomas. An epidural hematoma occurs between the dura mater and the skull. It is typically caused by damage to an artery or venous sinus as a result of severe trauma to the head. A subdural hematoma occurs between the dura mater and arachnoid mater. It is usually caused by head trauma that ruptures veins. A subdural hematoma can be acute and develop rapidly or it can develop slowly over a period of time.
Meningiomas are tumors that develop in the meninges. They originate in the arachnoid mater and put pressure on the brain and spinal cord as they grow larger. Most meningiomas are benign and grow slowly, however, some may develop rapidly and become cancerous. Meningiomas can grow to become very large and treatment often involves surgical removal.