How Meninges Support the Brain and Nervous System

Surrounding the brain and spinal cord are three layers of protective tissue, collectively called the meninges. Meninges are a necessary cushion between these vital organs and the cranium (or skull) and vertebrae (spine). They also prevent cerebrospinal fluid (CSF), the clear fluid the brain and spinal cord sit in, from leaking, while providing structural support for important blood vessels and nerves.   

Given their critical role, it’s little wonder that anatomical variations and health issues in the meninges can have a serious impact. Birth defects, such as spina bifida and anencephaly, as well as infection (meningitis) and bleeding (as in cerebral hematoma or hemorrhage), can lead to chronic disability or become fatal.

Anatomy

Location

Broadly speaking, the meninges, though completely interconnected, are identified by the part of the body these membranes cover. The portion that surrounds the brain is the cranial meninges. The spinal meninges, as the name suggests, surrounding the spinal cord.

Spinal meninges also encase the cauda equina, the bundle of nerves and nerve roots at the base of the spine. This includes nerves corresponding to lumbar (low back) vertebrae, as well as the sacrum (the bony, triangular structure at the base of the spine).     

Structure

Both cranial and spinal meninges are broken down into three layers, with the inner two collectively referred to as the leptomeninges:

• Outermost (dura mater): Also known as the pachymeninx or dura mater, the outermost layer is tough and fibrous. The cranial dura mater is, itself, composed of two layers: the periosteal cranial dura on the outside, and the meningeal covering the middle layers. In the spine, it consists only of a meningeal layer.   
• Middle layers (arachnoid): Resembling spider webs, the arachnoid or middle layers of the meninges project arachnoid trabeculae, which are strands of connective tissue to the innermost layer. This is membrane is marked by granulations, which are protrusions at areas of contact with the outermost layer. Unlike the innermost layer, the cranial middle layers straddle the edges of the cortical sulci, or depressions in the brain.
• Innermost (pia mater): The pia mater, or innermost membrane, follows the shape of the brain, and lines the spine. It consists of two layers: the outer (epipial) and inner (intima pia). The outer layer has stiffer fibers that support vessels that pass through the space between the innermost and middle layers. The pia intima is more elastic and attaches to the outer parts of the brain and spinal cord.     

In places, the meningeal layers are separated by gaps, and there also are spaces between the surrounding bone, as well as the enveloped brain and spinal cord. These important anatomical features are:

• Epidural space: Separating the dura mater and the bones and tissues surrounding it is the epidural space. The cranial epidural space separates the inside of the skull and the outermost layer, whereas in the spine, additional tissues line the area.   
• Subdural space: Beneath the outermost layer, and above the middle layers, you find the subdural space, which is continuous between spinal and cranial meninges. It’s a thin layer, with some suggesting it’s filled with fluid.  
• Subarachnoid space: The innermost layer of the meninges is separated from the surface of the brain and spinal cord by the subarachnoid space. Filled with cerebrospinal fluid (CSF), it allows major blood vessels to pass. In areas where the innermost layer completely separates from the middle layers, it has projections called cisterns.

Nerve and Blood Supply

The cranial meninges primarily receive blood from by the middle meningeal artery (MMA), which is a branch of the internal carotid artery, which progresses up the neck. On each side, the MMA enters the skull through an opening in its side called the foramen spinosum and continues through the epidural space.

At the same opening—and running alongside the MMA—is the meningeal branch of the mandibular nerve. Its two branches transmit signaling between the outermost meningeal layer and the brain, with its anterior branch communicating with the meningeal branch of the maxillary nerve.

Blood supply for the spinal meninges comes from a single anterior artery, as well as two paired posterior spinal arteries. Branches of the vertebral arteries, they arise at the base of the neck and take an upward course.       

Anatomical Variations

As with all parts of the body, the meninges can be significantly impacted by genetic mutations. These coding errors lead to neural tube defects, in which the meninges never fully forms. These potentially very severe diseases of the meninges include:

• Spina bifida: This condition occurs when a part of the neural tube surrounding the spine—including the meninges—never forms completely. Without adequate protection, spinal fluid-filled sacs form and even stick out from the spine. Especially in its most severe form, myelomeningocele, spina bifida can lead to skin problems, paralysis, neurological issues, incontinence, and learning disabilities, among other symptoms.    
• Anencephaly: This is another neural tube defect. It affects the development of the brain, skull, spine, and spinal cord in the embryo. Because the meninges never fully form while the fetus is still in the womb, developing neurons degenerate and die. As a result, those born with anencephaly lack significant portions of the brain, including those involved with thinking, hearing, vision, emotion, and movement. Bones of the skull may be malformed or missing.  
• Encephalocele: This is a rare condition. During the third and fourth weeks of pregnancy, malformation of the neural tube causes babies to be born with parts of the brain, meninges, or sacs of CSF poking through an opening in the skull. This sac usually protrudes from the back of the head but is able to emerge from anywhere along its centerline. Encephalocele can cause a range of symptoms, including developmental delays, seizures, microcephaly (a skull that is too small), intellectual disabilities, vision problems, and many others.

Function

The primary task of the meninges is to provide protection and support for the brain and spine. This involves:

• Structural support: The tough meningeal membranes in the spine help attach it to the surrounding spinal column, keeping the spinal cord in position and appropriately shaped. This is done by fibrous projections from the inner layer (pia), called denticulate ligaments, which pass through the middle layers and attach to the outer layer.  
• Nutrition support: The subarachnoid space—between the middle and inner layers—contains CSF, which is necessary for providing the brain and spinal cord with nutrition and removing waste.
• Protection: In providing space for CSF to flow—essentially keeping your brain and spine covered in a liquid that cushions impact—the meningeal layers help protect the central nervous system. The inner layer of the cerebral meninges serves to keep vessels separate from neural tissues. This forms an essential part of the blood-brain barrier. This structure works to prevent infections or pathogens from accessing the brain.          

Associated Conditions

When the health of the meninges is affected, the consequences can be severe. This part of the body can be affected by bleeds and blood circulation problems, infections, and cancer, among other medical conditions. Skull or spinal fracture can also significantly damage these membranes and warrants medical attention.  

Hemorrhage and Hematoma

Two closely related types of blood circulation issues that affect the meninges are hemorrhage and , hematoma. A hemorrhage occurs when there is bleeding in the brain or spine. Hematoma is a complication of a hemorrhage, where blood pools outside of blood vessels. These potentially serious bleeding problems may arise due to trauma, bone fracture, or other health conditions.

These intracranial bleeding problems are dangerous because they raise intracranial pressure (ICP), or blood pressure within the skull, which can damage the brain. These issues are categorized based on the meningeal space being affected.

Here’s a quick breakdown:

• Epidural hematoma arises when bleeding in the arteries or veins, often the MMA, causes blood to fill in the epidural space between the outer layer and the skull. It’s often associated with blunt trauma to the head or skull fracture.
• Subdural hematoma occurs when there’s bleeding in the subdural space due to tearing and hemorrhage in a blood vessel, usually a vein. A type of traumatic brain injury (TBI), it occurs in about 25% of cases of blunt trauma. This bleeding can be slow and gradual or very rapid, with the latter being a medical emergency.
• Subarachnoid hematoma is pooling of blood due to bleeding in the subarachnoid space. This can arise due to trauma or can occur due to spontaneous ruptures at weak points in vessels. This condition is often associated with cases of brain aneurysm, in which a cerebral artery (which brings oxygen from the heart to the brain) balloons or even bursts due to structural insufficiency.
• Intraparenchymal hematoma, sometimes called intracerebral hemorrhage or hematoma, is caused by bleeding in the parenchyma. This potentially fatal condition can also cause impairment of cognitive ability.     

Infection

Infection of the meninges, known as meningitis, leads to inflammation of these membranes, causing a range of significant and dangerous health effects. Meningitis is classified based on what’s causing these issues:

• Bacterial meningitis: Several types of bacteria, including two strains of streptococcus and Neisseria meningitides, can be spread and infect the meninges. Associated with sepsis (bacterial infection of the blood), inflammation due to this condition can be fatal or lead to permanent disability.
• Viral meningitis: Infection of the meninges can also occur due to non-polio enteroviruses, as well as others, including viruses that cause mumps, measles, herpes, and influenza (the flu), among others. This type tends to be milder than bacterial meningitis, with most able to recover on their own.
• Fungal meningitis: Meningitis due to fungal infection is very rare but arises when spores of cryptococcus, histoplasma, Blastomyces, or coccidioide are breathed in. Another fungus that lives in the body, candida, can also enter the bloodstream and infect the meninges.
• Parasitic and amebic meningitis: When certain infectious parasites, as found in infected eggs, produce, freshwater fish, poultry, among other foods, are ingested, noncommunicable parasitic meningitis arises. Another cause is brain infection with Naegleria fowler, a tiny amoeba, invisible to the human eyes, that lives warmer, fresh waters. Both of these types of meningitis are seldomly reported in the US.
• Non-infectious meningitis: Cancers, autoimmune disorders like lupus, and some medications (including non-steroidal anti-inflammatory drugs (NSAIDs), certain antibiotics, immunosuppressant drugs, and others) can also cause meningitis. This condition may also arise following head injury and brain surgery, which is why these patients are closely monitored during recovery. 

Tests

Many factors determine the specific approach to clinical testing and diagnosis of meningeal health conditions. Doctors may need to figure out the root causes of any infection or inflammation, assess the extent of congenital malformation, or take on hematoma and bleeding. Luckily, there are many tests and examinations they can use, including:

• Physical and neurological exams entail testing vision, balance, coordination, and cognitive function, as well as blood pressure and other vital checks. Doctors also assess medications you’re taking, lifestyle factors, as well as any family history of problems.
• Imaging, such as magnetic resonance imaging (MRI) or computerized tomography (CT or CAT) scanning may be used to let doctors identify the hematoma or hemorrhage or assess the scope of damage or bleeding.   
• Blood or CSF tests help doctors diagnoses causes of meningitis. Small samples of these fluids are taken to a lab to isolate the bacteria, virus, fungus, or other cause of the infection.
• Maternal blood alpha fetoprotein (MSAF) screening is performed at 16 to 18 weeks of pregnancy. This detects the presence of the alpha-fetoprotein (AFP) in fetuses or embryo’s blood or amniotic fluid (amniocentesis), which can be signs of spina bifida.
• Ultrasound or other imaging techniques are used during pregnancy to assess any congenital malformations of the neural tube due to spina bifida or other inborn conditions. This is especially indicated if AFP levels are higher.
• Genetic testing is considered when neural tube defects are detected. Though environmental factors are always relevant, mutations in certain genes can lead to spina bifida, among other congenital disorders. Samples of fetal or parental blood are clinically tested.

A Word From Verywell

There’s no doubt that the health of the meninges is closely associated with overall wellness. These layers are tasked with the essential job of protecting and nourishing the brain and spine. No doubt the burden due to congenital conditions, infections, injuries, or other disorders of these crucial membranes can be heavy.

But as researchers learn more about causes and effects of meningeal problems, and as doctors develop even better treatment approaches, prognoses and outlook for those suffering with them will continue improve. Management of conditions impacting the meninges requires the help of dedicated medical staff, as well as the support of family, friends and loved ones.    

If you’re concerned about meningitis, congenital issues during pregnancy, or other issues impacting this crucial part of the body, be sure to talk to your doctor.