In the space between the arachnoid mater and the pia mater (called the "subarachnoid space"), there is cerebrospinal fluid (CSF). CSF is a special fluid which bathes the brain and spinal cord. The meninges and the CSF work together to cushion and protect the central nervous system (the brain and spinal cord).
Layers of the meninges[change | change source]
Pia mater[change | change source]
The pia mater (or "pia") is the layer of the meninges that is closest to the brain and spinal cord. It is a thin, delicate layer ("pia mater" means "tender mother" in Latin). The pia mater sticks very closely to both the brain and spinal cord. The pia mater is made of cells that fluid cannot get through. This prevents cerebrospinal fluid from reaching the brain.
The smallest type of blood vessels, called capillaries, go through the pia mater and bring the brain the things it needs, like blood and oxygen. The pia's capillaries also make up an important part of the blood-brain barrier. This is the brain's "security system." It controls what can leave the bloodstream and get into the brain's nerve cells, and what cannot.
The pia's capillaries are lined with cells that are packed very closely together. Only certain things can get through these cells and reach the brain. This helps block things like bacteria and some poisons from getting into the brain. It also lets in the things that the brain needs to survive, like glucose (sugar), water, and white blood cells (which fight infection).
Arachnoid mater[change | change source]
The arachnoid mater is the middle layer of the meninges. It is a thin, clear membrane that fits loosely over the pia mater. This leaves a space in between these two layers called the "subarachnoid space." ("Subarachnoid space" means "the space under the arachnoid.") Cerebrospinal fluid flows through the subarachnoid space.
One of the arachnoid layer's most important jobs is to cushion the brain. Like the pia mater, the arachnoid mater is made of cells that fluid cannot get through. Because fluid cannot get through either of these layers, the cerebrospinal fluid stays in the subarachnoid space and does not leak out. Having this layer of fluid helps protect the brain, like a car's airbag. Without this cushion, every time a person moved their head, their brain would hit the inside of the skull and would get hurt.
The arachnoid mater also has an important job in the blood-brain barrier. Blood runs through the dura mater, the last layer of the meninges. The arachnoid mater keeps blood from getting through into the cerebrospinal fluid. This is important, because if blood gets through the arachnoid mater into the cerebrospinal fluid, the brain and spinal cord can get irritated and infected. Like all parts of the body, the arachnoid mater needs blood and oxygen to survive. Small blood vessels bring these things to the arachnoid layer. However, the arachnoid layer prevents the blood from getting through to the cerebrospinal fluid.
The arachnoid mater is attached to the last layer of the meninges: the dura mater.
Dura mater[change | change source]
The dura mater (or "dura") is the layer of the meninges that is closest to the skull and the spinal cord. It is a thick, protective layer. ("Dura mater" means "tough mother" in Latin.) The dura protects the brain from being scraped and hurt by the skull, which is very rough.
The dura mater has the largest blood vessels of all three layers of the meninges. The veins in the dura carry blood from the brain to the heart after the brain has used up the oxygen in the blood. The arteries in the dura carry oxygen-rich blood from the heart. These arteries split into the capillaries in the pia mater.
Mnemonic[change | change source]
- Pia mater: closest to the brain and spinal cord
- Arachnoid mater: middle layer
- Dura mater: farthest from the brain and spinal cord
Problems with the meninges[change | change source]
Because the meninges do many important things for the central nervous system, problems with the meninges can be very dangerous. The most common problems with the meninges are caused by infections or bleeding in the meninges.
Bleeding problems[change | change source]
When blood vessels in the meninges break or are injured, those blood vessels will cause bleeding in the meninges. Bleeding in the meninges is a type of stroke called a hemorrhagic stroke. ("Hemorrhagic" means "caused by dangerous bleeding (hemorrhage)"). Bleeding in the meninges is very dangerous, because if enough blood builds up, the blood can squeeze or crush the brain.
The three most common types of bleeding problems in the meninges are epidural hematomas, subdural hematomas, and subarachnoid hemorrhages.
Epidural hematoma[change | change source]
An epidural hematoma is a collection of blood between the dura mater and the skull. ("Epidural" means "on top of the dura.") Usually, injuries to the head cause epidural hematomas by tearing arteries in the dura. Because the arteries in the dura are large, they carry a lot of blood. If one of these arteries is injured, it can bleed very quickly, and an epidural hematoma can build up almost right away.
About 15% to 20% of people with epidural hematomas die from them.
Because the dura also covers the spinal cord, an epidural hematoma can also happen between the dura and the spine. Like with the brain, the blood can squeeze the spinal cord and cause serious problems.
Subdural hematoma[change | change source]
A subdural hematoma is a collection of blood under the dura mater, between the dura and the arachnoid layer. Usually, subdural hematomas are caused by injuries to the head that tear veins. Because these veins are smaller, and because veins do not bleed as fast as arteries, subdural hematomas may not bleed as quickly as epidural hematomas.
Subdural hematomas can happen right away after a bad injury to the head. These are called acute subdural hematomas. These are the most dangerous of all head injuries, and kill about 60% to 80% of people who have them.
Subdural hematomas can also be chronic. This means they take days or weeks to happen, and are usually caused by injuries to the head that are not very bad. Sometimes, if a person has a vein in their dura that slowly leaks blood every now and then, chronic subdural hematomas can take years to build up.
Subarachnoid hemorrhage[change | change source]
A subarachnoid hemorrhage is bleeding into the subarachnoid space (the space between the arachnoid and the pia mater). ("Subarachnoid" means "under the arachnoid"; "hemorrhage" means "dangerous bleeding.") Because the subarachnoid space holds cerebrospinal fluid, bleeding here makes blood mix into the cerebrospinal fluid. The blood irritates the brain and spinal cord, and causes symptoms like a very bad headache and a stiff neck.
Subarachnoid hemorrhages are very dangerous. About half of all people with subarachnoid hemorrhages die from them. About 10% to 15% die before they even get to the hospital. Many of the people who survive have brain damage.
Infections[change | change source]
The blood-brain barrier in the meninges' blood vessels protects the brain from most pathogens (things that cause infection). Because of this, infections in the brain are not very common. This is important, because antibodies - which the body's immune system makes to fight off infections - cannot get past the blood-brain barrier into the brain. Neither can most antibiotic medications. This means that when a person does get a brain infection, the body usually cannot fight off the infection on its own, and most of the medications that doctors use to kill infections cannot get into the brain to kill the pathogens inside.
However, some viruses, bacteria, and other germs are able to get through the blood-brain barrier. Some of these pathogens cause meningitis - inflammation (swelling) of the meninges. Meningitis is a medical emergency because the meninges are so close to the brain and the spinal cord. Meningitis almost always causes a high fever, a very bad headache, and a stiff neck. If it is not treated right away, meningitis can cause serious problems - like deafness, epilepsy, and brain damage - or death. In 2013, about 303,000 people in the world died from meningitis.
Some types of bacteria called spirochaetes can also force their way through the blood-brain barrier and infect the brain. Examples include Borrelia bacteria, which causes Lyme disease, and Treponema pallidum bacteria, which causes syphilis.
Related pages[change | change source]
References[change | change source]
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