Synonyms
Medical: Meninx encephali
Definition
The meninges are a connective tissue layer that surrounds the brain. In the spinal canal, it merges into the spinal cord skin. The human being has three meninges. From outside to inside, these are the hard meninges (dura mater or leptomeninx encephali), and the soft meninges (pia mater or pachymeninx encephali), as well as the cobweb (arachnoidea mater), which lies between them.
Function
There are three different meninges that surround the brain and perform different tasks. In general, the meninges serve to protect the brain. The spaces between them absorb shocks and changes in volume.
They also play an important role in supplying nutrients to the brain’s nerve cells. The outer hard meninges (dura mater) mainly serve to protect the brain. It also contains blood vessels in its invaginations that drain blood from the brain.
The hard meninges contain many pain receptors, which is why they are very sensitive to pain. The so-called spider’s web skin (arachnoidea) contains many smaller blood vessels to supply the brain. In addition, it fulfils the function of exchange between the cerebrospinal fluid (liquor) and the blood.
Here, the cerebrospinal fluid is absorbed in the area of special bulges of the meninges (arachnoid villi) and passed on to the blood vessels that drain it off. The soft cerebral membrane (pia mater) is closest to the brain tissue. It serves to supply the brain tissue with nutrients.
The dura mater forms a rough skin between the skull bone and the brain surface. It is divided into two leaves, the outer leaf forming the inner periosteum of the skull and the inner leaf merging with the cobweb skin (arachnoidea). Under physiological conditions there is therefore no space between the hard meninges and the skull bone.
However, a so-called epidural space can form under pathological conditions, such as bleeding or trauma. In the area of the spinal cord there is a physiological epidural space which is filled with fatty tissue. The hard meninges do not nestle into the individual retractions and coils (gyri and sulci) of the brain, but they form the so-called durasepts at larger gaps.
The largest septum is the falx cerebri, which runs in a sickle shape from front to back in the middle of the upper skull and separates the two cerebral hemispheres. The two halves of the cerebellum (cerebellum) are also separated by a duraseptum, the falx cerebelli is located in the rear part of the skull calotte. Below the pituitary gland, the hard meninges form the diaphragm sellae with an opening for the style of the pituitary gland.
Between the occipital lobe (occipital lobe) of the cerebrum and the cerebellum it finally forms the tent-shaped tentorium cerebelli. In addition to the durasepts, the hard meninges form so-called sinuses through duplications, which have a similar surface lining to blood vessels. They act as venous blood collection vessels that drain the blood from the meninges and brain to the internal jugular vein.
The most important of these are the superior sagittal sinus in the upper rim, the inferior sagittal sinus in the lower rim of the falx cerebri and the transverse sinus, which runs in a semicircle in the posterior, lower skull base. The hard meninges serve to protect the brain tissue by mechanically stabilizing it during rapid movements or traumas. Furthermore, its duplications contain large, draining blood vessels that ensure the flow of blood from the brain via the jugular vein into the superior vena cava and thus into the heart.
The cobweb skin forms a fine layer underneath the dura mater, against whose underside it completely clings. In this way, it also helps to shape all durasepts. There is therefore no subdural space in itself.
However, the blood vessels of the brain surface run below the translucent arachnoidea. The fine veins that carry the blood away from the brain pass through the arachnoid and the inner leaf of the dura mater to reach the sagittal sinus and the transverse sinus. These vessels, the bridging veins, may rupture and bleed under certain circumstances, resulting in a subdural bleeding (cerebral hemorrhage) and creating a gap between the dura mater and the cobweb skin.
Below the cobweb skin lies the physiological subarachnoid space, which is the outer cerebrospinal fluid space of the brain. This is where the cerebrospinal fluid flows, which cushions the brain and also the spinal cord during jerky movements or impacts.The subarachnoid space is divided by connective tissue septa that connect the arachnoidea with the underlying pia mater. The superficial blood vessels of the brain run between these septa in the subarachnoid space.
The arachnoid fulfills two important tasks that are indispensable for the proper functioning of our brain. Firstly, it forms fine protuberances that extend through the inner leaf of the hard meninges into the sinus veins. These so-called Pacchioni granulations (Granulationes arachnoideae) absorb the cerebrospinal fluid from the subarachnoid space and release it into the sinus veins in the dura mater.
The choroidal plexus in the inner cerebrospinal fluid space constantly produces new cerebrospinal fluid, so that the cerebrospinal fluid is constantly circulating and renewed. Furthermore, the upper layer, which is directly adjacent to the dura, forms the blood-brain barrier. Through tight junctions, i.e. very tightly fused cell connections, a barrier is created through which no blood components can pass into the cerebrospinal fluid. This is so important because some substances that occur in the blood would be toxic (poisonous) for the nerve tissue. Also, many drugs cannot pass the blood-cerebrospinal fluid barrier and must be extra-molecularly converted in order to be effective in the brain.