Arachnoid mater (Latin for cobweb skin) refers to a component of the meninges. The human brain has three meninges, of which the spider webs are the middle one. The name comes from its thin and white collagen fibers reminiscent of spider webs.
What is the arachnoid mater?
As a component of the meninges, the arachnoid mater definitionally represents a part of the leptomeninx encephali (Greek for soft meninges). Its short name is arachnoid and it is basically the middle meninges located between the dura mater encephali (hard meninges) and the pia mater encephali (soft meninges). Here, the dura mater lies on the outside, while the arachnoid lies immediately adjacent. The pia mater lies furthest inward. Between the arachnoid mater and the pia mater lies the spatium subarachnoideum (subarachnoid space). The two internal meninges, arachnoid mater and pia mater, are also grouped together as the soft meninges or leptomeninx.
Anatomy and structure
Anatomically, the sphenoid membrane represents a fine, thin, semitransparent membrane without vessels. The namesake whitish collagen fibers are also known as trabeculae or trabeculae. The structure, reminiscent of a spider web, stabilizes the brain and spinal cord within the fluid cushion. The arachnoid mater is again internally divided into two components, which have different locations. The arachnoid mater encephali is the variant of the arachnoid surrounding the brain. It is also known as the arachnoid mater cranialis. On the other hand, the variant surrounding the spinal cord is called arachnoidea mater spinalis according to the Latin name for the spinal cord (lat. spinalis: concerning the spinal cord). Here, the arachnoidea mater cranialis follows the contour of the brain, although it does not extend into the furrows (sulci) of the brain. The subarachnoid space located below the arachnoid mater belongs to the external cerebrospinal fluid space and is filled with cerebrospinal fluid (CSF). Opposite, the arachnoid mater shows protrusions into the cerebral sinuses (venous blood conduits) of the dura mater. This is done by the arachnoid villi (small outpouchings), with the support of which the cerebrospinal fluid is reabsorbed. The arachnoid covers the brain as a relatively smooth layer and, like the dura mater, does not pull through the cerebral grooves.
Function and Tasks
The arachnoid is essentially responsible for two functions that are indispensable for the functioning of the human brain. A central task of the arachnoid mater is found in the blood supply to the brain. The arachnoid mater is involved in this through a large number of smaller blood vessels. It also has the important task of exchanging cerebrospinal fluid (CSF) with the blood. Via the arachnoid villi, cerebrospinal fluid is absorbed and this cerebrospinal fluid is then passed on to the draining blood vessels. The fine protuberances extend through the inner region of the dura mater into the sinus veins. The reabsorption of CSF from the subarachnoid space is supported by the choroid plexus (a plexus of veins in the ventricle of the brain) in the inner CSF space. Through this, new cerebrospinal fluid is continuously generated. This ensures circulation and constant renewal of the CSF. The upper layer adjacent to the dura mater forms the blood-brain barrier. The so-called tight junctions are also important in the brain. They represent particularly tightly interwoven cell connections. They create a barrier that prevents blood components from passing into the cerebrospinal fluid. Since some blood components can have a toxic effect on the nerve tissue, this blood-liquor barrier is particularly important. It should be noted that many drugs are also unable to cross this barrier. Therefore, to show efficacy in the brain, molecular remodeling of the drugs is required.
Diseases
Because of the central functions of the arachnoid mater for the human brain, damage to this meningeal component is particularly critical. One disease in which the arachnoid is often dangerously affected is found in meningitis (meningitis). Some variants of meningitis lead to life-threatening complications. Infections leading to meningitis can be bacterial or viral, whereby especially bacterial variants can quickly become dangerous.Symptoms include neck stiffness, headache, dizziness, fever episodes, and also neurological abnormalities or deficits. A particularly problematic variant is meningococcal meningitis, in connection with which about one-third develop sepsis (blood poisoning). In the case of injuries in the various meninges, bleeding usually occurs. Thus, craniocerebral trauma often causes epidural hemorrhage (bleeding in the area of the dura mater). Problematic about brain hemorrhages are the symptoms that appear in free intervals, through which affected persons often believe themselves to be in a false sense of security. After a short period of unconsciousness, patients often feel better subjectively before a further loss of consciousness can occur. As a result, about one third of those affected die from the consequences of the hemorrhage. In the specific context of the arachnoid mater, subarachnoid hemorrhages (bleeding in the space between the arachnoid mater and the pia mater or spatium subarachnoideum) are dangerous. The causes of such hemorrhage are often aneurysms, i.e. ruptures of vascular outpouchings. In this case, this concerns vessels of the arachnoid mater or the pia mater. Symptomatic of such a rupture are severe headaches, increase in intracranial pressure with a drop in blood pressure, as well as impaired consciousness and vomiting. Cerebral hemorrhage in the subarachnoid space is survived without sequelae by only about one-third of those affected. One-third of patients do not reach emergency care in time, while the second third die or suffer severe disability in the hospital.
