The arachnoid is the intermediate meninges and fulfills very important functions in the central nervous system, especially in the transport of cerebrospinal fluid.
The arachnoid is the intermediate meninges that protects the central nervous system. It is located between the dura mater and the pia mater , and is considered the most delicate of the three meninges that protect the human central nervous system.
To accurately dissect this type of tissue and understand its functions, it is necessary to take an interdisciplinary approach, that is, to know where they are, what they do and what happens when they fail .
For this reason, in this article we explain in detail the morphology, functioning and possible pathologies (arachnoiditis and arachnoid cysts) that can occur in this extremely useful intermediate meninges. If you want to know more about this protective structure, continue reading.
- We recommend you read: “Meninges: anatomy, characteristics and functions”
What is the arachnoid?
According to the Royal Academy of the Spanish Language, the arachnoid is defined as follows: “Said of a meninge: Which is one of the three that amphibians, reptiles, birds and mammals have, placed between the dura mater and the pia mater, and formed by a clear and serous tissue that mimics spider webs.”
From a slightly less linguistic approach, we can define it as one of the membranes (meninges) that surround the spinal cord and the brain , in addition to dividing the spaces through which the cerebrospinal or cerebrospinal fluid (CSF) passes. As we can see, there are several terms that we must define before entering fully into the morphology of the arachnoid. Go for it.
The three meninges of the nervous system
The meninges are defined as the connective tissue membranes (that is, of a filling and supporting nature) that surround and cover the entire central nervous system. Its function is to protect and provide the necessary nutrients for nerve impulses to be transmitted from one neuron to another . In mammals, three different layers are distinguished, and we will show you below starting from the inside (in contact with the nervous system) to the outside (in contact with the bone system):
- Pia mater , highly vascularized layer found near nerve structures. Lines the convolutions of the brain.
- Subarachnoid space, which contains cerebrospinal fluid and reduces the likelihood of damage from trauma.
- Arachnoid , intermediate layer between the two spaces that is responsible for absorbing the impact generated in these areas.
- Subdural space, a very small “gap” that allows slippage between the arachnoid and the dura mater.
- Dura mater , the outermost layer, characterized by its fibrous, thick, and solid nature when it surrounds the spine.
As we can see, we are facing a concatenation of spaces and membranes that are responsible for absorbing physical injuries to the nerve structures and preventing the entry of harmful agents into the central nervous system. They represent an effective barrier against bacteria, viruses, tumor cells and other pathogenic parasites (such as the Trypanosoma protists, which attack the cerebrospinal fluid and the central nervous system, causing sleeping sickness).
In this case, we are dealing with a colorless fluid that bathes the brain and spinal cord. It is necessary to dwell briefly on this liquid, because after all it is transported through the subarachnoid space (directly connected to the arachnoid).
This cerebrospinal fluid has various functions, including providing cushioning to the central nervous system against trauma, providing the brain with hydropneumatic support, eliminating metabolites, acting as a reservoir of substances, and many other functions. Who would have thought that a liquid that occurs in a volume of 100 to 150 milliliters in a healthy human would have so many functions?
Arachnoid morphology
Once we have made a quick tour of the rest of the meninges and the cerebrospinal fluid that circulates through their spaces, it is time to fully enter into the detailed morphology of the arachnoid tissue.
In the first place, going to the etymological root of the word is essential for its understanding. “Arachnoid” comes from the ancient Greek “arachnoiedes”, which means “like a spider’s web” (arachne: spider, noides: similar to), referring to the multitude of trabeculae that this meningeal membrane projects on the pia mater.
Histological studies provide us with the following data about this formation composed of connective tissue and serous compounds:
- The arachnoid lamina has a thickness of 35-40 micrometers.
- The arachnoid lamina occupies 10% of the total dural space.
- In its external portion are endothelial sheets of the subdural compartment.
- Its internal portion is formed by a cellular plane 5-8 μm thick, made up of the superimposition of 4-5 barrier-forming arachnoid cells.
- The intercellular space of this plane is 0.02-0.03 μm in length.
- Desmosomal and tight junctions are found between the cells of the arachnoid tissue.
Throughout this histological conglomerate we wanted to reflect that we are facing a membrane of a complex nature with multiple peculiarities. In addition, this structure is formed by two horizons: the lamina or arachnoid itself, and the already named subarachnoid space through which the cerebrospinal fluid circulates .
Thus, the arachnoid has both a “highway” and a “barrier” functionality , since it allows the flow of cerebrospinal fluid with all the benefits it brings, but it also provides a mechanical shock absorber against injuries and external forces. Therefore, arachnoid cells have structural characteristics that ensure the barrier function of the human dural sac. That is, they are closely linked to each other and do not allow the passage of foreign substances.
Arachnoiditis is defined as a chronic inflammation of the arachnoid layer of the meninges, that is, the membrane that concerns us here. Its most severe form is adhesive arachnoiditis, which causes debilitating and intractable neurogenic pain in the patient’s back and extremities. This continued inflammation causes various physiological complications, including scarring and unintentional union of nerve roots and blood vessels. Some of the symptoms that characterize this pathology are the following:
- Tingling and strange sensations in the extremities.
- Severe sudden pain.
- General itching and burning.
- Muscle cramps, uncontrollable spasms and tremors.
As we can see, we are facing a disease that responds to an inflammation of the protective tissue of the central nervous system, which manifests itself in various ways in the individual’s body. It should be noted that this pathology can be caused by chemical compounds, viral and bacterial infections, as a result of injuries or poorly executed surgeries .
Another pathology that can occur in this delicate membrane is the appearance of arachnoid cysts, bags filled with cerebrospinal fluid that are located between the brain or spinal cord and the arachnoid membrane. The patient who presents these formations experiences headaches, nausea, seizures, auditory and ophthalmic alterations and many other derived problems.
Of course, as we have seen in this little journey through the pathologies of this membrane, we are dealing with a highly sensitive tissue that, if affected, can generate various symptoms of a debilitating nature in the patient.
- We recommend you read: “Meningitis, symptoms, types, causes and treatment”
Conclusions
The human brain contains an average of 86,000 million neurons , so it is impossible to doubt the complexity and essentiality of the central nervous system in our daily lives. This complex machinery, made up of the brain and spinal cord, is responsible for perceiving stimuli from the outside world, processing information and transmitting impulses to nerves and muscles. That is, without this system, life as we know it would not be possible.
Therefore, the existence of the three layers that act as a barrier is essential: the pia mater, arachnoid mater and the dura mater. All three are of essential importance for the mechanical and molecular isolation of the central nervous system, thus allowing the integrity of this delicate machinery throughout our lives. The arachnoid, the intermediate membrane, despite being the most delicate layer, has an essential function and when imbalances occur in it, the patient’s symptoms can include a wide variety of signs of varying severity. Of course, spaces like this show that, in the human body, each cell counts for the correct functioning of our organism and the performance of the functions that characterize us.
Bibliographic references
- What are arachnoid cysts? National Institute of Neurological Disorders and Stroke. (NIH) Collected on September 16 at https://espanol.ninds.nih.gov/trastornos/quistes_arachnoideos.htm#:~:text=Los%20cysts%20arachnoideos%20son%20bolsas,cerebro%20y%20la%20m %C3%A9dula%20spinal.
- Arachnoiditis, Orphanet. Collected on September 16 at https://www.orpha.net/consor/cgi-bin/OC_Exp.php?lng=ES&Expert=137817#:~:text=Es%20una%20inflamaci%C3%B3n%20cr%C3 %B3unique%20of, variety%20of%20additional%20neurol%C3%B3gic%20problems.
- Reina, MA, Prats-Galino, A., Sola, RG, Puigdellívol-Sánchez, A., Navarro, RA, & De Andrés, JA (2010). Morphology of the human spinal arachnoid lamina. Barrier that limits the permeability of the dural sac. Spanish Journal of Anesthesiology and Resuscitation, 57(8), 486-492.
- Reina, M.A., Villanueva, M.C., & Lopez, A. (2008). Trabecular arachnoid, human spinal pia mater and subarachnoid anesthesia. Rev Arg Anestesiol, 66(2), 111-33.
- Vega, GE, Vilanova, MC, Ferrer, MM, Rodrigo, JG, Calvet, CL, Ruiz-Capillas, JJ, & Quiñones, D. (2001). Arachnoiditis and intramedullary injury. Spanish Clinical Journal, 201(10), 575-578.
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