Generality
The meninges are the three superimposed laminar membranes, which take place between the components of the central nervous system (brain and spinal cord) and the surrounding bony structures (skull, brain, and spine, for the spinal cord).
Known as dura mater (outermost meninge), arachnoid (intermediate meninge) and pia mater (outermost meninge), the 3 meninges have the important task of contributing to the protection of the brain and spinal cord.
Meninges are involved in various medical conditions, including: meningitis, subarachnoid hemorrhage, subdural hematoma, epidural hematoma and meningioma.
Short review of the Central Nervous System (CNS)
The central nervous system, or CNS, is the most important set of organs in the nervous system of all vertebral bodies, including human beings.
The central nervous system is composed mainly of the brain and the spinal cord .
Protected by a very resistant bone structure (the skull, for the encephalon, and the spinal column, for the spinal cord) and immersed in a liquid also with a protective function (the so-called cerebrospinal fluid), the central nervous system has a vast network of neurons, which allows him to analyze in detail all the information coming from the internal and external environment to the organism, and to elaborate the most appropriate answers (to the aforementioned information).
What are Meninges?
The meninges are the 3 superimposed membranes, which, in the human being, cover the neurocranium and the vertebral canal internally, and cover, with protective purposes, the elements of the central nervous system, ie the brain and the spinal cord.
Composed essentially of connective tissue, the meninges are the laminar nerve structures also known as: dura mater, arachnoid (or arachnoid mother ) and pia mater .
To understand: what are neurocranium and vertebral canal?
- "Neurocranium" is the anatomical term that identifies the complex of skull bones placed to protect the brain.
- "Vertebral canal" and its synonym " spinal canal " are the anatomical terms that indicate the space inside the vertebral column, resulting from the superposition of the so-called vertebral holes present in the individual vertebrae.
Anatomy
Arranged in overlapping layers, the meninges reside below the bone structures that cover the brain and spinal cord. Starting from the outside (therefore from the bony part) and proceeding towards the inside (therefore towards the nervous structures), the first meninge is the dura mater, the second is the arachnoid and the third is the pia mater.
Hard Mother
The dura mater is the outermost meninge; therefore, it is the meningal closest to the bones of the skull (in the brain) and to the vertebrae (in the spinal cord), on the external face, and the meningin bordering on the arachnoid, on the inner face.
Composed of a dense fibrous tissue particularly flat cells, the dura mater is a very thick and resistant meninge.
On the dura mater, important arterial vessels take place, from which the capillaries of the pia mater derive; moreover, there is an intricate network of venous vessels - called dural sinuses - whose job is to drain the oxygen-poor blood leaving the central nervous system and direct it to the heart.
The dura mater of the brain has some substantial differences from the dura mater of the spinal cord; these differences will be discussed in the next two sections of this article.
HARD MOTHER OF THE ACTION (DURA MOTHER ENCEPHALIC)
The dura mater of the brain (or dura mater encephalic ) is a double-layer (bi-lamellar) meninge, in which the outer layer acts as a lining of the inner surface of the skull (" endosteal layer " or " periosteal dura mater "), while the inner layer covers the role of covering the outer surface of the brain (" meningeal layer " or " dura mater meningea ").
The dura mater of the encephalon has some characteristic folds, called folds of reflections, deriving from the adaptation of the meningeal layer to the typical grooves and cavities present in the brain; in number 4, these reflection folds are:
- The cerebral sickle (or large sickle ). It is the fold of reflection of the dura mater that comes between the two cerebral hemispheres; runs from the frontal bone to the occipital bone.
- The tentorium of the cerebellum (or cerebellar tentorium ). Similar to a crescent, it is the fold of reflection of the dura mater that separates the occipital lobes of the brain from the cerebellum.
- The cerebellar sickle (or sickle of the cerebellum ). It is the fold of reflection of the dura mater that separates the two hemispheres of the cerebellum; resides below the cerebellar tentorium.
- The sellar diaphragm (or diaphragm of the Turkish saddle ). It is the fold of reflection of the dura mater that covers the pituitary gland (hypophysis) and the sella turcica.
HARD MOTHER OF SPINAL CORD
Also known as dural sac, the dura mater of the spinal cord is, for all intents and purposes, a hollow cylinder, whose course downwards begins from the posterior cranial fossa, involves the crossing of the foramen magno and ends at the level of the vertebra S2 (second sacral vertebra).
Inside the vertebral column, the spinal cord extends from the C1 vertebra (first cervical vertebra) to the space between the L1 and L2 vertebrae (respectively, first and second lumbar vertebrae). This means two things: the spinal cord is not as long as the vertebral column that contains it; the dura mater of the spinal cord is longer than the latter, since, as previously stated, it ends at the level of the vertebra S2.
The dura mater of the spinal cord does not adhere directly to the vertebral holes of the vertebral canal, but is separated from them by a space rich in adipose tissue and arterial and venous blood vessels; this separation space is called an epidural space or a peridural space .
Did you know that ...
The epidural space is the site of injection of anesthetics and sedatives, on the occasion of epidural (or simply epidural ) anesthesia ; different from spinal anesthesia, epidural anesthesia is a type of local anesthesia, which allows to cancel the sensitivity to pain in a large part of the bust and along both lower limbs.
arachnoid
The arachnoid, or arachnoid mother, is the intermediate meninge; therefore, it is the meninge interposed between the dura mater, superiorly, and the pia mater, inferiorly.
Thin and transparent, the arachnoid is a meninge composed of fibrous tissue with flat cells (similar to those of the dura father), which guarantee waterproof properties.
While at the top the arachnoid is in close contact with the dura mater, below it presents a space of separation from the pia mater, which takes the name of subarachnoid space (literally is "space under the arachnoid").
The subarachnoid space is filled with cefalorachidian liquor, a very particular fluid that serves to improve the protective functions of the meninges.
In the subarachnoid space, moreover, there are recognizable filaments of connection, called arachnoid trabeculae, which pass between the lower surface of the arachnoid and the upper surface of the pia mater, and which form a web similar to a spider web.
Did you know that ...
The arachnoid mother owes its name to the network of filaments that connect its lower surface to the upper surface of the pia mater. As stated earlier, in fact, this network of filaments (the so-called arachnoid trabeculae) resemble the cobwebs woven by the most common arachnids: spiders.
Finally, it is important to note that the arachnoid is provided with a series of perforations, through which the cranial nerves (in the brain), the spinal nerves (in the spinal cord), and the arterial and venous blood vessels pass.
Deepening: what is cefalorachidian liquor?
The result of a process of ultrafiltration of the blood plasma, the cephalorachidian liquor (or cerebrospinal fluid), is a transparent fluid, free of red blood cells, rich in white blood cells and poor in plasma proteins, which has the task of:
- Protect brain and spinal cord,
- Create an ideal environment for nerve cells to function,
- Provide nourishment to the central nervous system,
- Adjust intracranial pressure,
- Promote the removal of waste products from the central nervous system.
Pia Madre
The pious mother is the innermost meninge; therefore, it is the meningus underlying the arachnoid and adhering to the upper surface of the brain and spinal cord.
Composed of a flat cell fibrous tissue, the pia mater is a thin and very delicate meninge, which, at an encephalic level, adapts perfectly to the furrows and convolutions of the brain and cerebellum.
At the level of the pia mater, the arteries intended to nourish the brain and the spinal cord become arterioles and capillaries .
As stated in the section dedicated to the arachnoid, on the upper side of the pia mater extends the subarachnoid space filled with cerebrospinal fluid.
Leptomeningi: What are they?
In human anatomy, the union between the arachnoid and pia mater meninges is called leptomeninge .
In other words, the anatomical term leptomeninge indicates the complex mother arachnoid-pia mater.
Histology
As reiterated in more than one circumstance, the meninges consist of fibrous tissue with flat cells ; these flat cells give them the impermeability they need, to contain the cerebrospinal fluid in the subarachnoid space.
Blood Spraying
Of the 3 meninges that cover the central nervous system, only the dura mater has a noteworthy supply of oxygenated blood; arachnoid and pia mater, in fact, are definitely not sprayed.
Going into details, to supply the dura mater of oxygen-rich blood are:
- The middle meningeal artery ;
- The so-called meningeal branches of the ophthalmic, anterior ethmoid and posterior ethmoid arteries ;
- The meningeal branches of the internal carotid artery ;
- The accessory meningeal artery ;
- The ascending pharyngeal artery ;
- The anterior and posterior branches of the middle meningeal artery.
Did you know that ...
The middle meningeal artery is a branch (NB: branch means branch) of the maxillary artery, one of the two terminal branches of the external carotid artery (the other is the superficial temporal artery).
innervation
As for blood circulation, even with regard to innervation, the only meninge to become interesting is the dura mater.
Specifically, the branches and sub-branches of the trigeminal nerve innervate the dura mater.
Function
The meninges have a protective function ; their purpose, in fact, is to protect the brain and the spinal cord from physical insults (eg: trauma to the head) and from any toxic or in any case dangerous substances which, through the blood, could reach the central nervous system.
It is important to remind readers that to support the meninges in their defensive action of brain and spinal cord is the CSF (the functions of which are reported in a previous in-depth box).
diseases
Meninges are involved in different medical conditions; among the latter, they deserve a particular signaling:
- Meningitis . "Meningitis" is the medical term for inflammation of the meninges.
As a rule, meningitis episodes are infectious, ie due to an infection.
Infectious agents capable of causing meningitis include bacteria (eg: Meningococcus ), viruses (eg: Enterovirus ) and fungi (eg: Cryptococcus neoformans ).
Meningitis of bacterial origin due to meningococcus is particularly dangerous and feared; this form of inflammation of the meninges can have permanent consequences on the affected individual and can even cause death.
- Subarachnoid hemorrhage . With subarachnoid hemorrhage, the doctors intend a blood spill in the space between the arachnoid meninges (intermediate meninges) and the pia mater (innermost meninges).
The episodes of subarachnoid hemorrhage can be the result of spontaneous processes, cranial traumas or ruptures of a cerebral aneurysm.
- Subdural hematoma . Subdural hematoma is a blood spill in the space between the meningeal dura mater (outer most meningeal) and arachnoid (intermediate meningeal).
In most cases, episodes of subdural hematoma are the result of secondary head injuries to car accidents, falls from great heights or violent attacks.
- Epidural hematoma . With an epidural hematoma, doctors mean a blood supply in the space between the dura mater (outermost meninge) and the adjacent bone structure.
The main causes of epidural hematoma include head traumas following motor vehicle accidents.
- The meningioma . Meningioma is a type of brain tumor that originates from the uncontrolled proliferation of a meninges cell.
Due to causes still little known, meningioma is in 90% of cases a benign, slow-growing tumor, and only in the remaining 10% of cases is a fast-growing tumor of a malignant type.
Clinical meaning
The meninges have a role of some importance also in the clinical-diagnostic and clinical-therapeutic field. The meninges, in fact, are involved in the execution of the so-called lumbar puncture and in the practice of two anesthetic techniques already mentioned above: epidural anesthesia and spinal anesthesia .
Lumbar Puncture: What is it?
The lumbar puncture consists in the removal from the subarachnoid space of the spinal cord of a portion of CSF and in the subsequent laboratory analyzes of this altitude.
Lumbar puncture is a fundamental test to detect the presence of infectious agents in the spinal cord (and in the central nervous system in general) and to understand if a local inflammation is in progress.
Epidural Anesthesia: What is it?
Epidural anesthesia (or epidural anesthesia ) is a type of local anesthesia, which involves the injection of anesthetics and analgesics into the epidural space of the spinal cord, in order to eliminate the painful sensation in the lower back and along both lower limbs (NB: we remind you that the space separating the dura mater of the spinal cord from the vertebral canal is called epidural).
Did you know that ...
Epidural anesthesia is an anesthetic practice used to alleviate birth pains in pregnant women.
Spinal Anesthesia: What is it?
Spinal anesthesia is a type of local anesthesia, which involves the injection of anesthetics and analgesics into the subarachnoid space of the spinal cord meninges, in order to eliminate the painful sensation in the lower back and along both lower limbs (NB: it should be remembered that the space between the arachnoid meninges and the pious mother is called arachnoid.
