Membrane Surrounding Brain (Dura, Arachnoid, Pia Mater)

The brain’s surrounding membrane has three concentric layers called meninges. Meninges is derived from Ancient Greek, plural of meninx, meaning membrane. Three membranes surround the brain and spinal cord: the dura mater, the arachnoid mater, and the pia mater.  

The membranes surrounding the brain are crucial for supporting the brain. The three cranial meninges, the dura, arachnoid, and pia mater, extend continuously in concentric layers.  A medically critical space connects each layer. These are the epidural, subdural, and subarachnoid spaces.

The meninges are essential and serve the brain and spinal in several ways. They safeguard vital organs from mechanical trauma and sustain the blood vessels that provide brain tissue with nourishing blood. They also create continuous cavities or passages in which CSF passes.

The Brain’s Meninges

The brain’s meninges consist of three main layers: the dura, arachnoid, and pia mater, and these individual membranes serve the brain in particular ways. The cranial meninges are differentiated from spinal meninges.

The CSF travels between the inner two meningeal layers- the arachnoid and the pia mater. Combined, these two meninges are referred to as the leptomeninges. Both the arachnoid mater and pia mater are byproducts of the neural crest. This contrasts with the dura mater, which arises from the embryonic mesoderm.

The Function Of The Brain’s Meninges

The three membranes of the brain anchor and support it. The meninges provide a support system to the CSF, which covers the central nervous system (CNS), the nerves, blood vessels, and lymphatics. The meninges also act as a drainage system via the meningeal lymphatic vessels.

The meninges play an integral role in the anatomy of brain protection. They act as a shock absorber and mitigate head injuries like a blow to the head. The meninges secure the CNS and prevent the brain from moving around in the skull. These membranes envelop the cranial structures to prevent their direct contact with bones.

These three membranes or meninges contribute to the blood-brain barrier function. The brain-blood barrier serves as a defense against pathogen-laden blood and toxicity. This meningeal function is incredibly complex and involves the tight junctions of endothelial and the epithelial cellular lining.

The meningeal lymphatic system plays an incredibly intricate and vital role in CSF and interstitial fluid (ISF) circulation. The meningeal lymphatic vessels are aligned to the dural venous sinuses and middle meningeal artery.

The meningeal lymphatic system is tasked with the drainage of extra fluid, immune cells, and small solutes of the CNS. The CSF and ISF are circulated and then drained by the meningeal lymphatic vessels.

The Brain’s Membrane- The Dura Mater

The dura mater, or dura, has its etymology from Latin, which translates to tough mother. The medical reference for this outermost meninge is pachymeninx (patchy is Greek for thick) or meninx fibrosa. This layer or lamellae of sturdy connective tissue comprises fibroblasts, extensive amounts of extracellular collagen, and elastic fibers.

Although the spinal dura mater’s composition is to a large extent like the cranial dura mater, they differ. The differences are minute yet significant. Three concentric lamellae structure the spinal dura mater.

A random composition of elastic fibers characterizes the spinal dura mater’s exterior layer. The middle layer is essentially fibrous tissue, and the inner cellular layer is called the dural border cell layer.   

The dura border cell layer has multiple interdigitating cell processes, no extracellular collagen, numerous extracellular openings, and limited cell junctions. It’s composed of modified flattened cells with sinuous processes and is attached to the arachnoid by random cell junctions.

The biophysical features of the spinal cord dura mater are relevant because pain medicine is injected into the epidural space (discussed below), and complications could potentially arise. An important example is creating an intradural space which could be fatal.

The Structure Of The Dura Mater

The dura mater is arranged in two distinct tiers: the periosteal (or endosteal) layer and the meningeal layer. The dura mater’s periosteal layer is a basic periosteum sheath that is secured to the periosteum bones of the skull’s inner surface.

The periosteal layer doesn’t continue past the foramen magnum and doesn’t make up the covering of the spinal cord. Therefore, the spinal cord dura mater doesn’t have a periosteal layer. This tubular sheath protects the cranial nerves. When the cranial nerves leave the foramen, the periosteal layer merges with the epineurium of nerves.

The meningeal layer is a tough, thick, and fibrous layer that extends past the foramen magnum and covers the spinal cord. The dura mater obtains its blood supply from the middle meningeal artery of the meningeal layer. This lining fashions dura folds, which allows space that facilitates cranial communication.

The trigeminal nerve travels through the dura mater. It has three sensory branches that deliver sensory innervation. The trigeminovascular system is composed of neurons that innervate cerebral blood vessels. 

The dura mater folds inward that fashions four distinct septa: the falx cerebri, tentorium cerebelli, cerebellar falx, and diaphragma sellae. These infoldings are dura reflections, creating divisions among the brain’s different hemispheres.

The Function Of The Dura Mater

The dura mater is the only vascularized membrane of all three meninges. The vascular system, called the circulatory system, involves a rich blood supply from various arteries. Therefore, the dura mater is an innervated connective tissue.

As mentioned above, the dura mater is energized by distributing nutrients and oxygen from the middle meningeal artery. The dura mater envelops and sustains the dural sinuses (the dural venous sinuses), which carry blood from the brain to the heart.

The dural venous sinuses are the dura mater’s drainage system. These cerebral sinuses are a network of venous channels located amid two layers of the dura mater- the endosteal and the meningeal layer.

The dural venous sinuses differ from other veins throughout the body as they are arranged alone and not parallel to arteries. In addition, they don’t possess valves, enabling bidirectional blood flow from and into intracranial veins.

The dura venous sinuses form the main drainage network from the brain to the internal jugular veins. When understanding the function of the dura mater, it’s crucial to remember that the draining structure of the intracranial veins is unlike the major cerebral arteries.

The dura mater’s drainage system permits blood to exit the brain and CSF to re-enter circulation. The dura mater is significant in maintaining the homeostasis of the CNS.

The Epidural Space

The epidural space is the region between the skull and the dura mater, the dura mater of the spinal cord, and the vertebrae. Epi is a prefix in Greek for on top. Lumbar punctures, commonly referred to as a spinal tap, is a diagnostic test that accesses the CSF in the spinal column.    

The spinal cord ends between the first and second lumbar vertebrae- here, in this space, there is only CSF. It’s in this location where lumbar punctures are executed. Pain medicines- analgesics and anesthesia, are occasionally injected into this space. The epidural space typically doesn’t exist unless there’s trauma or pathology.

The Brain’s Membrane- Arachnoid

The arachnoid mater is so-called due to its web-like appearance. It’s one of the three supportive membranes that guards the brain and spinal cord. It’s tasked with maintaining the CSF. This fibrous membrane is roughly secured around the brain and the spinal cord.

Unlike the dura mater, the arachnoid mater is avascular. Avascular entails the absence of blood vessels and inadequate distribution of blood supply. The arachnoid mater facilitates CSF metabolism through the subarachnoid space.

The Structure Of The Arachnoid Mater

The arachnoid mater is a fragile, transparent, and intricate connective tissue secured against the dura mater. This mid membrane has projections that fasten it to the pia mater.

This web-like membrane is unlike the pia mater because it doesn’t tightly envelop and follow the direction of the brain’s folds or sulci. However, it does bridge the sulci. It lines along with the depth of the longitudinal fissure- this separates the left and right cranial hemispheres.

A superficial mesothelial layer structures the arachnoid mater. This layer is lined directly below the dura mater. The mid network of the arachnoid mater consists of cells combined with junction proteins. The deeper aspect comprises loosely aligned cells with numerous collagen fibers secured in their intercellular space.

The CSF flows within the space beneath the arachnoid. In this space, there is a random interlacing of arachnoid trabeculae in which the CSF circulates. The arachnoid trabeculae are fine filaments and, in a mesh-like sequence, delicately breach the divide between the pia mater and arachnoid mater.

The arachnoid layer consists of collagen and elastic fibers. This netted membrane differs in thickness, and the outer facet is more even than the inner. The uneven distribution is due to the convoluted netting of the arachnoid trabeculae– which facilitate the brain’s cranial suspension.

The Function Of The Arachnoid Mater

The arachnoid mater has numerous bouquets of villa. The arachnoid villa or granulations project into the dura mater. The granulations act like a network that communicates between the systemic venous system and CSF.

The CNS is to an extent alienated from the body, and it’s constantly regulated in some measure by various diffusion barriers. These diffusion barriers are found between the extracellular spaces within the nervous system and other body regions.

The Subdural Space

The subdural space (epiarachnoid) is the zone between the dura and arachnoid mater. It’s flanked by the meningeal layer of the dura mater and the inner arachnoid mater of the leptomeninges.

Like the epidural space, ordinarily, this area isn’t a space. However, it will open to accommodate trauma, like brain bleed. It is filled with CSF that cushions the brain. Therefore, the subarachnoid mater doesn’t naturally occur and is apparent when there’s trauma (pathology).

The Brain’s Membrane- The Pia Mater

When the arachnoid and pia mater function as a single entity, it’s called leptomeninges. The leptomeninges is a singular operational unit set over against the pachymeninx. This section is the beginning of the more delicate meningeal coverings.

The pia mater gets its name from medieval Lati- tender mother. It’s the deepest layer of tenuous connective tissue with numerous minuscule blood vessels, and it completely envelops the surface of the CNS. The pia mater is characterized by its delicacy and tight adherence to the brain.

The Structure Of The Pia Mater

The pia mater is the only membrane that adheres to the brain in a snug, continuous layering that follows all the convulsions. It’s a diaphanous fibrous connective tissue that permits the movement of water and small solutes through its membrane.  

The pia mater is situated close to the cerebral blood vessel branches. These blood vessels penetrate the brain’s exterior and extend into the inner structures. The artery circuits (posterior and anterior) are responsible for meeting the brain’s high oxygen and nutrient demand.

The pia mater has two essential extensions: the denticulate ligament and the filum terminale. The denticulate ligament gives horizontal support to the spinal cord; denticulate comes from the Latin word for little teeth. These bilateral ligaments are the collagen components of the pia mater.

The denticulate ligaments have a lateral extension that fastens to the dura mater. This extension suspends the spinal cord in the subarachnoid fluid within the subarachnoid space. Therefore, they provide horizontal support.

The filum terminale is secured to the coccygeal bone, offering vertical support to the spinal cord. The filum terminale is divided into two sections: the filum terminale internum and the filum terminale externum.

The filum terminale internum is the upper three-quarters of the filum, and the dura mater and arachnoid meninges envelop it. While the filum terminale externum is the lower quarter, it merges with the dura mater and carries on inferiorly to fasten to the dorsal coccyx.

The Function Of The Pia Mater

The pia mater works alongside the other meninges to safeguard and feed the CNS. The pia mater sustains the blood vessels and enfolds the venous sinuses near the CNS.

This delicate innermost membrane envelops the venous sinuses to hold the CSF and create divisions with the skull. The CSF is frequently referred to as the fourth layer of meninges.

The Subarachnoid Space

Cerebral veins and arteries are organized in the subarachnoid space and are entirely covered by the pia mater. The subarachnoid area is the region between the arachnoid and pia mater, filled with CSF. The CSF guards and cushions the brain.

The CSF resides in the brain and spinal cord via three cranial features: the subarachnoid space, ventricles, and cisterns. There’s approximately 5 oz that’s consistently distributed and maintained throughout the CSF.

The CSF is predominantly secreted by the choroid plexus ( or plica choroidea). These are an intricate interlacing of capillaries lined by specialized epithelial tissue called the ependyma. The ependymal cells line the ventricles and the central canal of the spinal cord.

CSF travels across the ventricular system. CSF is generated in the lateral ventricles and flows via the interventricular foramina to the third ventricle. The CSF continues to the fourth ventricle through the cerebral aqueduct. It then flows across the median aperture (foramen of Magendie) and into the subarachnoid space at the brain’s base.

The Subarachnoid Cisterns

Subarachnoid cisterns are openings in the subarachnoid space. These openings are anatomic spaces in various regions of the brain’s arachnoid meninges. The cisterns are filled with CSF, and their interconnected presence is essential for CSF circulation. Some cisterns have nerves running through them.

As the pia mater tightly follows the brain’s surface, it fastens against the contours of the gyri and sulci. However, the arachnoid covering is superficial as it connects the gyri. Therefore, vast spaces remain. These spaces are subarachnoid cisterns.

The subarachnoid cisterns are divided by a trabeculated porous partition, and each is a different-sized cavity. There are numerous cranial cisterns, and a handful of large ones are medically significant.

Medical Conditions Concerning The Brain’s Meninges

The meninges play a vital role in protecting and supporting the brain and the spinal cord. Therefore, ailments that affect these critical membranes can become a serious concern.

Meningitis

Meningitis is a serious medical condition involving the meninges and the CSF. Meningitis presents as inflammation and could result in permanent brain damage. Therefore, early identification and treatment are crucial. An infection of the CSF triggers this condition.

Intracranial Hematomas

Brain blood vessel injury will result in an accumulation of blood in the brain tissue and cavities, forming hematomas. Hematomas give rise to harmful swelling. Blood applies pressure on the brain. As hematomas increase, it could result in loss of consciousness and eventual death.

Meningeal Carcinomatoses

Meningeal Carcinomatoses are cancerous growths developed from their original tumor site (meningiomas) and are a dangerous meninges medical condition. These cranial malignant cells occupy the meninges and spinal subarachnoid and could potentially invade the CNS.

Conclusion

The membranes surrounding the brain are made up of three meninges: the dura, arachnoid, and pia mater. These play an integral role in supporting the brain, facilitating the brain-blood barrier, and creating cavities filled with CSF.

Three divisions or spaces between each membrane play a medically significant role. These are the epidural, subdural, and subarachnoid. The epidural and subdural don’t present unless in the case of trauma. The subarachnoid space has cisterns, and these are responsible for CSF circulation.

References

https://my.clevelandclinic.org/health/articles/22266-meninges

https://www.mayoclinic.org/diseases-conditions/meningioma/multimedia/meninges/img-20008665

https://en.wikipedia.org/wiki/Meninges

https://www.britannica.com/science/meninges

https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/meninges

https://www.thoughtco.com/brain-anatomy-meninges-4018883

https://www.verywellhealth.com/meninges-anatomy-function-conditions-5190214

https://study.com/learn/lesson/meninges-layers-function-anatomy.html

https://www.frontiersin.org/articles/10.3389/fncel.2021.703944/full

Theodore T.

Theodore is a professional psychology educator with over 10 years of experience creating educational content on the internet. PracticalPsychology started as a helpful collection of psychological articles to help other students, which has expanded to a Youtube channel with over 2,000,000 subscribers and an online website with 500+ posts.