Spinal meninges

Classification and layers of the meninges

The neural tissue of the spinal cord and brain is covered in layers by three connective tissue membranes that follow from the outside inward in a strict order. In anatomy, it is customary to combine these membranes into two functional and structural groups. The first group is the pachymeninx (pachimeninx), or the so-called tough membrane (dura). The second group is called the leptomeninges, which include thinner structures.

• Dura mater (dura mater). This is the outermost, dense, and durable membrane. From a histological standpoint, it consists of dense fibrous connective tissue. Within the cranial cavity, it has specific features: it is formed by two layers. The outer layer essentially acts as the periosteum lining the inner surface of the bones, and the inner layer is the dura mater proper.
• Arachnoid mater (arachnoidea mater). This is the middle connective tissue layer that is part of the delicate leptomeningeal membranes.
• Pia mater (pia mater). The innermost membrane, which directly adheres to the surface of the brain tissue and closely envelopes the structures of the central nervous system.

Topography and intermeningeal spaces

Specialized intermeningeal spaces form between the connective tissue layers, playing a key role in the physiology of the central nervous system. When considering the topography in a coronal section from outside inward, the sequence of layers is as follows:

1. Epidural space — located external to the dura mater.
2. Dura mater — includes the inner and outer layers (in the skull).
3. Subdural space — located between the dura mater and the arachnoid mater. The outer boundary of this extremely narrow space is the inner layer of the dura mater. Normally, this space is largely a potential (virtual) space.
4. Arachnoid mater.
5. Subarachnoid space — located strictly between the arachnoid mater and the pia mater. This exact space is filled with cerebrospinal fluid (CSF).
6. Pia mater.
7. Brain tissue (e.g., cerebral cortex).

Structural features of the dura mater and dural venous sinuses

The inner and outer layers of the dura mater closely adhere to each other along most of their extent. However, in certain areas, they split and separate, creating space for the formation of venous sinuses.

In addition, the inner layer of the dura mater is capable of forming folds. For example, between the superior and inferior sagittal sinuses, a double fold known as the falx cerebri (falx cerebri) is stretched. The sinuses serve as collectors for blood collection and pathways for the resorption of cerebrospinal fluid.

Cerebrospinal fluid (CSF) and its circulation

The normal functioning of the central nervous system is impossible without the proper circulation of CSF. This process is divided into three main stages:

• Production. CSF is produced in the choroid plexuses of the ventricular system of the brain (right and left lateral, third, and fourth ventricles).
• Circulation. From the internal CSF space (ventricles), the fluid flows out into the external CSF space — the subarachnoid space, which completely surrounds the brain and spinal cord.
• Resorption (absorption). Used CSF is absorbed into the venous system. This occurs through arachnoid granulations (Pacchionian granulations), which project into the lumen of the sinuses (specifically, near the superior sagittal sinus). At the level of the spinal cord, absorption occurs through perineural spaces (perineural sheaths).

Disruption of this balance leads to a serious pathology of the CSF system — hydrocephalus. This condition manifests as an increase in CSF pressure and pathological enlargement of the brain ventricles. The pathogenesis of hydrocephalus is most often rooted in increased CSF production, which increases its total volume, and somewhat less frequently in decreased absorption into the venous system.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Anatomy of the ventricular system of the brain
• Structure and topography of the dural venous sinuses
• Histological structure of the choroid plexuses of the ventricles
• Mechanisms of development and types of hydrocephalus

Other topics in the section "DeMyer's The Neurologic Examination" / "Duus' Topical Diagnosis in Neurology"

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