Brain Meninges

Every organ in the body has some protective mechanism that shields it from external trauma or injury. Some are protected by bones while others just have thick capsule around them.  Others are covered by both of them.

The same is true for the central nervous system. Both the brain and spinal cord are provided protection by thick bony framework of the skull and vertebral column, respectively. In addition, both these structures have additional protection provided by capsule-like thick membranes called meninges.

Meninges not only provide protection from physical injury and trauma to the brain and spinal cord but also perform several other functions that are essential for the survival of these organs.

The meninges of the brain and spinal cord include
three layers; dura mater, arachnoid mater and pia mater.

In this article, we will talk about various aspects of
meninges. We will discuss the structure, blood supply and nerve supply,
functions, and clinical significance of each layer in three sections. We will
also conclude our discussion in the form of summary at the end of this article.

Dura Mater

It is the outer most layer of meninges present in close relation to the bony structures. It is further divided into two layers; endosteal layer and meningeal layer.

Endosteal Layer

It is simply the periosteum that covers the inner surface of the skull bones. The endosteal layer is continuous with the periosteum on the external surface of the skull around the margins of foramina.

It does not continue with the dura of the spinal cord. It also fuses with sutural ligaments at the sutures of the skull.

Meningeal Layer

It is also called dura mater proper. The meningeal layer is a dense, fibrous layer that covers the brain and spinal cord. The endosteal layers of the brain and spinal cord dura are continuous with each other at the foramen magnum.

The meningeal layer sends inward septa that divide the
cranial cavity into different chambers. These chambers lodge different parts of
the brain such as the cerebellum and cerebral hemispheres. These septa are
formed by the folds of the meningeal layer of dura mater. The four septa are
discussed below.

Falx Cerebri

It is a sickle-shaped fold of dura mater. It lies in
the midline between two cerebral hemispheres. The posterior broad part of falx
cerebri blends with the upper surface of the tentorium cerebelli.

Falx cerebri contains three sinuses.

• Superior sagittal sinus in the
  upper margin
• Inferior sagittal sinus in the
  anterior part of lower free margin
• Straight sinus in the lower margin
  attached to tentorium cerebelli

Tentorium Cerebelli

It is a crescent-shaped fold of dura mater. It roofs over the posterior cranial fossa. It separates the cerebellum from the cerebral hemispheres and supports the occipital lobes of the cerebral hemispheres. The anterior end of tentorium cerebelli has a gap for the passage of midbrain.

Falx Cerebelli

This fold of dura matter is attached to the internal
cerebral crust. It is a small sickle-shaped fold present between the two
cerebellar hemispheres. The fixed posterior margin of falx cerebelli contains
the occipital sinus.

Diaphragma Sella  

This fold of dura matter roofs the sella turcica. It
has a small opening for the passage of hypophysis cerebri.

Blood Supply

The blood supply to the dura mater is provided by the branches of the internal carotid artery, ascending pharyngeal artery, occipital artery, maxillary artery, and vertebral arteries. The most important of these is the middle meningeal artery, which can be damaged by head injury and has clinical significance.

Nerve Supply

The dura mater is supplied by the trigeminal nerve,
vagus nerve, and first three cervical nerves (C1, C2, C3). It is also supplied
by the branches from the sympathetic trunk.

Functions

The dura mater provides protection from trauma and
injury to the brain and spinal cord.

It divides the cranial cavity into different spaces
and provides support to different parts of the brain.

It contains the dural venous sinuses that carry the
blood back from the brain to the heart.

Arachnoid Mater

It is a delicate membrane present between the dura
mater and pia mater. It covers both the brain and spinal cord.

Spaces

The arachnoid mater is separated from the dura mater by a fluid-filled space called subdural space. It is also separated from the pia mater by a space called subarachnoid space. The subarachnoid space is filled with cerebrospinal fluid.

Subarachnoid Cisternae

In certain areas, the arachnoid is widely separated
from the pia to form subarachnoid cisternae. These cisternae can freely
communicate with one another and the other subarachnoid spaces. They are filled
with the cerebrospinal fluid.

Two important subarachnoid cisternae are:

• Cisterna Cerebellomedullaris,
  present between the inferior surface of the cerebellum and fourth ventricle
• Cisterna interpeduncular, present
  between the cerebellar peduncles

Arachnoid Villi and Granulations

The finger-like projections of arachnoid into the
dural venous sinuses are called arachnoid villi. The aggregates of these villi
are called arachnoid granulations. The arachnoid villi allow the CSF to diffuse
into the blood.

Subarachnoid Space

As stated earlier, the subarachnoid space separates
the arachnoid from the pia mater. It is filled with CSF.

All the structures that pass to the brain such as nerves, ligaments, arteries, etc. are contained in the subarachnoid space. All the cerebral arteries, veins and nerves enter and exit through this space.

At the point of exit, the arachnoid fuses with the epineurium of the nerves.  It also forms a sheath around the optic nerve, called the optic sheath. This sheath extends up to the eyeball and fuses with the sclera.

Blood and Nerve Supply

The arachnoid mater is not supplied by any nerve. Thus, it is not sensitive to pain.

The nutrition is mainly derived from the CSF and the
blood in the subdural venous sinuses. So, there is no blood supply of the
arachnoid mater.  

Pia Mater

It is the innermost layer of meninges. It is a very delicate membrane present in close contact with the brain and spinal cord.

In the case of the brain, it invests the brain
completely, covering the gyri and extending deep into the sulci. It forms the
tela choroidea of the third and fourth ventricles of the brain. it also fuses
with the ependyma to form the choroid plexus in the lateral, third and fourth
ventricles.

In the spinal cord, the pia mater is thickened on both sides to form ligamentum denticulatum between the nerve roots. This ligament is connected laterally to the dura mater and arachnoid mater. In this way, the spinal cord is suspended in the middle of the dural sheath.

Clinical Significance

The clinically important conditions associated with
the meninges are discussed in this section.

Meningitis

Meningitis refers to the inflammation of meninges. It is characterized by fever, headache, vomiting, severe pain and stiffness of the neck. Meningitis can be caused by bacterial, viral or fungal infections. It can also be a result of certain drugs and cancer.

Hemorrhages

There are two types of hemorrhages that can affect the meninges. These are epidural and subdural hemorrhages.

The epidural hemorrhage results from the injury to the meningeal arteries or veins. The most commonly damaged artery is the middle meningeal artery. It can result in bleeding and hematoma formation, known as epidural hematoma.

Subdural
hemorrhage is due tearing of the superior cerebral veins. It can result in
profuse intra-cranial bleeding and the formation of subdural hematoma. The
subdural hematoma can severely compress the cerebral tissue and increase
intracranial pressure resulting in mental retardation, coma or even death.

Meningioma

Meningioma is basically the benign tumors of the meninges. They can arise from any of the three layers of the meninges. Although they can occur anywhere in the CNS, most of them occur in the brain. Most of the meningiomas are asymptomatic but can prove fatal sometimes. They are more common in women. 

Conclusion/Summary

Meninges are the capsule-like coverings that provide protection to the brain and spinal cord. They include three membranes; dura mater, arachnoid mater and pia mater.

The dura mater is the most external and toughest layer
among the meninges. It has two layers;

• Endosteal layer, that covers the
  inner surface of the skull bones
• Meningeal layer, that covers the
  brain and forms folds.

The four important folds of the meningeal layer are

• Falx cerebri, that lies between two
  cerebral hemispheres
• Tentorium cerebelli, that forms the
  roof of posterior cranial fossa
• Falx cerebelli, that lies between
  the two cerebellar hemispheres
• Diaphragma sella, that bridges the
  sella turcica

Dura mater is sensitive to pain and is supplied by various nerves.

It receives nutrition from the blood supplied by
various dural blood vessels.

Dural venous sinuses are present in the folds of the endosteal
layer.

The arachnoid mater is the middle layer of the meninges.

It is separated from the dura mater by subdural space,
and from pia mater by subarachnoid space.

The arachnoid mater also projects into the dural
venous sinuses in the form of arachnoid villi and granulations.

Pia mater is the innermost layer, that directly invests the brain and spinal cord. It forms tela choroidea and choroid plexus in the brain. in the spinal cord, it forms modifications such as ligamentum flavum and ligamentum denticulatum.

The important clinical conditions associated with the
dura mater include:

• Meningitis
• Meningiomas
• Hemorrhages and Hematomas

References

1. "Scalp Anatomy: Structure, Nerve Supply, Arterial Supply". 20 June 2017.
2. Kumar, Vinay (2015). Robbins and Cotran Pathologic Mechanisms of Disease (9th ed.). Philadelphia: Elsevier Saunders. p. 1273. OCLC 892583347. In acute meningitis, an exudate is evident within the leptomeninges over the surface of the brain (Fig. 28-21).
3. "Overview of Adult Traumatic Brain Injuries" (PDF). Orlando Regional Healthcare, Education and Development. 2004. Archived from the original (PDF) on February 27, 2008.
4. van Gijn J, Kerr RS, Rinkel GJ (2007). "Subarachnoid haemorrhage". Lancet. 369 (9558): 306–18. doi:10.1016/S0140-6736(07)60153-6. PMID 17258671.