Meninges of the Brain and cerebrospinal fluid

Meninges of the Brain and Cerebrospinal Fluid

Central Nervous system is protected by:

    ·         Cranial Cavity

·         Meninges

    

    ·         Fluid layers in between (Cerebrospinal Fluid)

 *Brain floats in Cerebrospinal Fluid (CSF). It acts like a cushion for the brain. That’s why the brain  does not put weight on the neck.

Meninges (3 membranous coverings)

    ·         Outer most Meninx is called as Dura Matter (Separates right and left hemisphere of Cerebrum and   is partition between Cerebrum, Cerebellum and Hypophysis Cerebri)

   

    ·         Middle layer is called as Arachnoid Matter

·            Inner most layer is called as Pia Matter

Introduction

1.       Brain is very important and delicate organ. It is protected by the following coverings:

·         Bony covering of the Cranium (Cranial Cavity)

·         Three membranous coverings (Meninges)

o   Outer Dura Matter (Pachymeninx)

o   Middle Arachnoid Matter

o   Inner Pia Matter

* The Arachnoid Matter and Pia Matter together is known as Leptomeninges.

o   Cerebrospinal Fluid fills the space between the Arachnoid and the pia (Subarachnoid Space) and acts as a water cushion.

DURA MATTER

   ·         It is a thick membrane that is the outermost of the 3 layers of the meninges that surrounds the brain and the spinal cord.

   ·         It is derived from Mesoderm.

   

   ·         Extends into the skull cavity as Falx Cerebri (Crescent shaped fold of meningeal layer of Dura Matter that descends vertically in the longitudinal fissure between Cerebral Hemispheres of Human Brain) and Tentorium Cerebelli (Separates Cerebellum from the inferior portion of the Occipital Lobes).

ARACHNOID MATTER

   ·   Thin transparent membrane that loosely surrounds the Brain without dipping into Sulci (Groove or furrow on the surface of the Brain)

   

   ·         Bridges all the irregularities of the Brain; expect of the stem of the Lateral Sulcus (Divides both frontal lobe and parietal lobe above from the temporal lobe below)

·         Cannot identified in Hypophyseal Fossa.

   

   ·         Arachnoid Matter is separated from Dura Matter by the Subdural Space and from Pia Matter by Subarachnoid Space containing Cerebrospinal Fluid.

   ·         It provides sheaths for the cranial nerves as far as their exit from the skull.

   

   ·         Arachnoid villi are small, finger like processes of arachnoid tissue, projecting into the cranial venous sinuses. They absorb CSF. With advancing age the arachnoid villi enlarges to form pedunculated tufts called Arachnoid Granulations. These granulations may produce depressions in bone.

Pia Matter

   ·         Thin vascular membrane which closely invests the brain.

   ·         Dipping into various sulci and other irregularities of its surface

   ·         Better defined around brain stem

   ·         Provides sheaths for the cranial nerves merging with the epineurium around them.

   ·         Provides perivascular sheath for the minute vessels entering and leaving the brain substance.

   ·         Folds of pia mater enclosing tuft of capillaries form the Telachoroidea. Such pia mater lined by secretory ependyma form the Choroid Plexus.

Folds	Shape	Attachments	Venous Sinuses enclosed
Falx Cerebri	Sickle shaped, separates the right and the left cerebral hemisphere	Superior: - convex margins attached to the sides of the groove lodging the superior sagittal sinus. Inferior: - concave margin is free. Anterior: - attached to Crista Galli Posterior: - upper surface of Tentorium Cerebelli	·         Superior Sagittal Sinus ·         Inferior sagittal sinus ·         Straight sinus
Tentorium Cerebelli	Tent shaped, separates the cerebral hemispheres from hind brain and lower part of midbrain. Lifts of the weight of the occipital lobes from Cerebellum.	Has free Anterior margins. Its ends are attached to anterior clinoid processes. Rest is free and concave. Posterior margin is attached to the lips of the groove containing transverse sinuses, Superior Petroseal Sinuses and to the posterior clinoid processes.	Transverse sinuses, superior petroseal sinuses
Falx Cerebelli	Small sickle shaped fold, partly separating two cerebellar hemispheres	Base is attached to the posterior part of the inferior surface of Tentorium Cerebelli. Apex reaches till foramen magnum	Occipital Sinus
Diaphragm Sellae	Small horizontal fold	Anterior: - attaches to tubercullum sellae Posterior: - attaches to dorsum sellae Laterally: - continues with dura matter of middle cranial fossa.	Anterior and posterior intercavernous sinuses

EXTRADURAL (Epidural) AND SUBDURAL SPACES

  Extradural (Epidural) space: - Potential space between the inner aspects of the skull bone and the endosteal layer of the dura mater.

Subdural Space: - Potential space between the Dura and Arachnoid maters.

* These become actual spaces in pathological conditions.

SUBARACHNOID SPACE

·         It’s the space between the Arachnoid and the Pia mater.

·         Traversed by the network of the Arachnoid trabeculae which give sponge appearance.

·         Surrounds the brain and Spinal Cord

·         Ends below at the lower border of second sacral vertebra (S2 vertebra)

·         Contains CSF and large vessels of the brain

·         Cranial nerves pass through the space

·         At the base of the brain and around the brain stem the subarachnoid space forms the intercommunicating pools called Cisterns.

* Cisterns reinforce the protective effect of the CSF on the vital centres situated in the Medulla.

* Subarachnoid Cisterns are as follows: -

o   Cerebellomedullary Cisterns (Cisterna Magna): - Lies between the Medulla and the undersurface of the Cerebellum. Utilized for cisternal puncture. CSF passes from forth ventricle to this Cistern through medial and lateral apertures of fourth ventricle.

o   Pontine Cisterns (Cisterna Pontis): - Contains the vertebral and basilar arteries (supplies the brain with oxygen rich blood).

o   Interpeduncular Cisterns (Basal cistern): - Contains Circulus Arteriosus (Circle of Willis; it is a circulatory anastomosis that supplies the brain and the surrounding structures).

o   Cistern of the Lateral Sulcus: - Contains middle Cerebral Artery.

o   Cistern of the Great Cerebral Vein (Cisterna Ambiens): - Contains great cerebral vein.

o   Cisterna Chiasmatis: - Lies below and in front of the Optic Chiasma.

* The arterial pulsations within the Cisterns helps to force the CSF From the cisterns on to the surface of the hemispheres.

* Cisterns themselves form the cushions around Medulla.

* Subarachnoid space communicates with the ventricular system of the brain at:

  o   Median foramen (of Magendie)

  o   Two lateral foramina (of Luschka), situated in the roof of the fourth ventricle.

·         CSF passes through these foramina from fourth ventricle to the subarachnoid space.

* Prolongations: -

  

  o   Space is prolonged into the arachnoid sheaths around the nerves where it communicates with Neural Lymphatics, particularly around the first, second and eighth Cranial Nerves 

  

  o  It also extends into Pial sheaths around the vessels entering the brain substance (Perivascular Space). Thus CSF comes into direct contact with the nerve cells.

CEREBROSPINAL FLUID (CSF)

    ·         It’s a modified tissue fluid.

·           Contained in the Ventricular System of brain and in the Subarachnoid Space around the brain and  Spinal Cord.

·          CSF replaces lymph in the CNS.

Formation:

·        

        Bulk of CSF is formed by the choroid plexuses of the lateral ventricles and lesser amounts by the Choroid Plexuses of the third and fourth ventricles.

·         It is also formed by the capillaries on the surface of the brain and spinal cord.

* Total quantity of CSF is about 150 ml. It is formed at the rate of about 200 ml per hour or 5000 ml per day. The normal pressure of CSF is 60- 100 mm of CSF (or of water).

Circulation

·         CSF passes from lateral ventricle to the third ventricle through the Interventricular foramen (of Monro).

·         From the third it passes to the fourth ventricle through the Cerebral Aqueduct.

·         From fourth ventricle through the median and lateral apertures of the fourth ventricle.

Absorption:

·         

      CSF is absorbed chiefly through the Arachnoid villi and Granulations and thus drained into the Cranial Venous Sinuses.

·         It is also absorbed partly by the Perineural Lymphatics around the first, second and eighth Cranial nerves.

·         It is also absorbed by veins related to Spinal Nerves.

Functions:

·         Protects brain and spinal cord from trauma.

·         Supplies nutrients to nervous system tissue.

·         Removes waste products from Cerebral metabolism.