Meninges | Neurology | Anjani Mishra

Written by Anjani Mishra

Meninges

The meninges are the fibrous membranes which enclose and protect the brain and the spinal cord. It can be divided into spinal and cranial meninges. Both spinal and cranial meninges consists of three membranes; dura mater, arachnoid and pia mater.

A.    Spinal meninges

B.    Cranial meninges

A.   Spinal meninges

a)      Dura Mater- pachymeninx (tough fibrous nature)

b)     Arachnoid Mater- leptomeninx (thin delicate nature)

c)      Pia Mater- both arachnoid mater and pia mater are connected and are commonly  

                  known as leptomeninx.

a)     Spinal Dura Mater

It is a membrane which is separated from the periosteum of the vertebra by the epidural cavity. The epidural cavity is filled with fat and contains the internal vertebral venous plexus formerly referred to as the longitudinal vertebral venous sinuses.

It is a long cylindrical tube surrounding the spinal cord. Lateral tubular extensions cover the spinal nerve roots and accompany them to the intervertebral foramina. As the dorsal and ventral roots join to form the spinal nerve, the dura mater forms a single sheath which is continuous with the epineurium of the spinal nerve. These dura mater tubes are firmly attached to the periosteum around the intervertebral foramen and thus, aid in anchoring the dura mater and spinal cord in the spinal canal.

The cavity between the dura mater and the arachnoid is the subdural cavity, which contains a very small amount of fluid. In normal living animal the subdural cavity is a capillary cleft because the cerebrospinal fluid(CSF) in the subarachnoid cavity presses the arachnoid membrane against the inner surface of the dura mater. In the cadaver a portion of the cerebrospinal fluid(CSF) is lost by diffusion and the arachnoid membrane is not held against the duramater and the subdural cavity appears much larger.

b)     Spinal Arachnoid Mater

It is a thin, almost transparent tube which sorrounds the spinal cord and, like the dura mater, has a tubular extensions which cover the roots of the spinal nerves. The arachnoid is connected to the pia mater by connective tissue trabeculae which pass across the subarachnoid cavity. 

The spinal arachnoid is continuous with the cranial arachnoid at the level of the foramen magnum. Caudally it forms a cone-shaped sac conforms to the cone shape of the spinal dura mater.

 The subarachnoid cavity- is a cavity between the arachnoid and the pia mater.The cavity is traversed by trabeculae which connect the arachnoid to the pia mater. The subarachnoid cavity surrounds the spinal nerve roots. It is filled with cerebrospinal fluid (CSF) which functions to cushion the spinal cord from traumatic shock. The subarachnoid cavity is enlarged in the area of the cauda equina in the caudal lumbar and sacral regions to form the lumbar cistern. The cerebrospinal fluid can be obtained from the cistern by lumbar puncture.

c)   Spinal Pia Mater  

It is highly vascularized layer which is firmly attached to the spinal cord and spinal nerve roots. The blood vessels course over the surface of the spinal cord in the pia mater and give off branches which penetrate into the spinal cord. The pia mater is contiguous with the neuroglial limiting membrane of the spinal cord.

The dentate ligament is a condensation of the pia mater on the lateral sides of the spinal cord midway between the dorsal and ventral roots of the spinal nerves. It serves to anchor the spinal cord centrally in the subarachnoid cavity within the dura mater tube.

 

A.   Cranial meninges

a)      Cerebral Dura Mater

b)     Cerebral Arachnoid Mater

c)      Cerebral Pia Mater

a)     Cerebral Dura Mater

• The dura mater of the cranial cavity is closely united with the endosteum of the cranial cavity. The dura mater consists of two membranes;

                                                        i.            An external or endosteal layer, &

                                                     ii.            An internal or meningeal layer.

• These two layers are closely united. The meningeal layer of cerebral dura mater is  separated from endosteal layer, where cranial venous sinuses are located between them.

• The dura mater is continuous with sheaths of the cranial nerves at foramina of the  skull through which the nerves enter or leave the cranial cavity. It also continues with the spinal dura mater at the level of foramen magnum.

•  The cranial dura mater has three internal folds or processes which separate parts of brain;

                                                        i.            Falx cerebri,

                                                     ii.            Tentorium cerebelli, &

                                                   iii.            Diphragma selli.

        i.            Falx cerebri

• The falx cerebri is a dorsal, mid sagittal sickle shaped fold of dura mater which extends ventrally between the cerebral hemispheres of the brain. 

• It is attached to the crista galli(a ridge on the ethmoid bone) rostrally and joins the tentorium cerebelli caudally. 

• At the convex dorsal midline, the two layers of the falx cerebri are separated by the dorsal sagittal venous sinus. The dorsal sagittal sinus is bounded by the endosteum and bone dorsally.

• At the junction of falx cerebri with the tentorium cerebelli, the straight venous sinus joins the dorsal joins the dorsal sagittal sinus caudo-dorsally.

• The ventral border of the falx cerebri is 1-2 cm dorsal to the corpus callosum of the brain and does not contain the ventral sagittal sinus as in man.

        i.          Tentorium cerebelli

• The tentorium cerebelli is the transverse partition between the cerebellum and the occipital poles of the cerebral hemisphere.

• The internal concavw border is free and forms the tentorial notch, which partially encircles the mesencephalon(mid brain).

• At the junction of the tentorium cerebelli with the endosteum of the parietal occipital bones, the transverse venous sinus is located between the layers of the tentorium and the endosteum.

Diphragma selli

• The diphragma selli is a horizontal sheet of dura mater which bridges the sella turcica. It separates the hypophysis from the ventral surface of the diencephalon. It has an opening for the infundibular part of the hypophysis.

b)     Cerebral Arachnoid Mater

• The  cerebral arachnoid is a very delicate, thin membrane situated between the dura mater and the pia mater.

• It is connected to the pia mater by a thin connective tissue trabeculae which pass through the subarachnoid cavity.

• The cerebral arachnoid does not extend into the sulci on the surface of the brain. In certain locations the cerebral arachnoid is separated from the pia mater by a conciderable distance to form the subarachnoid cisterns. The main cisternae are;

                                i.            Cisterna cerebello medullaris

                             ii.            Cisterna fossae lateralis cerebri

                           iii.            Cisterna chiasmatis

 

                                i.            Cisterna cerebello medullaris

• It is located in the angle formed by the caudal surface of the cerebellum and the dorsal surface of the medulla oblongata. 

                             ii.            Cisterna fossae lateralis cerebri

• It is located over the area of the lateral cerebral fissure.

                           iii.            Cisterna chiasmatis

• It is located to the optic chiasma and between the cerebral crura of the mesencephalon. 
• The cranial subarachnoid cavity is filled with cerebrospinal fluid and is continuous with the spinal subarachnoid cavity at the foramen magnum.

c)     Cerebral Pia Mater

• The pia mater is a thin connective tissue membrane that adheres closely to the brain.

• It receives the arachnoid trabeculae and forms the deep wall of the subarachnoid cavity.

• The pia mater is highly vascularized and extend deeply into the sulci of the cerebral hemisphere and the folia of the cerebellum.

• The blood vessels penetrating the nervous tissue are surrounded by the pia mater and the perivascular space.

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