MENINGES

ANATOMY AND PHYSIOLOGY

Dr SHAROJ KHANMBBS (NGMC)

• Brain n and spinal cord are completely surrounded by three layers of tissue, the meninges, lying between the skull and the brain, and between the vertebral foramina and the spinal cord.

• The meninges are three layers of protective tissue called the dura mater, arachnoid mater, and pia mater.

• Named from outside inwards the layers of meninges are the:

• Dura mater• Arachnoid mater• Pia mater.

• Dura and arachnoid maters are separated by a potential space, the subdural space.

• Arachnoid and pia maters are separated by the subarachnoid space, containing cerebrospinal fluid.

DURA MATER

• The dura mater is the most superior of the meningeal layers.

• This forms several structures that separate the cranial cavity into compartments and protect the brain from displacement.

• The falx cerebri separates the hemispheres of the cerebrum.

• The  falx cerebelli separates the lobes of the cerebellum.

• The tentorium cerebelli separates the cerebrum from the cerebellum.

• The dura mater also forms several vein-like sinuses that carry blood (which has already given its supply of oxygen and nutrients to the brain) back to the heart.

• Venous blood from the brain drains into venous sinuses between the two layers of dura mater.

• Spinal dura mater forms a loose sheath round the spinal cord, extending from the foramen magnum to the 2nd sacral vertebra.

• Thereafter it encloses the filum terminale and fuses with the periosteum of the coccyx.

Arachnoid mater• Arachnoid mater is the middle layer of the meninges.

•  In some areas, it projects into the sinuses formed by the dura mater.

• These projections are the arachnoid granulation/arachnoid villi. They transfer cerebrospinal fluid from the ventricles back into the bloodstream.

•  Subarachanoid space lies between the arachnoid and pia mater.

• It is filled with cerebrospinal fluid. All blood vessels entering the brain, as well as cranial nerves pass through this space.

• It continues downwards to envelop the spinal cord and ends by merging with the dura mater at the level of the 2nd sacral vertebra.

•  The term arachnoid refers to the spider web like appearance of the blood vessels within the space.

Pia Mater• Pia mater is the innermost layer of the meninges.

•  Unlike the other layers, this tissue adheres closely to the brain, running down into the sulci and fissures of the cortex.

• It fuses with the ependyma, the membranous lining of the ventricles to form structures called the choroid plexes which produce cerebrospinal fluid.

Ventricles of the brain andthe cerebrospinal fluid

• The brain contains four irregular-shaped cavities, or ventricles, containing cerebrospinal fluid (CSF)

• They are:• Right and left lateral ventricles• Third ventricle• Fourth ventricle

• The lateral ventricles:These cavities lie within the cerebral hemispheres, oneon each side of the median plane just below the corpuscallosum.• They are separated from each other by a thin

membrane, the septum lucidum, and are lined with ciliated epithelium.

• They communicate with the third• ventricle by interventricular foramina.

• The third ventricle• Third ventricle is a cavity situated below the lateral

ventricles between the two parts of the thalamus. It communicates with the fourth ventricle by a canal, the cerebral aqueduct.

• The fourth ventricle- The fourth ventricle is a diamond-shaped cavity

situated below and behind the third ventricle, between the cerebellum and pons.

• It is continuous below with the central canal of the spinal cord and communicates with the subarachnoid space by foramina in its roof.

Cerebrospinal fluid (CSF)

• CSF is clear, colorless and transparent fluid Circulates through cavity of the Brain Subarachnoid space Central canal of spinal cord.

Functions of the Cerebrospinal Fluid

• Cushions and protects the central nervous system from trauma

• Provides mechanical buoyancy and support for the brain • Serves as a reservoir and assists in the regulation of the

contents of the skull • Nourishes the central nervous system • Removes metabolites from the central nervous system