Meninges: Protecting and Nourishing the Central Nervous System

Meninges: Protecting and Nourishing the Central Nervous System

The central nervous system, which includes the spinal cord and brain, is protected by a system of membranes called the meninges. This system consists of three layers of membranes, each with its own distinct structure and function.

1. Dura mater: The outermost layer, thick, tough, and inelastic. The outer surface of the dura mater is rough and adheres to the periosteum of the vertebral canal and skull, except in the region around the foramen magnum and around the skull. The inner surface of the dura mater is smooth, encasing the venous sinuses of the skull and covered by an endothelial layer. The dura mater contains many blood vessels.

Between the dura mater and the bony wall is the epidural space, containing fat tissue and the vertebral venous plexus. Between the dura mater and the arachnoid mater is the subdural space, which contains cerebrospinal fluid.

The dura mater splits into two layers at the venous sinuses of the dura mater, forming drainage tubes for venous blood. These tubes are lined with an endothelial layer.

2. Arachnoid mater: A thin, transparent layer of membrane located between the dura mater and the pia mater. The arachnoid mater does not penetrate the cerebral hemispheres and their sulci, but bridges over these sulci.

Between the arachnoid mater and the pia mater is the subarachnoid space, which contains cerebrospinal fluid. The subarachnoid space has delicate strands that connect the arachnoid mater to the pia mater. The subarachnoid space communicates with the ventricular system through the interventricular foramina and the lateral foramina of the roof of the fourth ventricle. The subarachnoid space also connects to the venous sinuses of the dura mater through arachnoid villi, which function to absorb cerebrospinal fluid.

3. Pia mater: The innermost layer, composed of loose connective tissue, containing many micro-blood vessels. The pia mater functions to nourish the brain and spinal cord. It covers the entire outer surface of the brain and spinal cord, and also penetrates deep into the sulci of the cerebral hemispheres. The pia mater, along with glial cells, helps to support and nourish neurons.

The pia mater forms the pial-glial membrane, which is the material basis of the blood-cerebrospinal fluid barrier. The subarachnoid space is the meshwork of the pial-glial membrane, containing cerebrospinal fluid.

Dural sac: The dural sac is the extension of the dura mater that encloses the spinal cord. Between the dural sac and the vertebral canal is the epidural space, containing fat tissue and the vertebral venous plexus. The base of the dural sac lies at the level of the 2nd sacral vertebra. The dura mater surrounds the cauda equina to form the dural sheath, which is firmly attached to the back of the sacrum.

Subarachnoid space of the spinal cord: The subarachnoid space in the spinal cord lies between the outer and inner layers of the arachnoid mater. The subarachnoid space contains connective strands, the denticulate ligaments, roots of the spinal nerves, and blood vessels. The subarachnoid space has two enlarged areas: the cisterna magna and the lumbar cistern. The lumbar cistern contains the cauda equina and lies at the base of the dural sac.

Denticulate ligaments: Denticulate ligaments are a band of pia mater that splits into two branches on each side of the spinal cord. The function of the denticulate ligaments is to connect the outer surface of the pia mater to the dura mater. The denticulate ligaments run from the foramen magnum to the conus medullaris of the spinal cord. Between the teeth of the denticulate ligaments are spaces for the anterior and posterior roots of the spinal nerves to connect with each other.

Meninges and nerves: The distal part of the spinal ganglion is covered by the dura mater. The proximal part of the spinal ganglion is covered by the arachnoid mater, separating the two roots. The pia mater forms the innermost layer around each spinal nerve root, extending to the intervertebral foramen and merging into the outer covering of the nerve.

Dura mater of the brain: The dura mater of the brain adheres to the bony wall and does not have an epidural space. The inner surface of the dura mater forms folds, creating the venous sinuses of the dura mater. The dura mater is firmly attached to the skull base in the region of the cranial vault. The area of the dura mater that is easily detachable in the cranial vault is from the posterior border of the lesser wing of the sphenoid bone to the occipital protuberance.

Folds of the dura mater of the brain:

• Falx cerebri: Shaped like a sickle, it adheres to the entire cranial vault, along the midline and extends into the longitudinal fissure between the two cerebral hemispheres.

• Tentorium cerebelli: Forms the roof over the posterior cranial fossa, separating the upper surface of the cerebellum from the occipital lobe of the cerebral hemispheres.

• Falx cerebelli: A small part of the dura mater, located in the occipital protuberance.

• Diaphragma sellae: A quadrilateral shape, forming the roof of the hypophyseal fossa, allowing the infundibulum of the pituitary gland to pass through.

Vascular system of the meninges: The dura mater of the brain is supplied by the middle meningeal artery, a branch of the maxillary artery, the posterior meningeal artery, and a branch of the vertebral artery. The venous blood of the meninges flows in two systems: the deep system and the superficial system. The deep system includes the basilar sinuses. The superficial system includes the superior sagittal sinus, the cavernous venous plexus, and the venous sinuses of the dura mater.

Subarachnoid space of the skull: The subarachnoid space of the skull expands to form cisterns: the cerebellomedullary cistern, the lateral cerebral fossa, the chiasmatic cistern, and the pontine cistern.

Cerebrospinal fluid: Cerebrospinal fluid is secreted by the choroid plexuses in the ventricles. Cerebrospinal fluid is contained within the ventricles and the subarachnoid space. Cerebrospinal fluid has different properties from plasma, being colorless, transparent, low in protein, low in glucose, high in Cl- and K+.

Functions of the meninges:

• Protection: The meninges protect the brain and spinal cord from mechanical, chemical, and infectious influences.

• Nourishment: The pia mater supplies blood and nutrients to the brain and spinal cord.

• Drainage of cerebrospinal fluid: The venous sinuses system of the dura mater drains venous blood from the brain. Arachnoid villi help to absorb cerebrospinal fluid.

The meninges are an important structure, ensuring the normal functioning of the central nervous system.