The Neuroscience on the Web Series:
CMSD 620 Neuroanatomy of Speech, Swallowing and Language

CSU, Chico, Patrick McCaffrey, Ph.D.


Chapter 3. The Meninges and Cerebrospinal Fluid


The Meninges

The meninges are three layers of protective tissue called the dura mater, arachnoid mater, and pia mater that surround the neuraxis. The meninges of the brain and spinal cord are continuous, being linked through the magnum foramen.

Dura Mater

The dura mater is the most superior of the meningeal layers. Its name means "hard mother" in Latin and it is tough and inflexible. This tissue 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.

The superior sagittal sinus runs across the top of the brain in an anterior-posterior direction.

Other sinuses include the straight sinus, the inferior sinus, and the transverse sinus.

The epidural space is a potential space between the dura mater and the skull. If there is hemorrhaging in the brain, blood may collect here. Adults are more likely than children to bleed here as a result of closed head injury.

The subdural space is another potential space. It is between the dura mater and the middle layer of the meninges, the arachnoid mater. When bleeding occurs in the cranium, blood may collect here and push down on the lower layers of the meninges. If bleeding continues, brain damage will result from this pressure. Children are especially likely to have bleeding in the subdural space in cases of head injury.

Arachnoid Mater

The arachnoid or 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.

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

Pia Mater

The 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.

Cerebrospinal Fluid

Purpose

Cerebrospinal fluid is a clear liquid produced within spaces in the brain called ventricles. Like saliva it is a filtrate of blood. It is also found inside the subarachnoid space of the meninges which surrounds both the brain and the spinal chord. In addition, a space inside the spinal chord called the central canal also contains cerebrospinal fluid.

It acts as a cushion for the neuraxis, also bringing nutrients to the brain and spinal cord and removing waste from the system.

Choroid Plexus

All of the ventricles contain choroid plexuses which produce cerebrospinal fluid by allowing certain components of blood to enter the ventricles. The choroid plexuses are formed by the fusion of the pia mater, the most internal layer of the meninges and the ependyma, the lining of the ventricles.

The Ventricles

These four spaces are filled with cerebrospinal fluid and protect the brain by cushioning it and supporting its weight.

The two lateral ventricles extend across a large area of the brain. The anterior horns of these structures are located in the frontal lobes. They extend posteriorly into the parietal lobes and their inferior horns are found in the temporal lobes.

The third ventricle lies between the two thalamic bodies. The massa intermedia passes through it and the hypothalamus forms its floor and part of its lateral walls.

The fourth ventricle is located between the cerebellum and the pons.

The four ventricles are connected to one another.

The two foramina of Munro, which are also know as the interventricular foramina, link the lateral ventricles to the third ventricle.

The Aqueduct of Sylvius which is also called the cerebral aqueduct connects the third and fourth ventricles.

The fourth ventricle is connected to the subarachnoid space via two lateral foramina of Luschka and by one medial foramen of Magendie.

Subarachnoid Space

Although cerebrospinal fluid is manufactured in all of the ventricles, it circulates through the system in a specific pattern, moving from the lateral ventricle to the third, and then from the third to the fourth. From the fourth ventricle, the cerebrospinal fluid passes into the subarachnoid space where it circulates around the outside of the brain and spinal cord and eventually makes its way to the superior sagittal sinus via the arachnoid granulations also called arachnoid villi. In the superior sagittal sinus, the cerebrospinal fluid is reabsorbed into the blood stream.

The cerebrospinal fluid of the neuraxis is regenerated several times every twenty-four hours.

Endolymph and perilymph, the fluids of the inner ear, are derived from cerebrospinal fluid. Currently, there is no consensus regarding the manner in which cerebrospinal fluid enters the inner ear. Osmosis may be involved.

A condition called hydrocephalus occurs when, for some reason, too much cerebrospinal fluid is produced and the ventricles swell, putting pressure on the tissue of the brain. Tumors are one potential cause of an over-production of cerebrospinal fluid.

Hydrocephalus should not be confused with hydroencephali. The term hydroencephali literally means "water brain" and refers to a rare birth defect in which the cerebrum is absent and the space where it should be is entirely filled with cerebrospinal fluid.

In the past, before CT and MRI technology existed, a technique involving cerebrospinal fluid called pneumoencephalography was used to view the brain. A small amount of cerebrospinal fluid was removed from the ventricular system and replaced with air or some other inert gas. This allowed the examiner to view the ventricles in a scan and make inferences about brain pathology. Tumors and hemorrhages could sometimes be located by examining the shapes and sizes of the ventricles. Because space within the cranium is limited, growths or coagulated blood (hematoma) will displace white and gray matter, pushing them into the ventricular system.

Cerebrospinal fluid can be analyzed to make judgments about a person's general health as can blood and saliva .A sample is taken from the spinal cord via a lumbar puncture which is also known as a spinal tap.


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Other courses in the Neuroscience on the Web series:
CMSD 636 Neuropathologies of Language and Cognition | CMSD 642 (Neuropathologies of Swallowing and Speech)

Copyright, 1997-2008. Patrick McCaffrey, Ph. D. This page is freely distributable.