Coverings and Fluids of the Brain
Objective 8
Describe the blood supply of the brain and the distribution of cerebrospinal fluid (CSF).
These layers are collectively called the meninges (singular meninx).
From the skull to the surface of the brain, the meninges comprise:
- dura mater (Latin: “tough mother”), a leathery covering;
- arachonoid mater (“spiderweb mother”), which resembles a wet spider web and is normally filled with cerebrospinal fluid (CSF);
- pia mater (“delicate mother”), like a coating of paint on the surface of the brain, which cannot be separated from the substance (parenchyma) of the brain.
How in the world did these meninges get such strange names? They were originally named by a Persian anatomist, Ali ibn Abbas, in the 10th century. There is no Arabic word for “membrane” so he wrote al umm, “the mother”, “the womb”. The European translators rendered this literally into the Latin “mother” instead of figuratively into the original meaning of mother as a womb, swaddling or covering.
The term epidural, as in epidural anesthesia, means “on top of the dura”. Epidural anesthesia is a deadening agent applied on top of (“epi-”) the dura mater. Patients who suffer bleeding into the brain may have blood collecting in the epidural space (between the skull and dura); tearing apart the subdural region (between the dura and arachnoid); or filling the subarachnoid space (between the arachnoid and the pial surface of the brain).
The brain has a series of hollow spaces. These are called ventricles.
Together, the ventricles and subarachnoid space are filled with cerebrospinal fluid (CSF), a filtrate of blood plasma that cushions and supports the brain, as well as supplying a favorable ionic environment for neurons (that is, isotonic and mostly NaCl). Scientists estimate that the brain, at an average 1400 g, weighs only 50 g when buoyed by CSF.
Cerebrospinal fluid is made in the choroid plexus, a specialized tissue that is found mainly in the lateral ventricles and fourth ventricle. There, the blood is filtered by ependymal cells, a type of glial cell.
CSF is almost identical to plasma; it is slightly lower in glucose and amino acids than plasma, and much lower in protein.
CSF circulates and is turned over about four times a day. It can be sampled with relatively little risk to the patient from the lumbar cistern, a large sac containing the cauda equina floating in CSF which is accessed from the intervertebral space between L2 and L3. Because of the circulation of CSF, a lumbar puncture may detect bleeding that originated (for example) in the subarachnoid space around frontal cortex.
The resorption of CSF is thought to occur in the arachnoid villi, a collection of structures along the midline of the superior aspect of the skull.
The ventricles include:
- Lateral ventricles, shaped like the letter C with a tail. This pair of ventricles is located beneath the white matter of cerebral cortex. There is no “first” or “second” ventricle, but there s