Our Three (Brain) Mothers

Protecting our brain and central nervous system are the meninges, derived from the Greek term for “membrane”. You may have heard of meningitis - this is when the innermost layer of the meninges swells, often due to infection, and can cause nerve or brain damage, and sometimes death.

There are three meningeal layers: the dura mater, arachnoid mater, and pia mater. In Latin, “mater” means “mother”. The term comes from the enveloping nature of these membranes, but we later learned how apt it was, because of how protective and essential the meningeal layers are.

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  • The dura mater is the outermost and toughest membrane. Its name means “tough mother”.

The dura is most important for keeping cerebrospinal fluid where it belongs, and for allowing the safe transport of blood to and from the brain. This layer is also water-tight - if it weren’t, our cerebrospinal fluid (CSF) would leak out, and our central nervous system would have no cushion! Its leathery qualities mean that even when the skull is broken, more often than not, the dura (and the brain it encases) is not punctured.

  • The arachnoid mater is the middle membrane. Its name means "spider-like mother", because of its web-like nature.

The arachnoid is attached directly to the deep side of the dura, and has small protrusions into the sinuses within the dura, which allows for CSF to return to the bloodstream and not become stagnant. It also has very fine, web-like projections downward, which attach to the pia mater. However, it doesn’t contact the pia mater in the same way as the dura: the CSF flows between the two meningeal layers, in the subarachnoid space. The major superficial blood vessels are on top of the arachnoid, and below the dura.

  • Pia mater is the innermost membrane, which follows the folds (sulci) of the brain and spinal cord most closely. Its name means “tender mother”.

The pia is what makes sure the CSF stays between the meninges, and doesn’t just get absorbed into the brain or spinal cord. It also allows for new CSF from the ventricles to be shunted into the subarachnoid space, and provides pathways for blood vessels to nourish the brain. While the pia mater is very thin, it is water-tight, just like the dura mater. The pia is also the primary blood-brain barrier, making sure that no plasma proteins or organic molecules penetrate into the CSF. 

Because of this barrier, medications which need to reach the brain or meninges must be administered directly into the CSF.

Images:
Anatomy: Practical and Surgical. Henry Gray, 1909.

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