Your brain is wrapped in a tough mother, a spider web and a tender mother
Three membranes sheathe the human brain and spinal cord. Their Latin names are surprisingly affectionate: the dura mater, or tough mother, lines the skull; the gauzy arachnoid is named for a spider's web; and the pia mater, the tender mother, clings to every fold of the brain's surface.
Collectively called the meninges, from the Greek for membrane, these layers form a physical and immune barrier around the central nervous system. They anchor delicate tissue, hold the cerebrospinal fluid, and carry the arteries and veins that feed the brain. Each layer has its own molecularly distinct fibroblasts. Evolution added them gradually: fish have a single primitive membrane, amphibians and reptiles two, and birds and mammals three.
The dura is thick collagen and elastin fastened to the inside of the skull, made of two sheets that part to enclose the venous sinuses draining blood back towards the heart. It folds inward in four places. The largest, the sickle-shaped falx cerebri, divides the two hemispheres, while the tentorium cerebelli forms a tent-like shelf over the cerebellum. Beneath lies the loose, see-through arachnoid, linked by fine threads across a fluid-filled gap to the pia, a very thin sheet that follows every ridge and groove yet contains no capillaries of its own.
The dura is dense with pain-sensing nerve endings, which puts the meninges at the centre of migraine research. The trigeminal nerve carries sensation from this tissue to the brainstem, and a signalling molecule called CGRP, released when that nerve fires, widens meningeal blood vessels and activates pain receptors in the dura. Rising activity in the hypothalamus appears to precede attacks. The meninges are also where meningitis takes hold, and meningiomas, which arise from arachnoid cells, are the most common primary brain tumours in adults.
Knowledge of these layers is ancient. The Edwin Smith Papyrus from Egypt contains the first known mention of the dura, Hippocrates urged surgeons to keep it intact, and Galen described the pia in the second century AD. Gerardus Blasius identified the arachnoid in 1664, and Frederick Ruysch's 1699 description of its spidery look gave it its name.
Source: Meninges
