Overview

From outside in, the meninges are:
• the dura mater (pachymeninx, "tough mother"), a dense fibrous sheet;
• the arachnoid mater, a thin, avascular, cobweb-like layer;
• the pia mater, a delicate vascular membrane adherent to the brain and cord.
The arachnoid and pia together are the leptomeninges.

Around the brain the dura has two layers. The outer periosteal (endosteal) layer is the periosteum of the inner skull, and the inner meningeal layer folds inwards to form partitions (falx cerebri, tentorium cerebelli). Where the two layers separate they enclose the dural venous sinuses. Around the spinal cord the dura is a single meningeal tube, separated from the vertebrae by a true epidural space containing fat and the internal vertebral venous plexus.

Three spaces relate to the meninges:
• the epidural (extradural) space: potential in the skull, real in the vertebral canal;
• the subdural space: normally potential, opened by blood or fluid within the dural border cell layer;
• the subarachnoid space: real, filled with CSF, carrying the major cerebral arteries and veins and enlarging into cisterns.
Arachnoid trabeculae cross the subarachnoid space to the pia. Arachnoid granulations protrude into the venous sinuses to return CSF to the blood. Together with the CSF, the meninges cushion and suspend the brain, carry its vessels and wall it off from infection.

Location: Surrounding the entire brain and spinal cord; spinal dura and arachnoid end at the level of S2, the pia continuing as the filum terminale.

Parts & subdivisions

Dura mater
Outer fibrous layer; dural folds and venous sinuses.
Arachnoid mater
Avascular barrier layer; arachnoid trabeculae and granulations.
Pia mater
Adherent vascular inner layer; denticulate ligaments and filum terminale.
Epidural space
Between bone and dura. Potential in the skull (epidural haematoma); real in the vertebral canal (epidural anaesthesia).
Subdural space
Potential space at the dura–arachnoid interface (dural border cell layer), crossed by bridging veins (subdural haematoma).
Subarachnoid space
CSF-filled space between arachnoid and pia, with cisterns.
Dural venous sinuses
Endothelium-lined channels between the dural layers draining the brain to the internal jugular veins.

Anatomy

Development: The dura derives from paraxial mesoderm; the leptomeninges (arachnoid and pia) derive largely from neural crest in the forebrain region, with a mesodermal contribution elsewhere.

Function

Protect and support the CNS; anchor the brain and cord
Contain the CSF that cushions and buoys the brain
Carry blood vessels to and from the CNS surface
Provide venous drainage (dural sinuses) and CSF reabsorption (arachnoid granulations)
Limit brain movement within the skull (dural folds)

Clinical significance

Intracranial haemorrhage by compartment

• Epidural: arterial (middle meningeal artery), biconvex, limited by sutures.
• Subdural: venous (bridging veins), crescentic, crosses sutures.
• Subarachnoid: blood in the cisterns and sulci, usually from a berry aneurysm, causing a thunderclap headache.
See the system-level clinical notes for details.

Meningitis

Infection of the leptomeninges and CSF causes fever, headache, neck stiffness and photophobia. Neck stiffness and Kernig/Brudzinski signs reflect inflamed meninges stretched by flexion of the neck or hip. Bacterial meningitis is an emergency requiring immediate antibiotics and dexamethasone.

Meningioma

The most common primary intracranial tumour. It arises from arachnoid cap cells, is usually benign and slow-growing, and is commoner in women. Common sites are the parasagittal falx, the convexity and the sphenoid wing. Imaging shows an extra-axial, dural-based enhancing mass with a "dural tail", and histology shows whorls and psammoma bodies. Symptoms depend on compression (seizures, focal deficits).

Mnemonics

Order of the meninges

"PAD" from the brain outwards — Pia, Arachnoid, Dura (the brain is protected by a PAD).

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.