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The meninges refer to the membranous coverings of the brain and spinal cord. There are three layers of meninges, known as the dura mater, arachnoid mater and pia mater. These coverings have two major functions:
The meninges are often involved cerebral pathology, as a common site of infection (meningitis), and intracranial bleeds. In this article, we shall look at the anatomy of the three layers, and their clinical correlations. Fig 1 – Overview of the meninges, and their relationship to the skull and brain. Dura MaterThe dura mater is the outermost layer of the meninges and is located directly underneath the bones of the skull and vertebral column. It is thick, tough, and inextensible. The dura mater consists of two layered sheets of connective tissue:
The dural venous sinuses are located between the two layers of dura mater. They are responsible for the venous drainage of the cranium and empty into the internal jugular veins. The dura mater receives its own vascular supply – primarily from the middle meningeal artery and vein. It is innervated by the trigeminal nerve (V1, V2 and V3). Dural ReflectionsThe meningeal layer of dura mater folds inwards upon itself to form four dural reflections. These reflections project into the cranial cavity, dividing it into several compartments – each of which houses a subdivision of the brain. The four dural reflections are:
Clinical Relevance: Extradural and Subdural HaematomasA haematoma is a collection of blood. As the cranial cavity is effectively a closed box, a haematoma can cause a rapid increase in intra-cranial pressure. Death will result if untreated. There are two types of haematomas involving the dura mater:
Arachnoid MaterThe arachnoid mater is the middle layer of the meninges, lying directly underneath the dura mater. It consists of layers of connective tissue, is avascular, and does not receive any innervation. Underneath the arachnoid is a space known as the sub-arachnoid space. It contains cerebrospinal fluid, which acts to cushion the brain. Small projections of arachnoid mater into the dura (known as arachnoid granulations) allow CSF to re-enter the circulation via the dural venous sinuses. Fig 3 – Coronal section of the skull, meninges and cerebrum. An arachnoid granulation is visible in the centre. Pia MaterThe pia mater is located underneath the sub-arachnoid space. It is very thin, and tightly adhered to the surface of the brain and spinal cord. It is the only covering to follow the contours of the brain (the gyri and fissures). Like the dura mater, it is highly vascularised, with blood vessels perforating through the membrane to supply the underlying neural tissue. Fig 4 – Autopsy of a patient with meningitis. The dura mater is being retracted to show a grossly swollen cerebrum with pus accumulation. Clinical Relevance: MeningitisMeningitis refers to inflammation of the meninges. It is usually caused by pathogens, but can be drug induced. Bacteria are the most common infective cause. The most common organisms are Neisseria meningitidis and Streptococcus pneumoniae. The immune response to the infection causes cerebral oedema, consequently raising intra-cranial pressure. This has two main effects:
Both complications can rapidly result in death. What is the order of the layer of meninges from the innermost to the outermost?Three layers of membranes known as meninges protect the brain and spinal cord. The delicate inner layer is the pia mater. The middle layer is the arachnoid, a web-like structure filled with fluid that cushions the brain. The tough outer layer is called the dura mater.
What is the order of meninges starting at the skull quizlet?What are meninges? 1) Dura mater - the dura mater layer is the thickest, outermost layer. 2) Arachnoid mater - this layer has a weblike appearance. 3) Pia mater - this is the innermost layer and is very thin at only one cell thick.
What is the correct order of meninges from superficial to deep?Three layers called the meninges encase the brain and spinal cord. From superficial to deep, these layers are the dura mater, arachnoid mater, and pia mater.
What is the outermost covering of the brain quizlet?The cranial meninges are continuous with the spinal meninges, have the same basic structure, and bear the same names: the outer dura mater, the middle arachnoid mater , and the inner pia mater The cranial dura mater has two layers; the spinal dura mater has only one.
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