meninges

(anatomy) The membranes that cover the brain and spinal cord; there are three in mammals and one or two in submammalian forms

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In mammals, the three membranes that cover the brain and spinal cord: the dura mater, the arachnoid membrane, and the pia mater. The outermost, the dura mater, is a tough, fibrous, double-layered structure that is adherent to the skull. The inner layer of the dura mater sends separating sheets between the cerebral hemispheres and between the cerebrum and cerebellum. It also contains large venous sinuses and forms sheaths for nerves leaving the skull. The middle layer, the arachnoid, is a delicate serous layer loosely investing the brain. Below this is the spongy subarachnoid cavity which contains the circulating cerebrospinal fluid. The innermost layer, the pia mater, is a vascular layer which closely follows each convolution of the brain. Together the meninges furnish protection, blood supply, drainage, and cerebrospinal channels for the brain. See also Nervous system (vertebrate).

Meninges is the plural of meninx — Greek for membrane. The term encompasses a group of three membranes that provide mechanical protection and support to the delicate tissue of the central nervous system — the brain and the spinal cord. Moving inwards from the skull towards the brain, or from the vertebral canal towards the spinal cord, the three meninges are: dura mater or pachymeninx (Greek pachy, meaning thick), arachnoid mater, and pia mater. Arachnoid and pia are also called leptomeninges (thin membranes).

The dura consists of an outer layer, rich in blood vessels and nerves, and an inner layer, firmly attached to the arachnoid. At some sites within the skull, these two layers are separated, forming channels for blood draining from the brain into the veins — the venous sinuses. The sagittal sinus, for example, curves from front to back over the midline of the brain — ‘in the line of an arrow’ (Latin sagitta, an arrow). The dura also extends into membranes that subdivide the cranial cavity into compartments: the central, vertical falx cerebri (Latin falx — a sickle — which describes its shape), which separates the cerebral hemispheres, and the tentorium cerebelli, a ‘tent’ stretched over the cerebellum, forming the roof of the posterior fossa of the skull, which contains the cerebellum and brain stem. The free front margin of the tentorium fits closely round the back of the brain stem; the brain stem can be damaged, for example if a tumour growing above or below the ‘tent’ encroaches on this narrow space. The dura also forms a diaphragm above the pituitary gland, through which passes the pituitary stalk, joining the gland to the hypothalamus.

The blood supply to the dura is provided by an artery (middle meningeal) that is vulnerable to laceration by fracture of the skull; this can cause an epidural haemorrhage between the skull and the dura.

The dura of the brain is in continuation with the dura of the spinal cord, which is separated from the periosteum (the covering of the vertebral bones) by a narrow epidural space. Epidural anaesthesia to eliminate sensation from lower regions of the body, especially in childbirth, involves injection of drugs into this space. The dura tapers at the lower end of the spinal cord, forming a sheath around its thin remnant (the filum terminale). At the gaps between the vertebral bones, on each side, the dura forms a sleeve around the nerve roots that carry sensory and motor information to and from the spinal cord down its whole length.

The arachnoid surrounds the brain (bridging the sulci — the furrows on the surface of the cerebral cortex), and also the spinal cord and the cranial and spinal nerves. The space between arachnoid and pia is called the subarachnoid space, and contains the cerebrospinal fluid (CSF), which drains out of the cerebral ventricles. The space is narrow over the convexity of the brain and wider below and around the brain stem, where large spaces are formed, called cisternae by obvious reference to water cisterns.

The subarachnoid space surrounds the spinal cord and extends beyond its lower end, forming a ‘cistern’ that extends from the upper lumbar to the sacral part of the bony canal. This provides a site from which CSF can be sampled through a hollow needle for diagnostic purposes — the so-called lumbar puncture.

The subarachnoid space contains arteries and veins, which can be the site of abnormalities such as cerebral aneurysms — ‘blow-outs’ of artery walls, or malformations of the blood vessels. Rupture of these is the cause of subarachnoid haemorrhage.

The subarachnoid space is continuous with the cavities of the cerebral ventricles. The rate of formation of new CSF within the cerebral ventricles is matched by a continuous flow through the subarachnoid space, back into the bloodstream. The route for this is provided by protrusions of the leptomeninges into the sagittal venous sinus. These protrusions are the arachnoid villi. They act as passive, pressure-dependent valves that discharge the CSF from the subarachnoid space into the sinus.

The innermost meningeal layer, the pia mater, is closely applied to the surface of the brain tissue and carries many small arteries and veins. The pia and arachnoid follow the branches of the surface blood vessels where they penetrate the brain tissue, so that a microscopic CSF-containing space surrounds them as far as the capillaries.

Meningitis — inflammation of the meninges — can be a dramatically severe and dangerous illness if due to infection by meningococcus bacteria — but is often relatively innocuous when (now most commonly) caused by one of many possible virus infections.

— Francesco Scaravilli

See also blood-brain barrier; cerebrospinal fluid; cerebral ventricles.

The membranes that surround the brain and spinal cord.

The three membranes covering the brain and spinal cord: the dura mater, arachnoid and pia mater. Consists of cranial and spinal membranes and special sections of it in diaphragma sellae, falx cerebri, tentorium cerebelli.

The three membranes enclosing the brain and the spinal cord, comprising the dura mater, the pia mater, and the arachnoid.

• Brain and Central Nervous System - meninges: sing. meninx; three membranes covering spinal cord and brain

Meninges
Meninges of the CNS
Gray's	subject #193 872
Artery	middle meningeal artery, meningeal branches of the ascending pharyngeal artery, accessory meningeal artery, branch of anterior ethmoidal artery, meningeal branches of vertebral artery
Nerve	middle meningeal nerve, nervus spinosus
MeSH	Meninges

The meninges (singular meninx from the Greek μῆνιγξ, "membrane"^[1]) is the system of membranes which envelopes the central nervous system. The meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater. The primary function of the meninges and of the cerebrospinal fluid is to protect the central nervous system.

Contents 1 Anatomy 1.1 Dura mater 1.2 Arachnoid mater 1.3 Pia mater 1.4 Spaces 2 Pathology 3 Additional images 4 References

Anatomy

Dura mater

The dura mater [Lt. Dura: Tough + mater: Mother] (also rarely called meninx fibrosa, or pachymeninx) is a thick, durable membrane, closest to the skull. It consists of two layers, the periosteal layer which lies closest to the calvaria, and the inner meningeal layer which lies closer to the brain. It contains larger blood vessels which split into the capillaries in the pia mater. It is composed of dense fibrous tissue, and its inner surface is covered by flattened cells like those present on the surfaces of the pia mater and arachnoid. The dura mater is a sac which envelops the arachnoid and has been modified to serve several functions. The dura mater surrounds and supports the large venous channels (dural sinuses) carrying blood from the brain toward the heart. The meningeal layer has dura infoldinings which include : 1) Falx cerebri, which is the largest sickle shaped separates the cerebral hemisphere. Starts from the frontal crest of frontal bone and the cristia galli runs till the occipital protuberance 2) Tentorium cerebelli, Second largest infoldining crescent shaped separates the occipital lobes from cerebellum the flax cerebri attachés to it giving a tent like appearance 3) Falx cerebelli, vertical infolding lies inferior to the Tentorium cerebelli, separates the cerebellar hemispheres 4) Diaphragma'' sellae, smallest infolding covering the pituitary gland attachments to the sella Turcia

Arachnoid mater

The middle element of the meninges is the arachnoid mater, so named because of its spider web-like appearance. It provides a cushioning effect for the central nervous system. The arachnoid mater exists as a thin, transparent membrane. It is composed of fibrous tissue and, like the pia mater, is covered by flat cells also thought to be impermeable to fluid. The arachnoid does not follow the convolutions of the surface of the brain and so looks like a loosely fitting sac. In the region of the brain, particularly, a large number of fine filaments called arachnoid trabeculae pass from the arachnoid through the subarachnoid space to blend with the tissue of the pia mater.

The arachnoid and pia mater are sometimes together called the leptomeninges.

Pia mater

The pia or pia mater[Lt. Pia: Soft + mater: Mother] is a very delicate membrane. It is the meningeal envelope which firmly adheres to the surface of the brain and spinal cord. As such it follows all the minor contours of the brain (gyri and sulci). It is a very thin membrane composed of fibrous tissue covered on its outer surface by a sheet of flat cells thought to be impermeable to fluid. The pia mater is pierced by blood vessels which travel to the brain and spinal cord, and its capillaries are responsible for nourishing the brain.

Spaces

The subarachnoid space is the space which normally exists between the arachnoid and the pia mater, which is filled with cerebrospinal fluid.

Normally, the dura mater is attached to the skull, or to the bones of the vertebral canal in the spinal cord. The arachnoid is attached to the dura mater, while the pia mater is attached to the central nervous system tissue. When the dura mater and the arachnoid separate through injury or illness, the space between them is the subdural space.

Pathology

There are three types of hemorrhage involving the meninges:^[2]

• A subarachnoid hemorrhage is acute bleeding under the arachnoid; it may occur spontaneously or as a result of trauma.
• A subdural hematoma is a hematoma (collection of blood) located in a separation of the arachnoid from the dura mater. The small veins which connect the dura mater and the arachnoid are torn, usually during an accident, and blood can leak into this area.
• An epidural hematoma similarly may arise after an accident or spontaneously.

Other medical conditions which affect the meninges include meningitis (usually from fungal, bacterial, or viral infection) and meningiomas arising from the meninges or from tumors formed elsewhere in the body which metastasize to the meninges.

Additional images

• 

  Diagrammatic representation of a section across the top of the skull

• 

  Diagrammatic section of scalp.

• 

References

1. ^ μήνιγξ, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
2. ^ Orlando Regional Healthcare, Education and Development. 2004. "Overview of Adult Traumatic Brain Injuries." Retrieved on January 16, 2008.

v d e Nervous system (TA A14, GA 9) Central nervous system Meninges Spinal cord Brain Rhombencephalon Medulla oblongata Pons Cerebellum Mesencephalon Prosencephalon Diencephalon Telencephalon Peripheral nervous system Somatic Sensory nerve Motor nerve Cranial nerve Spinal nerve Autonomic Sympathetic Parasympathetic Enteric nervous system M: CNS anat(n/s/m/p/4/e/b/d/c/a/f/l/g)/phys/devp noco(m/d/e/h/v/s)/cong/tumr, sysi/epon, injr proc, drug(N1A/2AB/C/3/4/7A/B/C/D) M: PNS anat(h/r/t/c/b/l/s/a)/phys(r)/devp/prot/nttr/nttm/ntrp noco/auto/cong/tumr, sysi/epon, injr proc, drug(N1B)

v d e Anatomy: meninges of the brain and medulla spinalis (TA A14.1.01, GA 9.749/9.872) Layers Dura mater Falx cerebri · Tentorium cerebelli · Falx cerebelli · Diaphragma sellae · Trigeminal cave Arachnoid mater Arachnoid granulation · Arachnoid trabeculae Subarachnoid cisterns: Cisterna magna · Pontine cistern · Interpeduncular cistern · Chiasmatic · Lateral cerebral fossa · Of great cerebral vein · Of lamina terminalis Pia mater Denticulate ligaments Tela chorioidea (Tela chorioidea of third ventricle, Tela chorioidea of fourth ventricle) · Choroid plexus Combined Filum terminale · Leptomeninges Spaces Epidural space · Subdural space · Subarachnoid space (Cerebrospinal fluid) M: CNS anat(n/s/m/p/4/e/b/d/c/a/f/l/g)/phys/devp noco(m/d/e/h/v/s)/cong/tumr, sysi/epon, injr proc, drug(N1A/2AB/C/3/4/7A/B/C/D)

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