Download Anatomy 3- Gross Brain, Meninges, and CSF Meninges The brain

Anatomy 3- Gross Brain, Meninges, and CSF
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Meninges
The brain and spinal cord are surrounded by a series of membranous coverings known as the meninges (s.
meninx)
– The term “meninx” is mod. L. (from Gr.) meaning “membrane”
Reference is made to both cranial meninges and spinal meninges
– There are some differences between the two
The meninges have two general functions
– They provide a supportive framework for vasculature and cerebrospinal fluid (CSF)
– They provide protection for the brain and spinal cord
The protection afforded by the meninges is twofold
– The brain and spinal cord are physically suspended, or tethered within (and by) the meninges
– The brain and spinal cord float in CSF
• Brain in air weighs 1500 g, in CSF this figure falls to 50 g due to buoyancy
The meninges consist of three individual layers
The arachnoid mater and the pia mater are continuous with each other, thus they are sometimes referred to
together as the leptomeninx
Dura Mater
• The dura mater is the outermost meningeal layer
• It is a tough, thick, collagenous membrane providing protective covering
• The dura mater is sensitive to pain
• The cranial dura mater is adherent to the inside of the calvaria
• It is especially tenacious along suture lines and the base of the cranium
• The dura mater surrounding the brain is divided into two layers
• Periosteal layer: This is simply the inner periosteum of the skull (continuous with outer periosteum)
• Meningeal layer
• The two layers of dura are tightly fused together (No pathological conditions are known to separate them)
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As mentioned previously, the two layers of cranial dura mater are fused together throughout most of the skull
– However, at certain points the two layers exhibit a normal separation, thus forming venous sinuses
At the major brain fissures, the meningeal layer projects inward to form infoldings/partitions/septa
Four infoldings divide the cranial cavity into communicating compartments
– Falx cerebri : Divides cerebral hemispheres
– Tentorium cerebelli : Divides cerebellum from cerebral hemispheres
– Falx cerebella: Divides cerebellar hemispheres
– Diaphragma sellae: Covers pituitary gland
Anatomy 3- Gross Brain, Meninges, and CSF
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The dural infoldings have two main purposes
– Division of the cavity into compartments
– Restrict rotary displacement of the brain
Dural Venous Sinuses
• The dural sinuses are endothelial lined spaces between the periosteal and meningeal layers of cranial dura
mater (They collect venous blood from the brain)
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Innervation
The brain and the leptomeninges are not sensitive to pain
The principal pain-sensitive intracranial structures are the dura mater and the proximal portions of blood vessels
Many headaches are thought to result from stimulation of nerve endings in the dura
Innervation of the cranial dura mater is from two general sources
• CN V: Branches of the trigeminal nerve primarily innervate the floor of the anterior and middle cranial
fossae, and the falx cerebri, and tentorium cerebelli
• C2 and C3 fibers: Primarily innervate the floor of the posterior cranial fossa
Blood Supply
The cranial dura mater is supplied by the middle meningeal artery and branches
– The middle meningeal is a branch of the maxillary artery, which itself is a terminal branch of the external
carotid artery
The middle meningeal artery enters the cranium through the foramen spinosum
– This artery and its branches travel in the periosteal layer of dura and principally supply bone
Anatomy 3- Gross Brain, Meninges, and CSF
Spinal Dura Mater
• The meningeal layer of cranial dura mater continues inferiorly to become the spinal dura mater (which consists
of only a meningeal layer)
• Strictly speaking, a periosteal layer of dura mater still exists in the spinal canal
– It is separated from the meningeal layer to create the epidural space
• The spinal dura mater is innervated by recurrent meningeal nerves from mixed spinal nerves
– These may become irritated in meningitis
• Blood supply to the spinal dura is from spinal branches of posterior intercostal arteries
Leptomeninges (Arachnoid Mater and Pia Mater)
• The pia mater and arachnoid mater are continuous, as they develop from a single layer of mesenchyme
surrounding the embryonic brain
• The continuity of the leptomeninges is defined in two basic ways
• The membrane is continuous in the sense that the pia is the visceral part, adherent to the brain/spinal
cord, while the arachnoid is the parietal part, adherent to the dura mater
• The arachnoid and the pia are also joined to each other by arachnoid trabeculae
• The trabeculae—the “tethers” referred to earlier—stabilize the brain’s position within the skull
Arachnoid
• The arachnoid is a thin, semitransparent, avascular membrane
• It is essentially the inner lining of the dura
– It is not attached to the dura, but instead is held against it by the pressure of CSF
• Spinal arachnoid is very similar to cranial arachnoid
Pia
• The pia mater, a continuation of the arachnoid mater, closely adheres to all contours of the brain and spinal
cord
– Cranial pia follows arteries into the cerebral cortex of the brain
• Unlike the arachnoid, the pia is highly vascularized by a network of fine blood vessels
Spinal Pia
• The spinal pia is relatively thick and gives rise to two important tethering structures
– The bilateral denticulate ligaments anchors the length of the spinal cord to the dura mater
– The filum terminale ultimately tethers the cord to the coccyx
Subarachnoid Cisterns
• In most parts of the brain the arachnoid mater is closely applied to the pia mater
– The subarachnoid space is narrow
• However, in some areas the arachnoid mater bridges over larger surface irregularities creating cisterns
• The cisterns are useful as radiological landmarks, and also represent potential sites for CSF sampling
Anatomy 3- Gross Brain, Meninges, and CSF
Meningeal Spaces
Extracerebral Hemorrhages
Intracranial bleeding may be extracerebral or intracerebral
By definition, extracerebral hemorrhage occurs between the skull and the brain
– Intracerebral hemorrhage is into the substance of the brain
• Three types of extracerebral hemorrhage occur; one in each of the three meningeal spaces
Subarachnoid Space
• A subarachnoid hemorrhage is usually the result of a ruptured aneurysm
– A sudden increase in blood pressure is often the cause of rupture
• Cardinal sign is excruciating headache and stiff neck
– Blood can be sampled from CSF
Epidural Space
• The epidural, or extradural space (more appropriately, dura-cranial interface) is a potential site of hemorrhage
• This often occurs subsequent to a skull fracture, which lacerates the middle meningeal artery
• Typically there is initial loss of consciousness, followed by a lucid interval (1-5 hrs), then subsequent decline of
consciousness
Subdural Space
• Hemorrhage into the subdural space (more appropriately, the dural border, between dura and arachnoid) is
often subsequent to head trauma that ruptures a cerebral vein
– Even relatively mild trauma may cause laceration
• Because the hemorrhage is venous, the hematoma may develop chronically (over weeks)
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Cerebrospinal Fluid
Cerebrospinal fluid (CSF) is a clear, colorless, watery liquid that bathes the brain and spinal cord
CSF is a partial filtrate of blood, and thus has a composition similar to plasma
– CSF contains very few white blood cells, no red blood cells, and is low in protein
– It is similar to plasma in terms of ionic content
• CSF has three main functions
– Buoyancy
– Cushion: Minimizes impact of brain against cranium
– Chemical stability: The CNS is critically dependent on a stable chemical environment
Formation
• CSF is formed in four internal chambers of the brain known as ventricles
• The ventricles arise from the cavity of the embryonic neural tube
• The ventricles are continuous with one another and with the central canal of the spinal cord
Ventricles
• Within each cerebral hemisphere is a relatively large lateral ventricle
• The third ventricle is a midline cavity within the diencephalon (between right and left halves of the
diencephalon)
• The fourth ventricle is sandwiched between the cerebellum posteriorly, and the brainstem anteriorly
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Anatomy 3- Gross Brain, Meninges, and CSF
Choroid Plexus
• The choroid plexuses are the specific sites of CSF formation within the ventricles
– They are epithelial protrusions into each of the ventricles
• Each ventricle (and the central canal of the spinal cord) is lined by a layer of ependymal cells (the ependyma)
• The ependymal cells of the choroid plexuses are continuous with regular ependymal cells, but are specialized to
produce CSF
• In embryological terms, the choroid plexus represents an invagination of the tela choroidea into the ventricles
– The tela choroidea consists of pia mater and associated capillaries, plus the adjacent ependymal cells
• Note that CSF is not simply a filtrate of blood. Its composition is actively controlled by ependymal cells
Circulation of CSF
• The amount of CSF in the average adult is about 150 mL (it is secreted at ~ 500 mL/day)
– ~125 mL is intracranial
• ~25 mL in ventricles
• ~100 mL in cranial subarachnoid space
• CSF leaves the lateral ventricles through the interventricular foramina and enters the 3rd ventricle
• It passes through the cerebral aqueduct into the 4th ventricle and exits via median and lateral apertures to enter
the subarachnoid space, which is continuous with the spinal cord and the cerebellum
• CSF returns to the blood via arachnoid villi—large clusters of which are called arachnoid granulations
• The villi are located within the dural sinuses—the superior sagittal sinus and its lateral lacunae in particular
• Each villus is essentially a herniation of arachnoid mater through the meningeal layer of dura mater
– The arachnoid mater in the villus is lined by a single layer of endothelium
• The flow of CSF is maintained by three main factors
– The pressure of CSF itself
– The action of ependymal cell cilia
– Rhythmic pulsations of brain blood flow
Hydrocephalus
• Hydrocephalus is the abnormal accumulation of CSF in the brain
– It usually results from blockage of flow (mainly in the interventricular foramina or cerebral aqueduct), or
of reabsorption of CSF
• Excess CSF dilates the ventricles, thins the cerebral cortex, and separates the bones of the calvaria in infants
Anatomy 3- Gross Brain, Meninges, and CSF