MENINGES AND CEREBROSPINAL FLUID

CNS Cisterns

* In several regions of the brain, the subarachnoid space is enlarged to form cisterns where pools of CSF form. * There are several cisterns - Cerebellomedullary cistern (Cisterna Magna) - spans the space over the cerebellum and medulla. It is the largest cistern - Pontine cistern - lies over the pons - Chiasmatic cistern - adjacent to the optic chiasm Interpeduncular cistern - lies over the interpeduncular fossa

Choroid Plexus

* Present in each ventricle and produces CSF * In the lateral ventricles, the choroid plexus is in the medial wall and extends from the tip of the inferior horn to the inter ventricular foramen

The 4th ventricle communicates with the subarachnoid space via

- 2 lateral apertures - Foramina of Luschka - 1 medial aperture - Foramina of Magendie - At the end of the 4th ventricle, a small central canal extends through the spinal cord

CSF - Formation

- 70% of the CSF in the brain and spinal cord is produced by the choroid plexus - The remaining 30% is secreted by the parenchymal cell of the brain crosses the ependyma and enters the ventricles - CSF production is active and involves the enzyme carbonic anhydrase and specific transport mechanisms - Involves the fenestration of blood through endothelial cells however movement of larger molecules into the filtrate is prevented by tight junctions.

CSF Formation (cont)

- Active transport is required to transport Na+ and Mg2+ ions into the CSF and remove K+ and Ca2+ ions - Water flows across the endothelium to maintain osmotic pressure - Normally, the rate of formation is 500 mL/day 90/140 mL in total volume - 23 mL in ventricles and remainder is in subarachnoid space

Herniation

- After skull sutures have closed, herniation occurs if there is a space-occupying lesion in the cranial vault (tumor, hydrocephalus, hematoma, hemorrhage) - Uncal herniation - the uncus (tiny structure of the medial temporal lobe) herniates into the tentorial notch and puts pressure the midbrain. - Decrease consciousness and eventually death - Dilation of the pupil on the side of the herniation (WHY the same side). If the herniation is bilateral both pupil with be dilated - If the pressure is not relieved, uncal herniation may advance to central herniation.

A choriod plexus consists of 3 layers of membranes

- An endothelial layer of the choroidal capillary wall which is fenestrated (openings) -A pia membrane - A layer of choroidal epithelial cells that contain numerous mitochondria and have many basal in folding and microvilli on the surface facing the ventricles - Tight junctions can be found between adjacent choroidal epithelial cells

- With an outermost layer of epithelial cells with tight junctions, large molecules can't enter the CSF - There are 7 structures (circumventricular organ) without BBB (tight junctions) and permit the passage of large molecules.

- Area postrema - Pineal body - Subcommissural organ - Organum vasculosum lamina terminalis (OVLT) - Neurohypophysis (posterior pituitary gland) - Medial eminence - Subfornical organ ****Which hormones are secreted by the posterior pituitary****

Arachnoid villi (granulation)

- At the sinuses (especially superior sagittal and dural sinuses ), minute pieces of arachnoid penetrate through apertures in the meningeal layer and protrude into the venous sinuses. - Main passage way for the CSF in the general systemic circulation - The pressure in the subarachnoid space is greater (200 mm H2O) then the dural sinuses (80 mm H2O). - If dura sinus pressure is higher, the blood can't flow into the subarachnoid space because the arachnoid villi tubules collapse - Groups of arachnoid villi form arachnoid granulation which can calcify with age.

Perivascular space

- Blood vessels of the brain run in the subarachnoid space on the surface of the pia before breaking up into branches. - When these blood vessels penetrate the brain tissue, the carry a cuff of pia and arachnoid into the brain for a short distance creating the perivascular space. - This space is continuous with the subarachnoid space - May serve a passage for CSF movement in the brain - The pia is also part of the choroid plexus, a structure which in involved in the production of CSF

CSF - Composition

- CSF is clear and colorless - Any change in color or composition is very important clinically - Pink or red - blood in the CSF usually due to rupture or bleeding from an aneurysm of vessels in the subarachnoid space - Yellow and clot spontaneously - increased protein content, sometimes due to tumors - Xanthrochromia - hours after subarachnoid hemorrhage, hemoglobin from lyzed RBC get broken down into bilirubin (yellow) - Increase in gamma globulin - patients with MS or chronic CNS infections

Meningitis

- Can be viral, bacterial and fungal induced and life-threatening if its not treated properly - Most common in infants and children but can occur at any age. - Can occurs from skull fracture following head injury - Commonly involves the arachnoid/subarachnoid space/pia (leptomeningitis) - Bacterial menigitis is most serious and needs prompt treatment (Streptococcus pneumonaie and Neisseria meningitidis) - Increase number in WBC count, CSF pressure, protein level - Treatment - antibiotic, corticosteriod, acetaminophen and anticonvulsant.

Increase in Intracranial Pressure

- Cause by the increase in size or volume of any of the constituents. Can be caused by - Volume increase due to cerebral edema - Intracranial hemorrhage or tumors - Obstruction in CSF flow - Increase blood volume in brain tissue due to venous obstruction

CSF- Composition (2)

- Cloudy or "white" - increase in the number of white blood cells which signals an infection (bacterial meningitis or viral encephalitis) - Changes in glucose level - Low - acute bacterial and chronic fungal infections of the CNS - Normal - viral infections

Spinal Arachnoid Mater

- Connected to the dura via connective tissue, trabecular - Rostrally, it passes though the foramen magnum to join the cranial arachnoid and caudally to surround the cauda equine - Cauda equina consists of a bundle of nerve roots of the spinal nerves caudal to the second lumbar vertebra

Spinal Dura Mater

- Consists of only the meningeal layer - Rostrally, the spinal dura joins the meningeal layer of the cranial dura at the margin of the foramen magnum - Caudally it end at the S2 vertebra - Becomes a thin extension and anchors the spinal dura sac to the vertebral canal

Covering of the Spinal Cord

- Conus Medullaris - (Conical-shaped caudal end of the spinal cord) is at the caudal edge of the 1st edge of the 2nd vertebra - Has a thin film enclosed in pia and consisting of ependymal cells and astrocytes emerge from the conus medullar is - The filament, called filum terminale internum, extends from the conus medullaris and passes through the caudal end of the dural sac at S2 - At S2, a caudal thin extension of the spinal dura, coccygeal ligament, surrounds the filum terminale.

Arachnoid Mater

- Delicate (spider-like) avascular membrane enveloping the brain and spinal cord (medulla spinalis). - Lies between the pia mater internally and the dura mater externally. - Follows closely the meningeal layer of the cranial dura and is separated from it by a narrow (or potential) space called the subdural space. - Separated from the pia mater by the subarachnoid cavity, which is filled with CSF.

Disorders of the CSF System - Hydrocephalus

- Dilation of the ventricles ensure when circulation of CSF is blocked or absorption is impeded. - Leads to increase in ventricular pressure and thus dilation - Can lead to impairment of structures such as the corticobulbar and corticospinal tracts with a progressive loss of motor functions. - Hydrocephalus can occur before birth and usually noted during the first few months of life

Tentorium Cerebelli

- Extends horizontally between the occipital lobes and the cerebellum and attached dorsally to the falx cerebri in the midline. Divides the brain transversely (top from bottom) - Forms the roof of the cerebellum - Anterior portion has an opening called the tentorial notch through the midbrain runs. - Supra- and infra-tentorial lesions (space-occupying lesions) are above and below the tentorium cerebelli respectively.

Subarachnoid Space

- Has a network of connective tissue or trabeculae which bridges the arachnoid and pia membranes. - Has CSF which bathes the brain and helps distribute and equalize pressure within the skull. - All major blood vessels of the brain are located in the subarachnoid space.

Symptoms may include

- Headache - stretching of cranial pain - sensitive mechanisms - Nausea and vomiting - activation of chemoreceptor trigger zone near area postrema - Bradychardia - increased pressure on the nucleus ambiguus and the dorsal motor nucleus of the vagus nerve in the medulla - Increase in systemic pressure - on ventrolateral medulla - Loss of consciousness - Elevation and blurring of the optic disc margin

Pia Mater

- Innermost meningeal layer - Thin vascular membrane the adheres closely to the surface of the brain and spinal cord - Some internal structures (ventricles) are also covered by pia - Can not be removed and gives brain its shiny appearance

The Blood-Brain Barrier and Blood-CSF Barrier

- Large molecules cannot pass from the blood into the interstitial fluid due to the existence of the blood-brain barrier - Located in the interface between capillary wall and brain tissue. Consists of - Endothelial cells lining the capillary wall with tight junctions between them - Processes of astrocytes abutting on the capillaries - A capillary basement membrane

Falx Cerebri

- Largest septum - Located in the longitudinally fissure between the two hemispheres. - Has a sickle-like form (Falx). - Attached to the superior and inferior sagittal sinus (to be discussed later)

CSF - Circulation

- Movement of CSF is due predominantly pulsations of the arteries in the subarachnoid space - Flows from each lateral ventricles through the interventricular foramen (hole in the septum pellucidum) where it mixes with more CSF in the 3rd ventricle. Then through the cerebral aqueduct into the 4th ventricle. - The fluid leaves the ventricular system via the Foramen of Luschka and Magendie and enter the cerebellomedullary cistern.

Straight sinus -

- Posterior continuation of the inferior sagittal sinus - Empties in the transverse (lateral) sinus (usually the left) - The Great Cerebral Vein of Galen (which collects venous blood from the internal cerebral veins), joins with the straight sinus. - Supratentorial space -occupying lesions can cause this vein to be compressed and impede the venous outflow of blood from the brain.

The 3rd ventricle

- The 3rd ventricle forms the medial surface of the thalamus and the hypothalamus - Anteriorly, a thin plate or wall called the organum vasculosum lamina terminalis (OVLT) forms the anterior end of the 3rd ventricle - The 3rd ventricle is connected to the 4th ventricle via the cerebral aqueduct (aqueduct of Sylvius)

The 4th ventricle

- The 4th ventricle is located posterior to the pons and the upper half of the medulla and ventral to the cerebellum - Its floor is flat and rhomboid-shape, and its roof is tent-shaped, with the peak of the tent projecting into the cerebellum

CSF - Circulation (2)

- The CSF travels rostrally over the cerebral hemispheres where it enters the arachnoid villi - Through the arachnoid villi, CSF can flow into the dural venous sinuses but can not in the opposite direction because the pressure in the subarachnoid space is higher. - From the cisterna magna, CSF also flows downward into the spinal subarachnoid space and then ascends along the ventral surface of the spinal cord into the basal part of the brain where it courses dorsally to empty into the dural sinuses

Coverings of the Brain - Dura Mater

- The outermost meninge - is composed of two layers of fibrous connective tissue which are fused together except where the dural venous sinuses are located - The outer layer - periosteal layer adheres to the inner surface of the cranial bone. This is not continuous with the spinal cord. - It is highly vasculated and innervated - Typically there is no space between the cranium and the dura except in pathological conditions (cranial epidural space) - The inner or meningeal layer is continuous with the dura of the spinal cord at the foramen magnum (open at the base of the occipital lobe through which the medulla is continuous with the spinal cord). - It is smooth and avascular and lined with mesothelium on its inner surface.

Covering of the Spinal Cord (contd)

- The spinal cord is also surrounded by 3 meninges like the brain but there are some differences. - Spinal dura is single-layered and lacks the periosteal layer of the cranial dura - Spinal epidural space is an actual space in which the venous plexuses are located and is used clinically for the administration of epidural anesthesia - paravertebral nerve block. - Spinal epidural space is located between the meningeal layer of the dura and the periosteum of the vertebra

Spinal Pia Mater

- Thicker compared to the cranial pia mater - Vasculated membrane and projects into the ventral fissure of the spinal cord - Toothed ligaments of the pia tissue (dentate ligaments) anchor the spinal cord to the arachnoid and the inner surface of the dura

Septa and Venous Sinuses of the Dura

- Two important structures are formed by the cranial dura mater (septa and venous sinuses) - Septa - sheet-like processes that are reflected inward and extend from the meningeal layer of the dura deep into the cranial cavity. - 4 locations in the brain - form freely communicating compartments that restricts the movement of the brain. - The function of the dural septa is to provide support and protection for the brain - Reduce or prevents displacement of the brain when the head moves - It is a means of protection for the brain against sudden and violent movements of the head

Head injury and resultant hemorrhage

- Usually fractures to the base of the skull which increases the risk of CSF from the nose, bleeding from auditory canal, meningeal bleeding, infections

Falx Cerebelli

- Vertically oriented triangular projection into the posterior fossa (portion of the brain below the tentorium - infra-tentorial space) - Dural fold between the folds of the cerebellum in the posterior fossa - Analogous to the Falx cerebri

Transverse (lateral) sinus

- continuation of the straight sinus around the lateral aspect of the hemispheres. Sigmoid sinus - a continuation of the transverse or lateral sinuses anteriorly. Confluence of sinuses - dilatation of the venous channels posteriorly where the superior sagittal, straight, and the two transverse (lateral) sinuses converge. Cavernous sinus - at the base of the brain and close to the internal carotid artery Receives blood from veins of the face, pharynx, nose and thus providing a route for CNS infection .

Inferior sagittal sinus

- extends along the inferior margin of the falx cerebra - Drains blood predominantly from the medial aspect of the brain

Lumbar cistern

- extends from the L1 to S2 of the vertebral column. Advantages of this area -Lumbar puncture or spinal tap (removal of CSF) at L3-L4 in adults and L4-L5 in children Administer drugs (anesthesia, antibiotic) -In adults, this region (cauda equina - horse's tail) contains only nerve root fibers so the chances of damage when needles are inserted -If intracranial is high, CSF withdrawal can lead to brain tissue herniation through the foramen magnum

Superior sagittal sinus -

- located within the attached borders of the falx cerebri and then deviates to posteriorly as the right transverse (lateral) sinus - Receives branches from the superior cerebral vein and veins from the meninges, scalp, and nose and thus provides a route for the spread of infections to the CNS CSF is returned to the general systemic circulation

CSF - Function

-4 main functions - Structures (brain and spinal cord) float in CSF because the specific gravities are the same. - Reduction of traction exerted on the nerves and blood vessels with the CNS - Cushioning of the CNS and dampens effects of trauma - Vehicle for the removal of metabolites - Provides a stable ionic environment.

Dementia Pulgilistica - common syndrome affecting professional boxers

-Generalized brain damage. Autopsy shows evidence of 1000s of small (petechial) brain hemorrhages Severe global cerebral atrophy -Develop thinking/memory loss and personality issues WHICH LOBES ARE AFFECTED? - Higher incidence of Parkinson Syndrome - damage to basal ganglia - More likely to develop symptoms characteristic of Alzheimer's disease - Contrecoup (other side) injury of the brain - Ataxia - lack of coordination of the limbs (Affected area?) - Visual problems including partial blindness

Meninges

-Special protection of the brain and spinal cord tissue is provided by the: a) Cranial vault b) The bony vertebral canal c) Three layers of connective tissue membranes made up of fibroblast and collagen fibrils (Meninges) d) Dura, arachnoid, and pia mater e)The arachnoid and pia mater are called leptomeninges (thin and fine in Greek).

Major types of Head injury and resultant hemorrhage

1) Intracerebral hemorrhage (ICH) - Occurs with the brain tissue - May occur during child birth - Extra or epidural hemorrhage - between the periosteal and meningeal layers of the cranial dura - Occurs when arteries tear (usually meningeal arteries) with skull fracture - Herniation and death can occur if hematoma is not addressed 2) Subdural hemorrhage - bleeding in the "potential" space between dura and arachnoid membranes - Most commonly results from the tearing of the superior cerebral vein - Usually sudden blow to the front of the head (anterior/posterior displacement)

There are 2 types of the CSF System Disorders

1) Non-communicating - Fluid flow out of one or more of the ventricles is blocked. Most common is the blockage of the cerebral aqueduct or the foramina of the 4th ventricle Two developmental malformation which blocks 4th ventricle a) Dandy-Walker malformation - cyst formation near the internal base of the skull, partial or complete absence of the area of the brain between the two cerebellar hemispheres and an enlargement of the fourth ventricle. b) Arnold Chiari malformation - the indented bony space at the lower rear of the skull is smaller than normal and pushed and elongates of the brainstem and cerebellum. If not corrected, brain damage and/or herniation and death can occur

There are 2 types of CSF System Disorder

2) Communicating - Over production of CSF and movement into the dural venous sinuses are obstructed usually at the level of the arachnoid villi - Often seen in newborns where there is more fluid made than reabsorbed - Choroid plexus are sometimes destroyed

Brain Ventricular System

4 cavities (ventricles) are located in the brain - 2 lateral ventricles - corresponds to the shape of the hemisphere in which it is located and consists of 4 parts - Anterior (frontal) horn - in the frontal lobe - Body - parietal lobe - Posterior (occipital) horn - occipital lobe - Inferior horn - more ventrally in the temporal lobe - Both lateral ventricles are connected to the 3rd ventricle via 2 short channels called the interventricular foramen

Diaphragma sellae

Located on the ventral aspect of the brain Contains a small hole to accommodate the infundibulum (pituitary stalk), which connects the hypothalamus and the hypophysis (pituitary gland).

CSF Composition

SEE MENINGES SLIDE 40

There are several (4) types of sinuses

Superior sagittal sinus Inferior sagittal sinus Straight sinus Transverse (lateral) sinus

The Blood-Brain Barrier and Blood-CSF Barrier (2)

The different cell arrangement prevents the passage of large molecules - BBB prevents the entry of blood borne foreign substances in to the brain tissue - BBB also prevents drug delivery into the CNS - Blood-CSF barrier - prevents large molecules from passing into the CSF from the blood - Tight junctions don't exit between neighboring capillary endothelial cells in the choroid plexus. The cells are fenestrated and allow passage of large molecules but the presence of epithelial cells with tight junctions prevents large molecules from entering CSF

Venous Sinuses

These are endothelial-lined channels formed between the 2 layers of the cranial dura which allows venous drainage from the brain. Have 2 important functions Provide for the venous drainage of blood from the brain mainly to the internal jugular vein Provide a route by which CSF is returned to the general systemic circulation

Types of brain herniation: 1. Uncal 6. Tonsillar

Tonsilar herniation - The tonsil ( a tiny structure medial to the cerebellum) herniates into the foramen magnum - Because of the medullary position of the reticular formation (controls all vital functions), this type of herniation is very serious - Rapid decrease in consciousness, abnormalities in heart rate and breathing - If pressure is not relieved, it will lead to death

Posterior part of the dura

is supplied by branches of the vertebral and occipital arteries.

The anterior part of the dura

is supplied by the anterior meningeal arteries.

Lateral cranial dura

is supplied by the middle meningeal artery and its branches.