The Cerebrospinalfluid

BY DR.A.SELVA AROCKIAM

Protection of the brain and spinal cord is provided by bone, fluid, & by the membranes called meninges that surround these structures.

Meninges (membranes – 3 layers) are located between the bone & the soft

tissues of the nervous system. They protect the brain & spinal cord.

Meninges of the Brain

Meninges The meninges have 3 layers: (meninx – singular)

• dura mater (outermost meninx) is made up of tough, white dense connective tissue, contains many blood vessels, it continues into the vertebral canal as a strong, tubular sheath that surrounds the spinal cord, has epidural space

• arachnoid mater (middle meninx) - thin, weblike membrane, & lacks blood vessels, cerebrospinal fluid is housed in the subarachnoid space (between the pia mater& the arachnoid mater).

• pia mater (innermost) - thin membrane with nerves and blood vessels, attached to the brain and spinal cord & contains many blood vessels (nourish underlying cells of the brain & spinal cord) & nerves.

Meninges- spinal cord

Ventricles

• The ventriclesventricles are a series of interconnected cavities within cerebral hemispheres and brain stem

• There are four ventricles

two lateral ventricles

third ventricle

fourth ventricle

Pair of C-shaped Lateral Ventricles : – largest – ventricles located in the cerebrum

Third ventricleThird ventricle – located in the midbrain, connected to the

fourth ventricle via the cerebral aqueduct– “wall” divides brain into symmetrical halves

Fourth ventricleFourth ventricle – located in the brain stemin the hindbrain

dorsal to the pons

Cerebral aqueduct: long tube that connects 3rd to 4th ventricle

Three-Dimensional Views of the Ventricles

Cerebrospinal Fluid

• The fluid contained in the central canal of spinal cord, subarachnoid space and cerebral ventricles is known as cerebrospinal fluid

• It is part of extracellular fluid

Formation of CSF:

CSF is formed by the choroid plexuses situated within the ventricles.

The choroid plexuses (red cauliflower like) are tuft of capillary projections present in the walls of ventricle

Covered by pia mater and ependymal covering secrete cerebrospinal fluid

This plexus projects into 1. temporal horns of both the lateral ventricles2. Posterior horn of the third ventricle3. Roof of the third ventricle

Properties of CSF:

• Clear.colourless, transparent fluid

• Normal Volumeis 150ml (varies between 100 – 200 ml)

• Rate of formation : 0.3ml /min (550ml/day)

• Specific gravity : 1005

• Reaction : alkaline

Composition of CSF:

water: 99.13 % solids: 0.87 %

Solids are:Organic : proteins, amino acids, glucose, cholesterol, urea,

uric acid, creatinine, lactic acidInorganic : Sodium, calcium, potassium, magnesium,

chlorides, phosphates, bicarbonates, sulfatesAlso contains lymphocytes: 5 lymphocytes / cu mm (are

added when they flow in the spinal cord)

Substance CSF PlasmaNa + (meq/kg of H2O) 147 150

K + (meq/kg of H2O) 2.9 4.6

Ca + (meq/kg of H2O) 2.3 4.7

Cl- (meq/kg of H2O) 113 99

Mg + (meq/kg of H2O) 2.2 1.6

HCO3- (meq/kg of H2O) 25.1 24.8

Protein (mg / dl) 20 6000

Glucose (mg / dl) 64 100

Cholestrol (mg / dl) 0.2 175

pH 7.33 7.40

Osmolality 289.0 289.0

Circulation of CSF:

Circulation of CSF:

CSF is formed in the LATERAL VENTRICLES

Passes through the foramen of Munro

THIRD VENTRICLE

Aqueduct of Sylvius

FOURTH VENTRICLE

Enters Cisterna magna and Cisterna lateralis through foramen of

Magendie and foramen of Lushka

A portion of the cisternal fluid circulates through the spinal subarachnoid space

Greater part of the fluid passes upward, over the brainstem to the surface of the cerebral hemispheres

Formation, Location, and Circulation of CSF

Absorption of CSF :

• CSF is mostly absorbed by the arachnoid villi into the dural sinuses and spinal veins

• Small maounts is absorbed along the perineural spaces into cervical lymphatics and into perivascular spaces

• Mechanism : Pressure gradient between hydrostatic pressure in the subarachnoid space fluid and the pressure that exists in the dural sinus blood facilitates filtration

Functions of Cerebrospinal Fluid:1. Supports the weight of the brain: The brain weighs

about 144 g in air, but in its “ water bath” of CSF it has a net weight of only 50 g. The buoyancy of the brain in the CSF permits its relatively flimsy attachments to suspend it very effectively.

2. Distributes the force of blows on the head: CSF surround the brain and spinal cord; it creates a watery cushion to absorb the normal shocks of everyday life

3. Mechanical shock absorber: When the head receives a blow, the arachnoid slides on the dura and the brain moves, but its motion is gently checked by the CSF cushion and by the arachnoid trabeculae

4. Maintains the intracranial pressure: by balancing the volume of blood and CSF:CSF thus regulates the volume of cranial contents. If the cranial contents increase ( e.g..; cerebral hemorrhage and brain tumors) increase in the absorption of substances into the venous sinuses→ Increase in intracranial pressure → decrease in the cerebral circulation → asphyxia

This is prevented by greater absorption of CSF to give space for the increasing cranial contents

5. Nutrient delivery :Glucose, oxygen and some amino acids are carried in CSF and delivered to neural tissues.

6. Removal of wastes:CO2 and waste products of normal metabolism and cell breakdown are removed by CSF

7. helps maintain stable ionic concentration in the CNS 8. Also supplies info about the internal environment to

autonomic centers in the hypothalamus & brain stem

Applied Physiology Lumbar Puncture:Procedure by which CSF is taken out from the

subarchnoid space .CSF is drawn by introducing a needle between the 3rd and

4th lumbar vertebrae.(because the spinal cord terminates at lower border of L1 &

subarachnoid space is wider in this site as the pia mater is reduced very much)Purpose of Lumbar puncture:• For diagnostic purposes• Spinal anesthesia• To measure CSF pressure( alternative cisternal puncture: between the occipital bone

and atlas from the cisterna magna))

• The normal Lumbar CSF Pressure is normally 70 – 180 mm CSF

Hydrochephalus: condition of increased intracranial

pressure due to defective absorption or circulation of CSF. Baby will show

enlargement of the cranial cavity and associated damage to the brain because of the pressure effect.

The blood brain barrier: hypothetical barrier present between the brain and the blood

More than 50 years ago when trypan blue , the acidic dye was injected into the living animals all tissues were stained by it except the brain and spinal cord. This suggested the presence of a barrier which prevented the diffusion of trypan blue into brain tissues from the capillaries

Structure of the blood brain barrier:1. The capillaries of the brain consist of endothelial lining

which have tight junctions which close the pores in the blood vessels

2. Astrocytes completely cover the capillaries and make it less porous

3. The blood vessels have a thick basement membrane.

BBB Very permeable to oxygen, CO2,water, alcohol

and most anesthetics Slightly permeable to Na+, K+ Cl-

Not permeable to blood proteins and non- lipid organic molecules

Importance of Blood brain barrier:1. Maintains constancy of environment for the neurons in

CNS2. Protects the brain from the effect of endogenous and

exogenous toxins in the blood3. Prevent escape of neurotransmitters from the CNS into

the general circulation Blood CSF barrier: barrier between the blood and CSF

exists at the choroid plexus whose function is similar to blood brain barrier. Doesn't allow the entry of substances into the CSF from the blood

Applied aspects:

• Kernicterus

• Penetration by various drugs

• Tends to break down in infections and injuries

The blood brain barrier exist in the capillary membrane of all parts of the brain except in some areas of the hypothalamus.

4 small areas in or near the brain stem stain like tissues outside the brain. These areas are

1. Posterior pituitary (neurohypophysis)2. Area postrema3. Organum vasculosum of the lamina terminalis

(OVLT)4. Subfornical organ (SFO)

These areas are referred to collectively as the circumventricular organs