Lecture 34: CSF Flashcards
What are the 2 ‘spaces’ between the meninges?
- Subdural space (thin) lies between dura and arachnoid mater.
Subarachnoid space lies between arachnoid and pia mater, contains CSF, (veins) and arteries, prone to infection (menigitis).
What are the two ‘types’ of meninges?
There are two types of meninges:
- Dura mater (pachymeninges): a thick, rigid membrane surrounding brain and spinal cord.
- Cranial dura consists of two layers including a thick periosteal layer and an inner meningeal layer. The two layers are ordinarily adherent, but they split to form venous sinuses.
- The falx (between heimsphere) and tentorium (between middle cranial fossa separate temporal lobe and cerebellum) are formed by reduplication of the inner meningeal layer.
- The inner layer of dura extends into the spinal canal where it surrounds the spinal cord and forms the lumbar theca sac.
- Leptomeninges: consist of two structurally similar layers, include arachnoid (outer layer) and pia (inner layer).
- Arachnoid is a thin layer of connective tissue that encloses the CSF in the subarachnoid space.
- Pia is adherent to the surface of the brain and spinal cord.
What is CBF?
The cerebrospinal fluid (CSF) is the fluid located in ventricles and subarachnoid space around brain and spinal cord.
Where is the CSF produced?
Most of the CSF is formed by choroid plexus, mainly in the lateral ventricles.
A small amount of CSF is produced directly from the interstitial fluid of the brain by bulk flow along perivascular spaces and along axon tracts.
This involves two processes in series
- (a) Ultrafiltration across choroidal capillary wall, which depends on hydrostatic pressure in the capillaries;
- (b) Active secretion by choroidal epithelium (endothelial and epithelial cells of choroid plexus have specific transport systems for a variety of compounds including ions, vitamins, nucleosides, purines, glucose and amino acids).
Describe the Choroid Plexus Structure
Fenestrated capillary network surrounded by a single r_ow of epithelial cells_ (part of the blood brain barrier)
Choroid plexus epithelial cells
- tight junctions between cells
- contain numerous vesicles, lysosomes
- ventricular surface of epithelial cells has a brush border of microvilli
Total CSF volume is usually _______\_
The CSF volume in ventricles is __________\_
Total CSF volume is usually ~150ml (can vary from 140 to 270ml in adults). Total CSF volume gradually increases with age.
The CSF volume in ventricles is 12-25ml.
- Most of the CSF (125ml) is in the subarachnoid space, of which 30-45 ml are in lumbar theca sac.
Describe how CSF are produced
Most of the CSF is formed by choroid plexus, mainly in the lateral ventricles.
This involves two processes in series
(a) Ultrafiltration across choroidal capillary wall (fenestrated), which depends on hydrostatic pressure in the capillaries;
* Fluid is pushed out into the interstitial space from the capillaries
(b) Active secretion by choroidal epithelium (endothelial and epithelial cells of choroid plexus have specific transport systems for a variety of compounds including ions, vitamins, nucleosides, purines, glucose and amino acids).
* The fluid needs to be actively secereted to become the CSF(found outside of the choroidal epithelium) because of the tight junctions
The flow of CSF from ventricles is fascilitated by …….
The flow of CSF from ventricles is facilitated by:
- Hydrostatic pressure created by CSF production in the choroid plexus (major);
- Arterial pulsations inside cranial cavity;
- Directional beating of the ependymal cilia.
Describe the direction of flow of the CSF
CSF flow follows the following route:
- lateral ventricles
- via interventricular foramina (foramina of Monro**) ®
- third ventricle (midline) ®
- via cerebral (Sylvian) aqueduct ®
- fourth ventricle ®
- via 1 midline foramen (foramen of Magendie) & 2 lateral foramina (foramina of Luschka) ®
- subarachnoid space (subarachnoid cisterns surrounding brainstem & subarachnoid space overlying spinal cord surface and brain convexities) ® Virchow-Robin (perivascular) spaces (in depths of cerebral cortex).
Describe where/how the CSF are absorbed
CSF returns to the vascular system by entering the dural venous sinuses via arachnoid granulations.[2] These are outpouchings of the arachnoid mater into the venous sinuses around the brain, with valves to ensure one-way drainage.
The major route of CSF absorption is via the small arachnoid villi and the larger arachnoid granulations.
The arachnoid villi are herniations of arachnoid membrane through dura mater into lumen of superior sagittal sinus and other cerebral veins.
- CSF absorption depends on hydrostatic pressure in
- subarachnoid space
- CSF absorption is not regulated by any transport process
- CSF absorption occurs by unidirectional bulk flow.
- Arachnoid villi act as one-way valves (allow CSF flow into veins), which are kept open (and dependent) by hydrostatic pressure in subarachnoid space.
CSF absorption is not regulated by any transport process.
Label 1 and 5
1) Third ventricle
5) Posterior horn of lateral ventricles
What is the turnover rate of CsF
3-4x per day
(600ml/day 0.35ml/min)
Most of the CSF is in the….
Subarachnoid space
What are Arachnoid villi?
- herniations of arachnoid mater through dura mater into lumen of superior sagittal sinus
- absorb CSF by unidirectional “bulk flow”
- function as one-way valves that allow flow of CSF into veins
CSF absorption is proportional to ___________\_
CSF absorption is proportional to CSF pressure (65-250 mmH2O).
Therefore, huge increase in CSF production is required to raise intracranial pressure to a level at which production greatly exceeds CSF absorption.
How can a sample of CSF be obtained?
A sample of CSF can be obtained from subarachnoid space in lumbar theca sac by lumbar puncture.
- _Lumbar puncture i_s usually performed using local anaesthesia with patient lying in the left lateral recumbent position (back in a straight line) with their neck, trunk, hips and knees flexed.
- The needle is inserted into L3/4 intervertebral space. When needle is in subarachnoid space, CSF pressure can be measured with a manometer and a sample of CSF can be removed for laboratory analysis.
CSF sample is most commonly examined for presence of cells (leukocytes, erythrocytes, tumour cells), concentrations of protein and glucose are measured. If infection is suspected (e.g. meningitis), CSF can be cultured. A variety of other tests can be done for special indications.
Which lumbar space do you obtain the CSF from?
L3/4 intervertebral space
What is the normal CSF composition (KNOW THE NUMBERS)
Normal cerebrospinal fluid appears clear and colourless:
- White blood cells <5x106/L (no polymorphs)
- No neutrophils
- No red blood cells
- Protein <0.45g/l
- Glucose >2.5mmol/l (depends on blood glucose and rate of brain glucose metabolism)
How much glucose should there be in CSF?
Glucose >2.5mmol/l (depends on blood glucose and rate of brain glucose metabolism
How much WBC should be in the CSF?
White blood cells <5x106/L (no polymorphs)
How much protein is there in the CSF?
Protein <0.45g/l
What does the Blood Brain Barrier consist of?
- specialised endothelial cells
- thick basement membranes
- astrocytic processes on capillaries
Compare the systemic and brain capillaries
Endothelial cells in blood brain barrier (CNS) differ from endothelial cells in other organs in several ways:
- Absence of fenestrations and presence of tight junctions, which limits movement of even small molecules between cells and effectively separates plasma from brain extracellular fluid.
- Brain specialized endothelial cells have fewer pinocytotic vesicles, thicker basement membranes and more mitochondria (reflecting higher metabolic rate required for active transport systems)
- Astrocytic foot processes are closely apposed to the abluminal walls of capillaries and have an important role in regulation of ions and volume of the brain.
Blood-CSF barrier is located in the choroid plexus and the meninges. Unlike the capillaries that form the blood brain barrier, the endothelial cells in the choroid plexus are fenestrated, but the barrier is provided by tight junctions between the epithelial cells of the choroid plexus. The epithelial cells in the choroid plexus have many mitochondria.
Decribe Cerebral herniations
If compensatory mechanisms fail to control ICP, brain tissue may be displaced:
- From one compartment of intracranial cavity to another,
- Or through foramen magnum into upper spinal canal.
These displacements are called cerebral herniations. Herniation of brain tissue can cause compression of part of brain and adjacent structures (cranial nerves and blood vessels), which in turn, causes secondary damage.
Describe the midline shift
Midline shift:
displacement of one cerebral hemisphere across midline -> compress midline diencephalic structures
Describe the Transtentorial herniation
Transtentorial herniation:
herniation of medial temporal lobe through tentorial notch ->
- c_ompress upper midbrain_ (containing reticular activating system, abnormal cause drowsiness),
- oculomotor nerves (III) (dilated pupils),
- posterior cerebral artery.
Describe the Tonsillar herniation
Tonsillar herniation:
herniation of inferior cerebellum (cerebellar tonsils) from posterior fossa into spinal canal -> compress underlying medulla (cause Crushing’s triad).
Where’s the tentorial notch?
Transtentorial herniation: herniation of medial
temporal lobe through tentorial notch →
compression of midbrain, oculomotor nerve, posterior
cerebral artery
Describe Cerebrovascular Autoregulation
Cerebral perfusion pressure (CPP) is determined by difference between mean arterial blood pressure (MAP) and i_ntracranial pressure (ICP)_:
CPP = MAP – ICP
In normal circumstances, cerebrovascular autoregulation maintains constant cerebral blood flow over a wide range of CPP (60-150mmHg). Cerebral blood vessels regulate blood supply to different parts of brain in direct response to metabolic demands for oxygen and glucose (i.e. neuronal activity).
- Vasoactive factors released by neurons mediate constriction and dilatation of small cerebral arteries.
- Direct innervation of cerebral blood vessels and astrocyte foot processes also may play a role in autoregulation.
If there is loss of cerebral autoregulation (e.g. cerebral infarct, tumours), _cerebral blood flow (_and cerebral blood volume) is proportional (vary passively) to arterial blood pressure. Important to maintain BP (avoid BP sudden drop) in strokes to avoid further infarct! (because cerebral blood flow will drop too)
