CSF, Intracranial Pressure and the Blood Brain Barrier

what is cerebrospinal fluid?

specialised type of extracellular fluid (ECF)

function of CSF

• cushioning for the brain (allows the brain to withstand normal minor traumas)

• provides constant external environment for neurons and glia

• major route for removing harmful brain metabolites

• distributes hormones (secreted by hypothalamic neurons)

where is CSF found?

• sub-arachnoid space

• ventricles (brain)

CSF is formed mainly by the choroid plexus, name another structure that is responsible for CSF formation

other brain capillaries

describe the circulation of CSF in the brain

• produced by choroid plexus

• flows through ventricles (lateral→ third → fourth)

• exits fourth ventricle (base of brain)

• flows into sub-arachnoid spaced

• absorbed into venous blood across arachnoid vili

explain how CSF is formed

CSF is formed as a filtrate following active transport and passive diffusion across the wall of the choroid plexus

choroid plexus permeability

permeable to lipid-soluble solutes (anaesthetics, O₂, CO₂)

what is the blood-CSF barrier?

Barrier formed by tight junctions between the ependymal cells of the choroid plexus

total volume of CSF

125-150ml

• 25ml in ventricles

• 125ml in sub-arachnoid space

rate of CSF production

~0.35 ml/min

CSF pressure

10 mmHg

hydrocephalus

build up of CSF in brain → enlargement of ventricles

→ brain compressed →brain damage

function of the blood brain barrier (BBB)

• excludes toxic substances from brain tissue

• protects brain from chemical fluctuations in blood

• protects brain from certain circulating neurotransmitters/hormones (e.g. after stress)

• maintains a constant environment for neurons to function effectively

brain capillary

• transport system (highly specific)

• no holes

• interconnected by tight junctions

• lack of vesicular transport

peripheral capillary

• non selective diffusion across capilary wall

• small holes (fenestra/ inter-endothelial clefts)

• pinocytotic vesicles

importance of BBB permeability

• protects brain from harmful substances

• limits use of drugs to treat CNS disorders

• CNS side effects can be avoided in peripheral drugs

difference of BCSF and BBB barriers

• BCSF: formed by tight junction between chroroidal epithelial/ependymal cells

• BBB: formed by tight junctions between endothelial lining of capillaries and surrounded by astrocytic foot processes

the rigid cranium contains

• brain

• blood in cerebral vessels

• CSF

intercranial pressure (ICP)

pressure exerted by fluids inside skill and on brain tissue

maintenance ICP

~10 mmHg (directly proportional to cranial contents volume)

monro-kellie doctrine

explains the relationship of the total cranial content (brain, blood and CSF)

• increase in the volume of one component causes a compensatory decrease in the volume of the others

increased ICP means

• cerebral vessels compressed

• reduced blood supply to brain → cerebral ischemia

• cushing’s reflex occurs

cushing’s reflex

NS response to increased ICP

• reduced blood supply to brain = increased CO₂, decreased O₂

• activates sympathetic and parasympathetic NS

• results in: increased arterial BP, reduced HR, reduced respiration