Lecture 27; Ischemia Brain Injury 4 Flashcards
What to we gain a lot of information from?
Knowing WHEN cells depolarise
What does knowing when cells depolarise tell us?
ECF [Na]/[K] (differential = RMP)
DC potential of single cells
DC potential of the tissue
No depolarisation = no death
Tissue Impedance (how its measured) (continuous picture)
Apparent diffusion coefficient of water
What is the RMP?
Cells maintain a negative RMP
Na/K ATPase pumps 3 Na+to 2 K+
Thus the cell: Low Na, high K, -70 mV potential vs outside
What happens to this Na/K ion distribution during ischemia?
- Decreasing BF = increasing Na/K ratio
i. e Na in, K out
Also water fluxes into the cell as gradient is lost, thus decreased ECF volume. Volume halves suddenly at 30ml/100g/min
This is shown in a stroke model, going penumbra to core
What happens to polarity during ischemia?
Cell depolarises to +30mV and remains there as no ATP to restore gradient
Normally would be dynamic rapid depolarisation and repolarisation
How else could we measure depolarisation?
Could measure change from resting ECF potential…Electrode in ECF Baseline =0
‘single cell = anoxic depolarisation’
Group of cells = electrode in ECF, ECF becomes relatively negative in ischemic conditions (DC potential)
What does a probe in the ECF really measure?
Tissue depolarization measured by DC potential is proportional to~
- the number of depolarized cells
- the mean change in membrane potential of cells
Remember: it is a bulk tissue measure, not single cells!
What was shown to occur in AFTER global ischemia of cats in terms DC potential?
- Even after 2hrs not all cells are repolarised
- Both grey and white matter depolarised (DC potential developed)
Summery;
Sudden decrease in DC potential after start of ischemia indicates that most cells in the region started to depolarize “BULK depolarisation”
What was shown to occur in AFTER global ischemia of cats in terms Ca levels?
- Ca 10000x gradient ECF:ICF normally
- Ca influx into cells during ischemia only occured in the grey matter! (synaptic opening), but in WM ECF Ca increased was water was flowing into cells
However Ca levels return to normal rapidly even though some cells take hours to repolarise
What happened when an AC current was passed through the brain?
Passing an alternating current through the brain
Resistance to AC = Impedance
5Khz = through ECF (Around cells), higher then goes through cells too
What did measuring impedance of 10min carotid artery occlusion in sheep show?
Completely resolved within 100mins (cell potential)
What did measuring impedance of 40min carotid artery occlusion in sheep show?
Did not resolve even after 100mins.
- Reperfusion isnt enough to restore cell polarity / ECF volume thats being measured by impedance b/c:
- Takes time for ATPase to fire up
- Microcirculation = reduced blood flow b/c pressure of cell swelling there takes time to restore these things
- Repolarisation takes 30mins to make an impact event if it isnt fully restored at 100mins
What is impedence proportional to?
Impedance is inversely proportional to ECF volume!
What are the pros and cons of impedence measurements?
Pros;
Continuous vs time
Fine discrimination of timing and pattern
Greater reflection of ECF volume in severe insults
Cons;
Need to know baseline (relative measure)
no regional data
Mostly: relative change
What does the ECF DC potential measure?
ECF DC potential~ number of depolarized cells the membrane potential of each cell
This is a bulk tissue measure!
i.e. sudden DC potential after start of ischemia indicates that most cells in the region started to depolarize
