Haemodynamics Flashcards
What are the metabolic demands of the brain?
Neural activity is metabolically expensive
Brain store very little energy
Continuous perfusion is needed for O2 and glucose supply
Metabolic demands met chiefly through vascular supply of O2 and glucose
How does the brain make energy?
Glucose and oxygen needed for ATP synthesis
1. Glucose transported from vascular network into brain cells
2. Glycolysis produces 2 ATP + pyruvate
3. If O2 not present, pyruvate is reduced to lactate
4. If O2 present, pyruvate enters TCA cycle whereby ADP is recycled into ATP
5. End products diffused into blood stream
What is ATP and what does it do?
Adenosine triphosphate
Principle energy currency of neurons - nucleotide consisting of 3 phosphate groups
Hydrolysis results in release of free energy in the form of a phosphate group
Phosphorylation = binding of phosphate temporarily alters structural conformation of protein modifying its functions
What is ATP used for presynaptically?
ATPase - group of enzymes which catalyze the hydrolysis of a phosphate bond in ATP
Sodium pump - extrudes Na+ ions creating an action potential and powers Ca2+ removal by Na+/Ca2+ exchange
Calcium-ATPase (sarcoplasmic endoreticulum calcium-ATPase) - in plasma membrane and lowers Ca2+ concentration
Vacuolar ATPase - energizes vesicle transmitter uptake
Motor proteins that move mitochondria and vesicles around the cell
What is ATP used for postsynaptically?
Greater use postsynaptically
Primarily used to pump ions to mediate synaptic currents
Smaller usage on returning Ca2+ to intracellular stores and on mitochondrial trafficking
How is ATP used in astrocytes?
Used largely on extruding Na+ to maintain the resting potential and to remove the ions driving glutamate uptake, and on conversion of glutamate to glutamine
What are the 3 key haemodynamic changes associated with ATP demands?
Blood flow
Vasodilation
Blood oxygenation changes
How was blood flow first discovered?
Human balance circulation - Angelo Mosso
Late 19th century
Placed participants on a balance table
Rationale = blood will flow to the brain when asked to do a task as the blood needs energy - this will cause the head to weigh more so the balance table will tip at the end with the head
Mosso rang a bell and found the participants would tip towards the head side
What is vasodilation and what is its function?
Widening of blood vessels as a result of the relaxation of the muscles of the blood vessel wall
During somatosensory stimulation, capillaries increase in diameter by up to 11%
Increased area for transmission of O2 and glucose
Decrease in metabolic transmit time by up to 20%
Increase the speed of blood cells by up to 33%
Accounts for up to 18% of change in cerebral blood volume
Provides the ultimate spatial resolution limit for haemodynamic imaging
What makes haemodynamic signalling possible?
Red blood cells are packed with haemoglobin
4 binding sites
2 states = oxygenated and deoxygenated
Possess different physical properties dependent on oxygenation - magnetic, chemical, optical
These different properties allow haemodynamic signalling depending on the oxygenation
How can we image blood flow using optical techniques?
Laser Doppler techniques
Explain laser Doppler techniques
Doppler shift is a frequency change imparted upon waves as a source moves
We can measure the Doppler shift induced by moving red blood cells
When light is shined on tissue, 3 things happen to photons, they are reflected, absorbed and scattered
Scattering is randomised in direction independent of blood flow and blood volume
Doppler shift imparted in principle by scattering of diffuse light from moving red blood cells
What is laser Doppler flowmetry?
Laser light illuminates area of tissue
Photons are scattered by static and dynamic particles
Dynamic particles (red blood cells) impart Doppler shift on photon
The light returned to the a photomultiplier unit is now a mixture of the original frequency and the Doppler shifted frequencies
Low pass filtering leaves only the different signal
What is a disadvantage of laser Doppler flowmetry?
It can only be measured in arbitrary voltage - not a quantitative measure
How do wavelengths allow laser Doppler flowmetry to work?
Tissue penetration of light differs based on wavelength
Affects depth to which back-scatter light is detected
- Green light = 0.5mm
- Red light = 3.5mm
- Infrared light = 5+mm
Green light more sensitive to changes in blood flow during vasodilation
