Lecture 3 Haemodynamics Flashcards
describe briefly the chain of events from sensory event occurring (flow chart)
sensory event–> neural activity–>metabolite recruitment–> increased blood flow–> deoxyhaemoglobin (short)–> oxyhaemoglobin (long)
explain the importance of metabolite recruitment to the brain
neural activity is metabolically expensive, despite only being 2% of body weight, the brain uses ~20% of the O2, blood flow and glucose.
the majority of these are used to produce ATP as the brain, unlike muscles, cannot store much energy so needs this constant fresh supply
briefly outline ATP synthesis/krebs cycle (flow chart)
glucose from blood is transported into cells–> glycolysis produces 2ATP+pyruvate –> in absence of O2 pyruvate is reduced to lactate, in presence of O2, pyruvate enters TCA cycle where 36 ADP are recycled to produce 36 ATP–> ATP diffuses into blood to supply energy to neural cells
how does ATP produce energy
it is hydrolysed when reacting with water in order to produce phosphate which binds and alters/activates proteins e.g. ATPase ion pumps
what neural processes is ATP involved in?
synaptic transmission, housekeeping, maintaining resting potential, allowing APs
what are the 3 exploitable physical properties of haemodynamics
blood flow
vasodilation
blood oxygen charges
briefly overview the vasculature of neural tissue
energy demands are chiefly met by dense capillary network- cells at cortical level are 1-2 cells away from capillary
describe blood flow
a robust physiological change associated with cellular activity, closely linked to vasodilation
describe vasodilation
capillaries increase diameter up to 11% increasing metabolite transit
describe blood oxygen changes
red blood cells are packed with haemoglobin, a complex protein that binds and changes with O2. It is in one of 2 states- oxygenation and deoxygenated.
haemoglobin oxygenation changes magnetic, chemical and optical properties
following a neural events, after 10-15 second delay, there’s a large peak in HBO2 and a drop in HBr
neurovascular coupling
a link between neural activity and blood flow/oxygenation are well-established, however, not well understood
doppler shift
change in frequency imparted upon waves as a source moves
when light is shone on tissue what happens
it is either absorbed, reflected or scattered
describe light/photon scatter
the directionally random scattering of photons independent of blood flow by moving RBCs
how does more RBCs affect doppler shifted photons
more doppler shifted photons
describe the technique used for laser doppler perfusion monitoring
a fibre optic probe passes laser light into an area of tissues, photons are scattered by static and dynamic particles, imparting doppler shift and light is return to a photoreceptor in a mixture of original frequency and doppler shifted frequencies
it is measured as arbitrary voltage
describe how different wavelengths are utilised
red wavelengths can penetrate deeper, so used to measure blood flow- laser doppler flowmetry
green light is more sensitive to changes in blood flow velocity due to vasodilation/constriction as it measures rate of RBC flow in the periphery of vessel- laser doppler velocimetry
what are the pros of laser-doppler measurements
non-invasive measurement of flow and velocity for peripheral activity
sensitive to high frequency perfusion changes
measurable in real time
what are the cons of laser-doppler methods
surrogate measure of neural activity (indirect)
sensitive to movement (artefacts)
invasive surgery for neural recordings
lacking quantitative units
difficulty localising depth
describe functional near infrared spectroscopy (fNIRS)
- human tissue has high penetration of NIR wavelengths
- oxy- and deoxyhaemoglobin have different chromophore properties, absorbing differening wavelengths preferentially (830nm HBO2 and 692nm HBr)
- when neural activity occurs, NIR at varying wavelengths shone through cap and detectors detect light- which will vary depending on the ratio of HBO2 and HBr
- banana shaped photon paths
-proportional- no set units
pros of fNIRS
- Biochemical specificity is high (good understanding of chromophores and wavelengths absorbed)
- high temporal resolution (for a blood-based measurement)
- transportable
- easy for use with children
cons of fNIRS
- same limitations of BOLD fMRI- surrogate measure/ delay/ ceiling effect
- limited to cortex (superficial penetration)
- issues with machine differences
