Lecture 3 Haemodynamics

Question 1

describe briefly the chain of events from sensory event occurring (flow chart)

Answer 1

sensory event–> neural activity–>metabolite recruitment–> increased blood flow–> deoxyhaemoglobin (short)–> oxyhaemoglobin (long)

Question 2

explain the importance of metabolite recruitment to the brain

Answer 2

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

Question 3

briefly outline ATP synthesis/krebs cycle (flow chart)

Answer 3

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

Question 4

how does ATP produce energy

Answer 4

it is hydrolysed when reacting with water in order to produce phosphate which binds and alters/activates proteins e.g. ATPase ion pumps

Question 5

what neural processes is ATP involved in?

Answer 5

synaptic transmission, housekeeping, maintaining resting potential, allowing APs

Question 6

what are the 3 exploitable physical properties of haemodynamics

Answer 6

blood flow
vasodilation
blood oxygen charges

Question 7

briefly overview the vasculature of neural tissue

Answer 7

energy demands are chiefly met by dense capillary network- cells at cortical level are 1-2 cells away from capillary

Question 8

describe blood flow

Answer 8

a robust physiological change associated with cellular activity, closely linked to vasodilation

Question 9

describe vasodilation

Answer 9

capillaries increase diameter up to 11% increasing metabolite transit

Question 10

describe blood oxygen changes

Answer 10

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

Question 11

neurovascular coupling

Answer 11

a link between neural activity and blood flow/oxygenation are well-established, however, not well understood

Question 12

doppler shift

Answer 12

change in frequency imparted upon waves as a source moves

Question 13

when light is shone on tissue what happens

Answer 13

it is either absorbed, reflected or scattered

Question 14

describe light/photon scatter

Answer 14

the directionally random scattering of photons independent of blood flow by moving RBCs

Question 15

how does more RBCs affect doppler shifted photons

Answer 15

more doppler shifted photons

Question 16

describe the technique used for laser doppler perfusion monitoring

Answer 16

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

Question 17

describe how different wavelengths are utilised

Answer 17

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

Question 18

what are the pros of laser-doppler measurements

Answer 18

non-invasive measurement of flow and velocity for peripheral activity

sensitive to high frequency perfusion changes

measurable in real time

Question 19

what are the cons of laser-doppler methods

Answer 19

surrogate measure of neural activity (indirect)
sensitive to movement (artefacts)
invasive surgery for neural recordings
lacking quantitative units
difficulty localising depth

Question 20

describe functional near infrared spectroscopy (fNIRS)

Answer 20

• 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

Question 21

pros of fNIRS

Answer 21

• 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

Question 22

cons of fNIRS

Answer 22

• same limitations of BOLD fMRI- surrogate measure/ delay/ ceiling effect
• limited to cortex (superficial penetration)
• issues with machine differences