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Learn fMRI (w02) 11 How neural activity leads to oxygenation changes (UCLA Monti PSYCH265) — Transcript

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0:00so let's dive a little deeper into
0:03the metabolic response and how it
0:05follows neural activity
0:07and importantly how does it give rise to
0:09the bold signal
0:11now typically if you were if you were to
0:14look at the saturation of oxygen
0:16inside the blood vessels in the vascular
0:19system
0:20um studies that have looked at this on
0:22average
0:23report that in the arterial side
0:25saturation is somewhere above
0:2890 percent saturation of oxygen
0:31once you get to the capillaries which
0:32obviously are are half arterial and half
0:35venous the oxygen saturation tends to be
0:39closer to eighty percent and then
0:42once you get to the venous side the
0:45saturation
0:46of oxygen falls down to 60 percent
0:49meaning another 60 oxygenated hemoglobin
0:52and 40 deoxygenated hemoglobin
0:55now studies that have looked at how
0:57these numbers change
0:59with neural activity have reported
1:03that generally
1:07have reported that the cerebral blood
1:10flow
1:11and the cerebral metabolic rate of
1:13glucose
1:14so the rate at which glucose is used is
1:17metabolized
1:18out of the out of the vascular system
1:24is just about 50 so with activity
1:27the the amount of flow
1:30of blood through um the vascular system
1:34increases and somewhere around
1:3750 of course this is an average
1:40and the same is true for glucose so the
1:42metabolism the rate at which glucose
1:44gets used
1:45the cerebral metabolic metabolic rate of
1:48glucose
1:50also increases by just about 50 percent
1:53what is somewhat
1:54odd and we will spend some time
1:56discussing this
1:57is that whereas cbf and cmr glucose
2:01increased by 50 percent
2:05the rate the metabolic rate of oxygen so
2:08the rate at which
2:09oxygen is is metabolized
2:13during activity only increases by five
2:15percent
2:16so in a sense there's a dissociation
2:18between the cbf
2:20and cmr glucose which increased by 50
2:23and the cmr oxygen the cerebral
2:26metabolic rate of oxygen
2:28which somehow only increases by 5
2:32um now overall uh when
2:36when people have measured the levels of
2:40saturation
2:41of oxygen in the vascular system they
2:43tend to report
2:45that on the arterial side during
2:48activity
2:49so in the area proximal to the neural
2:51stimula to the stimulation
2:53or to the neurons that that became
2:55active
2:56the saturation of oxygen inside
3:00in the arterial side increases maybe by
3:03three
3:03maybe by three percent on the other hand
3:06at the capillaries
3:08um the saturation of oxygen
3:12seems to increase by just about five
3:15percent
3:16finally on the venus side there is the
3:19largest
3:19increase with decrease going up by just
3:22about
3:2312 so going from 60
3:27to um 72 percent
3:32so it goes up by 12 percentage point i
3:34should say
3:35now um of course the question is why
3:38does this happen
3:39and in particular why does cbf go up so
3:42much
3:43but the rate of oxygen use only goes up
3:46by five percent
3:52a number of different theories have been
3:54proposed as to why there's such a change
3:57in the cereal blood flow fifty percent
4:00for only such a tiny change in the
4:02cerebral metabolic rate of oxygen
4:04fire just about five percent um the
4:07first
4:08idea came from fox and colleagues and
4:11and actually fox and colleagues are
4:13those
4:14who derived the measurements that i
4:17showed in the previous slide
4:19so they observed for the first time this
4:22big dissociation between cbf and cmro2
4:26with activation and they thought
4:30that maybe the two dissociate
4:33that the serial flow and and the use of
4:36oxygen dissociate
4:39probably because this is their
4:40speculation
4:42oxidative metabolism so aerobic
4:45metabolism might already be at ceiling
4:47during rest
4:48so in other words they thought that
4:50perhaps
4:52just at rest so with no extra activation
4:55the oxidative metabolism or the aerobic
4:58metabolism
4:59might just be maxed out already
5:01somewhere close to 99
5:03so whenever a neuron would suddenly
5:05start
5:07you know bursting with activity and
5:08require more atp
5:10to support its function this this this
5:13additional energetic requirement
5:15might be might be taken care of with an
5:19increase
5:20in the faster although less efficient
5:23metabolism so really through glycolysis
5:26so the idea
5:27is oxidative metabolism aerobic
5:30metabolism is just going on the whole
5:32time
5:34and whenever a neuron requires more atp
5:38so requires more energy that is
5:41addressed
5:41with an increase in glycolysis
5:44so the use of glucose transformed into
5:47two molecules of atp
5:49and pyruvate and then lactate indeed
5:52we do know that lactate which is a
5:55byproduct of
5:56glycolysis does increase in neural
5:58activity so
5:59this dissociation between cbf and cmro2
6:03might be due to the fact that
6:07um cbf even though it's carrying both
6:11oxygen and glucose uh it's really just
6:14only the glucose part of it gets used
6:16for extra energy
6:17through glycolysis the oxygen itself
6:21doesn't help because this was the
6:24hypothesis of fox and colleagues
6:26because the oxidative metabolism might
6:28already be
6:29doing might already be maxed out at
6:32basement
6:34so this was their theory now a different
6:37point of view was was proposed before
6:41that
6:41by malonic and greenbelt now malonik and
6:44greenveld
6:46they thought that the difference had to
6:49do
6:50with a difference in how local or
6:53coarse the control of metabolism can be
6:56so here's what they did they used a cat
6:59model
7:00and they implanted in the cats um a near
7:03infrared spectroscopy
7:05detector so this is just a different
7:08form of imaging
7:09but and it uses light it uses infrared
7:12light
7:12in order to detect concentrations of
7:16oxygenated hemoglobin and deoxygenated
7:18hemoglobin
7:19based on the fact that the two reflect
7:23differently light so you can shine light
7:25inside the brain
7:27and and the reflection according to the
7:29concentration of oxygen deoxyhemoglobin
7:32will will be different so you can tell
7:35what's the concentration of oxygen
7:36what's the concentration of
7:37deoxyhemoglobin
7:40and so they this was the the preparation
7:42and then they just simply showed
7:44a visual stimulus to the cat so for
7:46example these these
7:47vertical bars and then they observe the
7:50changes
7:51in concentration of oxy and the
7:53oxyhemoglobin in visual cortex
7:56and what they saw was that
8:00um deoxyhemoglobin
8:03the the uh the graph on the left
8:07so this is time and this is changing
8:10concentration from baseline
8:12now deoxyhemoglobin and the brown
8:17area is the this gray area sorry
8:20is the period during which stimulation
8:22was taking place
8:24so what this um what they observed is
8:27that deoxyhemoglobin
8:29had a quick spike and it makes sense
8:32because any oxygen lying around
8:34might be used for metabolism but then
8:37it would fall very quickly and then
8:41slowly go back to baseline on the other
8:43hand
8:44oxy the concentration of oxyhemoglobin
8:48would very slowly build over time
8:52peak after five to six seconds and then
8:56fall off again
8:58and on top of this they also noticed
9:00that this change in deoxyhemoglobin this
9:03increase in the oxyhemoglobin was very
9:06local in nature
9:07only happened nearby the neurons
9:12that were actually active nearby visual
9:14cortex
9:16and the specific neurons that processed
9:19the stimulus that they were showing the
9:20cat whereas
9:23oxyhemoglobin this increase in
9:25oxyhemoglobin
9:26appeared to be much broader space-wise
9:29so appeared this reaction appeared to
9:32take place
9:33in a broader area compared to where the
9:36active neurons were
9:38and so their thought was that cmr2
9:41the use of oxygen might be controlled at
9:44a finer scale
9:45might really only happen in those five
9:47neurons that are active
9:49let's say whereas cbf is a more systemic
9:52response
9:54and so if you want to if you need to
9:57increase cbf you just need to increase
9:59cdf sort of at a larger scale you can't
10:02only increase cbf and you know the one
10:04or two capillaries
10:06where the neurons that are active are
10:08and this
10:09this difference in control is what leads
10:12to a fifty percent increase of cbf
10:15for only a cmr a five percent increase
10:18in cmro2
10:19or the way they very poetically said in
10:22uh their paper uh you're really watering
10:25the guardian for
10:26sake of one thirsty neuron
10:31now the third theory which i also think
10:33is the one that most people tend to
10:35believe today
10:36is known as the oxygen limitation model
10:38and the main proponent of it
10:40is uh is baxter now
10:43the idea is the following the idea is
10:46that the large increase in cbf so the 50
10:49increase in cbf is is necessary
10:53is needed in order to get a 5
10:56change in the oxygen metabolism
11:00so the two are not dissociated or not
11:02decoupled the way that fox and
11:04colleagues said
11:06it's just that in order to get that
11:08little five percent increase in
11:10oxygen metabolism you need a 50
11:13increase of cerebral blood flow
11:16and the idea is the following at rest
11:21how can you calculate the rate what what
11:24is
11:24the rate of oxygen at rest well
11:28it's a function of the concentration of
11:32oxygen in the arteries
11:33for the sake of this example i'm going
11:35to say that that's constant okay
11:37then it's a function of the cerebral
11:39blood flow which means just
11:41how many oxygens are floating by bound
11:43to
11:44hemoglobin and the percent the so-called
11:47extraction fraction the percentage of
11:49oxygens that you pull out
11:52from the cerebral blood flow
11:55now we do know that at activation the
11:58cerebral blood flow goes up by 50
12:01however cmro2 only goes up
12:05by 5 so algebraically
12:08the only thing that could possibly be
12:10happening
12:11is that the extraction fraction is going
12:14down which means
12:16as a proportion of the number of
12:19hemoglobins and oxygens that are
12:21floating by
12:23at activation even though there are more
12:25floating by
12:26as a proportion of that more you're
12:29pulling out
12:30fewer than before maybe you are pulling
12:32out
12:3320 of um
12:36you're pulling out 20 of
12:39oxygens at rest and now you're only
12:42pulling out 14 or 15
12:44oxygens okay so cbf goes up so now there
12:47are more oxygens available
12:49proportionally you're pulling out fewer
12:51than before
12:53now if you pull out fewer oxygens than
12:55before
12:57what happens to those oxygens
13:00well as a consequence of this decrease
13:02in the extraction fraction
13:04all those oxygens that have increased
13:07because the cbf went up
13:08will now end up in the venous side of
13:11the blood vessel
13:13which means there will be a flooding of
13:15oxygen inside the venous side
13:18and this of course is the whole basis of
13:20bold
13:21right bold is a consequence after neural
13:24activation there's there's a change
13:27in the metabolic there's a metabolic
13:29response
13:30the consequence of that metabolic
13:31response is an increase in cdf an
13:34increase in
13:34cerebral blood volume the net result of
13:37that is what you see here
13:39there's a giant increase in the amount
13:41of oxygen
13:42that is inside the veins and you
13:44remember from the graph that i used at
13:46the beginning of this lecture and the
13:48previous one
13:50as you increase the proportion of oxygen
13:52in the blood vessel
13:53the magnetic field becomes smoother as
13:56the magnetic field becomes smoother
13:58protons resonate at the same alarm or
14:00frequency if they resonate at the same
14:02alarm or frequency
14:03it will take longer for protons to
14:05dephase so the t2 star signal will be
14:08stronger
14:09so the way in which bold happens to
14:12emerge
14:13from the from neural activation
14:16is that at activation we get we get this
14:19massive increase in in cerebral blood
14:21flow
14:22so more oxygens are going to a certain
14:24part of the brain
14:26proportionally we pull out fewer oxygens
14:29of those that are floating by so we end
14:30up with many more oxygens
14:32on the other side that is to say in the
14:34vein
14:36as we get this proportional increase of
14:38oxygenated hemoglobin in the veins
14:40compared to deoxygenated
14:43the magnetic field becomes smoother and
14:44we get the bold effect
14:46now the question of course is why is
14:49there
14:50this increase in why do you need this
14:53increase
14:54of this flooding of oxygen in the
14:57in the veins well the reason is the
15:00whole basis of the oxygen limitation
15:02model
15:04and the idea is that when oxygen
15:07when when oxygens are floating by inside
15:10a capillary
15:11they have two choices of course it's
15:14passive choices
15:15they can either keep floating down into
15:17the vein
15:18or they can float out into the
15:20mitochondria
15:21and both of these processes are passive
15:24okay it's not that any oxygen decides
15:26where to go
15:27it's a passive process now we want
15:31more oxygens to go out of the
15:34bloodstream
15:35into the mitochondria so one way to do
15:38that
15:39is to to increase the cerebral blood
15:41flow pack as much oxygen as possible in
15:44the vein
15:45and now by osmosis the next oxygen will
15:48not want to go down
15:50into the vein because there's already so
15:51much oxygen there it will want to
15:53float out in other words
15:56by by flooding the veins with oxygen
15:59you're increasing the oxygen pressure
16:02inside the venous side as that pressure
16:05grows
16:06the next oxygen will want to float out
16:10into the mitochondria instead of a float
16:12down
16:13into the into the vein
16:17so this is what buxton thought is the
16:20link
16:21between the increase in cbf which
16:23increases
16:24massively the presence of oxygen in the
16:26vein
16:27and the increase in cmro2 or the
16:30increase
16:31in number of oxygens that float out
16:34into the mitochondria so according to
16:37buxton this
16:38flooding is a functional role it
16:41increases the
16:42pressure of oxygen in the venous side
16:45and this increase
16:46leads to a greater diffusion across the
16:48membrane into the mitochondria
16:50of oxygen so the whole idea
16:54of this model is that the the the
16:57vascular system
16:58relaxes the smooth muscle around
17:02capillaries this leads to a
17:05giant increase in the amount of oxygens
17:08that float
17:08by as more and more oxygen floats by and
17:12accumulates in the venous side
17:14this will incentivize a couple more
17:18molecules of oxygen
17:20to float out into the mitochondria
17:24instead of down into the veins
17:27so there's a functional link between you
17:29need to increase by 50 percent the
17:31amount of oxygen
17:32floating by in order to convince
17:35five percent more uh oxygens to float
17:38out into the mitochondria
17:40rather than uh to keep floating down the
17:43blood vessel
17:44so the link between the 50 increase in
17:47cmro2
17:47is functional you need a 50 increase in
17:51cvf
17:52in order to get a 5
17:55increase in cmro2
18:00me show this with a graph and with some
18:01numbers okay
18:03um so what i'm showing here is uh
18:06up here let's just imagine that this is
18:08the arterial side
18:10of a blood vessel there's a capillary
18:12right here and then there's the vein
18:15and each of these little circles is
18:18an oxygenated hemoglobin for the sake of
18:21the example
18:22i'll just assume that the oxygenation in
18:24the artery is 100
18:26if you remember we said it's somewhere
18:27closer to 90 to 93
18:29but it just makes it easier to imagine
18:31that every single
18:33um hemoglobin is oxygenated in the
18:36arterial side
18:38so right now we have 16
18:41hemo molecules of the of oxyhemoglobin
18:44in here we know that the extraction rate
18:48is
18:48at rest somewhere around 20 so this
18:51means that
18:52as these protons as these um
18:56as these oxygenated hemoglobin molecules
18:59float by
19:01some 20 of oxygens will float out
19:04into the mitochondria okay so in our
19:07case given these numbers
19:08uh just about 3.2
19:11and of course the 0.2 is an
19:13approximation because of the numbers i'm
19:15using
19:16by the way i'm putting up here the exact
19:19same table
19:20but using a hundred
19:23using 100 as the incoming number of cbf
19:26okay just to make sort of the numbers
19:28a little easier to digest so here we are
19:31we have
19:3216 molecules of oxygen of
19:36oxygenated hemoglobin which means our
19:38cbf is 16.
19:40the extraction rate is typically 20 so
19:43just about 3.2 and again the 0.2 is just
19:46for the sakes of the example
19:48float out into the mitochondria which
19:50means that in the vein
19:53we we now have 12.8
19:57oxygenated hemoglobin and the 3.2
20:00deoxygenated hemoglobin
20:02okay so in other word d and these
20:05oxygenated hemoglobin are exerting
20:07pressure now let's see what happens with
20:11activation
20:12so with activation we know that the cbf
20:15goes up by 50
20:17so if here we had 16 oxygenated
20:21hemoglobins
20:22now then here we have 24. cbf went up by
20:2550
20:28then what happens well we said that
20:30cmro2
20:31the cerebral metabolic rate of oxygen
20:35changes only by five percent
20:38so really now we're extracting a little
20:41bit more
20:423.36 and again the 0.36 is just an
20:46approximation given the numbers that i'm
20:48using
20:50so now there's a few more oxygens
20:53trickling out right so
20:57as i said before the extraction rate so
20:59the number of oxygens that are that are
21:01floating out
21:02given how many oxygens are floating by
21:06is going down from 20 percent up here
21:093.2 of 16 versus
21:1214 which is 3.8
21:16or sorry 3.64
21:19of 24. so
21:22in the venous side right the extraction
21:24rate goes down
21:26so in the venous side now we have
21:2920.64 molecules of
21:33oxygenated hemoglobin and 3.36 molecules
21:37of deoxygenated hemoglobin
21:39and see now there's much more oxygenated
21:42hemoglobin in the vein than there was
21:44before
21:45so it's the fact that there's this this
21:48the cerebral
21:49flow increases we start packing
21:53the venous side with more oxygen
21:56as a result of that a few more oxygens
21:59will trickle out
22:01okay so so the series of events is that
22:04cbf goes up
22:06the cmro2 doesn't change for now as the
22:08vein
22:09sort of gets flooded with oxygen now
22:12there's more oxygen in the vein so the
22:14next
22:15molecule of oxygen will want to will
22:17have a greater tendency to diffuse out
22:20rather than to keep going straight
22:23and that is what makes is what leads
22:26to the slight increase the five percent
22:28increase in cmro2
22:30okay so the link here is that
22:34the cbf increases by 50
22:38so that it can pack oxygen in the venous
22:42side
22:43of the blood vessel and as it packs
22:45oxygen in there
22:47now a couple more oxygens
22:50will diffuse out towards the
22:52mitochondria as opposed to keep floating
22:55down the venous side of the blood vessel
22:57okay
22:58so the the 50 increase in oxygenation
23:01is needed in order to persuade five
23:05percent more
23:06oxygenated spike more five percent more
23:09molecules of oxygen
23:10to float out into the mitochondria

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