Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: nilmot on 23/05/2003 21:54:36
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I've troubled by this question. Lactic acid is poisonous to the body, and everytime we respire anaerobically cells produce lactic acid. How come we can excercise for longer through training...etc if lactic acid is still present in our muscle?
Tom
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quote:
Originally posted by nilmot
I've troubled by this question. Lactic acid is poisonous to the body, and everytime we respire anaerobically cells produce lactic acid. How come we can excercise for longer through training...etc if lactic acid is still present in our muscle?
Tom
We can't. Feel the burn.
PG
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if you do something called "warming down" which athletes do more effectively than us, you can reduce the levels of lactic acid in your muscles and thus reduce the ache. Warming down involves doing less strenuous exercises such as walking and stretching as this keeps a steady flow of oxygen flowing to the muscles, this in turn reduces the oxygen debt that has been created and overall, reduces levels of lactic acid.
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Dear Tom,
lactic acid, which is not toxic per se, is the product of anaerobic metabolism or energy which is produced in the absence of oxygen.
Energy is generated in cells by 3 sequential metabolic pathways. The first, called glycolysis, occurs in the absence of oxygen and breaks down the sugar glucose to produce a molecule with 2 carbon atoms called pyruvic acid, and a small amount of energy (ATP). This process uses a hydrogen carrier molecule called NAD, which is in limited supply, to remove hydrogen from the reaction (NAD + H = NADH).
The pyruvate is then used by the next reaction, called Krebs Cycle.
In the final pathway the hydrogen removed by NAD is added to oxygen to make water (2NADH + O = H2O + NAD), releasing large amounts of energy, and in the process it regenerates the NAD which can be used again.
You can see that whilst there is plenty of oxygen about it is easy to keep regenerating the NAD needed to burn glucose. But if oxygen starts to become limited then you would quickly run out of NAD (they are all queueing up as NADH waiting to get rid of their hydrogen to oxygen) and energy production would have to stop.
Instead a clever enzyme called Lactate Dehydrogenase takes the pyruvic acid we mentioned above and adds hydrogen from NADH to it, producing LACTIC ACID and NAD. Thus the NAD can be used again as described above, but what about the lactic acid ?
For a start it increases the acidity of the muscle, which hurts, accounting for the burning sensation you feel when you perform severe unaccustomed exercise.
But the lactic acid is also continuously removed by blood flow and joins the CORI cycle (named after Cori and Cori who discovered it). Lactic acid produced in muscles is transferred via the blood to the liver which picks it up and, again using lactate dehydrogenase, reverses the process that made it (Lactic acid + NAD = pyruvic acid and NADH). The pyruvic acid is then turned back into glucose (by reversing the steps we described above) which is pumped back out into the blood stream and returned to muscles again. Hence the Cori CYCLE.
When you exercise and produce lactic acid you are accumulating an oxygen debt which you repay later by increased respiration which helps to remove carbon dioxide from the blood stream. Since carbon dioxide is an acid, increased breathing is one way to remove the acid contributed by the lactate.
So now you ask why can training affect your ability to exercise.
Excellent question. The reason is that muscles contain a combination of fibre types with totally different metabolic profiles and characteristics. As you train you boost the number of fibres that work using oxygen (and DON'T produce lactate), and you reduce the number that work anaerobically and produce lactic acid.
In any muscle a certain subset of fibres are called 'fast twitch'. They are very good at producing vast amounts of power, very quickly, but for a short time only. They are specialised at working anaerobically (without oxygen) and so they have a relatively poor blood supply. If you look at a piece of Turkey, the white meat on the breast is fast twitch muscle - it's very pale.
The dark meat, on the other hand is that colour because it is rich in 'slow twitch fibres'. These fibres, whilst producing less force, are capable of sustaining their contraction for much longer because they work aerobically, using oxygen. As a result they have an excellent blood supply and contain lots of iron-rich proteins which are involved in oxidative metabolism. Hence they are dark to look at. This kind of muscle is usually used in postural muscles such as those that help us stand up.
This shift in favour of a more oxygen-dependent metabolism takes a few weeks to occur which is why you need to train to achieve it. The reason that you lose it very quickly if you stop is because high blood-flow muscle is very energy-costly to maintain so if you don't use it, you lose it !
Perhaps someone would like to hazard a guess at how the same muscles can be used to stand (slow twitch) and then hop (fast twitch) ?
TNS
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TNS, you said that fast twitch muscles are white is it because they lack of the pigment called myoglobin? Does myoglobin work the same way as haemaglobin?
Angel
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Exactly right. Myoglobin is a haemo-protein (contains iron) like haemoglobin but whilst haemoglobin is sequestered in red blood cells, myoglobin (as myo in the name suggests) is locked up in muscles. Its role it to pick up oxygen from the blood and yes, it contributes to the dark colour of slow twitch muscles. When I said "iron-rich proteins" I was alluding, in part, to this, but attempting not to confuse with overwhelming detail. You were just clever enough to pick it up.
Best
TNS
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TNS, you said "In any muscle a certain subset of fibres are called 'fast twitch'." My guess is that muscles would use the appropriate subset for the particular activity; "slow twitch" subset for aerobic activity, since oxygen is ideally in good supply to cleanly burn the glucose. When the oxygen supply dwindles, the lactate dehydrogenase initiates the Cori cycle and we switch to "fast twitch" subsets. At this point we would quickly tire and have to change to anaerobic activity.
If we perform anaerobic activity before the oxygen supply slows, then the "fast twitch" muscles would be activated, initiating the Cori cycle OR lactate dehydrogenase would be activated, which would iniate the Cori cycle.
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You are on the right lines Donnah.
As I said, a muscle contains different proportions of slow and fast twitch fibres. The bigger the fibres, the greater the power output and the more anaerobic the action becomes.
The key lies in the nerve fibres that activate them. Slow twitch fibres are supplied by small motor neurones which discharge more readily when stimulated (Hoffman's size principle) and have a higher tonic (basal) firing rate - in other words they 'talk' almost continuously to the muscles fibres they innervate.
Large anaerobic fibres are supplied by big motor neurones which have a very high input resistence meaning that they only 'fire' when the stimulus trying to activate them is very strong. These cells have a very low rate of tonic discharge.
When you are standing up the stimulus from the motor systems is small and hence the only motor neurones that respond are the small ones supplying the oxidative slow twitch fibres. When you want to hop, on the other hand, the descending motor pathways from the brain send a barrage of signals down the spinal cord sufficient to recruit not only the small motorneurones but the large ones too, activating the fast twitch (poorly oxidative) fibres.
Today's question : So what happens when you train a muscle ?
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Could you be a little more specific? Are you asking how muscle fibres expand?
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I did 2 hours in the gym yesterday and boy do i ache today! Bloody Lactic Acid!!!!
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quote:
Originally posted by NakedScientist
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Today's question : So what happens when you train a muscle ?
Does the muscle cell get bigger and so have more myoglobin? Muscle cell can store more oxygen and can sustain longer anaerobic exercise?
What about in reflexes? Does the impulse send to the fast twitch or slow twitch musles? I'd thought it's to the fast twitch muscles.
Exodus, did you warm down like you said in the previous post?
Angel
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i did yes, but it doesnt completely help... i did small stretching exercises while doing a slow walk. It was most probably because it was a beast of a session, abdomen is the worst but did do 250 sit ups.
I am under the impression that training leads to the burning of fat which produces less lactic acid... although i might be completely wrong. (https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.gamers-forums.com%2Fsmilies%2Fups%2FDeNiro%2Fidiot.gif&hash=55fc36c8012c4dfe85708d4be902f2b1)
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Yeah, marathon and cross country runner tend to burn fat for energy rather than glucose because fat releases lot more energy.
Don't know what type of training TNS is talking about, but if it's aerobic then what Exodus said is true (not sure about the bit with "produces very little lactic acid")
Woo...250 sit ups, your abdomen must be aching like hell!
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Edit: i meant to say "less lactic acid". Have changed it. And yes, the old abdomen is a little sore!
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Exodus - pleased to see that you have quite appropriately put yourself as coming from the VIRGIN islands !
chris
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- Groucho Marx
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What about cardiac muscles? They are said to be myogenic, which means they never get tired. So do they carried out aerobic respiration all the time? I'd thought so, don't like the thought of having lactic acid in the heart!
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quote:
Originally posted by chris
Exodus - pleased to see that you have quite appropriately put yourself as coming from the VIRGIN islands !
chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
Just visiting claire...
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quote:
Today's question : So what happens when you train a muscle ?
The muscle fibres tear as they are being worked. It takes about 48 hours to replenish the potassium used in the workout and repair the lesions, and the muscle fibres grow larger as they are repaired. That's why we alternate workouts, say upper body one day and lower body the next.
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I'll stay out of this one as macrobiology is not my thing :-s
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Thanks for all your reply. [:)]
I was thinking that say if you excerise and the same amount of lactic acid is produce; does your lung capacity increase (not too sure there). If it does you can taking in more O2 to pay back the oxygen debt. Therefore more acid is being broken down and you won't feel the burn that quickly.
Tom
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Tom
good thought, but no, not really.
Lung capacity does increase slightly, and there are some other physiological changes that occur within the respiratory system with training and also in response to low oxygen conditions which is why athletes train at altitude, but on the whole you are viewing 'oxygen debt' the wrong way. The blood that leaves your lungs (assuming that they are normal) is 100% saturated with oxygen and so breathing even harder, or even breathing 100% oxygen, will barely affect the amount of oxygen in the blood stream. It will, however, reduce the level of carbon dioxide in the blood, which partly helps to repay the oxygen debt and correct for the lactic acidosis which it produces.
Oxygen debt actually occurs because the metabolism occurring within the active muscles is outstripping your ability to deliver blood (carrying oxygen) quickly enough. Blood brings in oxygen and takes away carbon dioxide (and other waste including lactic acid), so once muscles begin to burn energy faster than you can supply oxygen to remove the hydrogens from NADH (see above) you are forced down an anaerobic pathway. Removing the lactate in the liver (the cori cycle) is an energy-dependent process requiring oxygen, hence oxygen-debt.
TNS
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So the whole point of breathing faster or deeper is not to get more oxygen in your blood is get carbon dioxide or other wastes that might be poisionous out of your body.
Tom
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Quite right. And the rate of ventilation is increased by the detection of high level of CO2 in blood and this cause the repiratory centre in the medulla to send more frequent impulses to the diaphragm and intercostal muscles, which initiate ventilation.
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Angel
good thought, but not strictly true.
For some reason there are no chemoreceptors in the venous circuit. The carotid body (which measures blood pH and CO2) is in the arterial circuit and if you measure arterial blood during severe exercise, CO2 does not go up. In fact it may even go down because efficient respiratory compensation blows off CO2. Blood pH can fall though, due to lactate production, which can tickle the carotid body and trigger increased respiration via the medulla, as you contend.
TNS
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TNS,
Does the same chemoreceptors in aorta and carotid artery send impulses to cardiovascular centre to increase(or decrease) heart beat?
I think the chemical released at the end of the sympathetic nerve is noradrenaline, this increase heart beat. And acetylcholine is released at the end of the parasympathetic nerve that cause heart beat to decrease.
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We've just started the upper sixth biology lesson and now I've found this page more understandable.
All the cycle and the stages, more familiar with the terms, also the last bit about chemoreceptor we've just done in an exam is Mod3.
Very useful as well. Very detailed and full of good arguement and explaination.
Tom