I come from Physics, not biology. My understanding is that without sufficient oxygen present, the Krebs cycle terminates in lactic acid formation for the body to later continue the reaction when oxygen becomes more available for oxidation.

If I understand correctly, for a period of time there will be an excess of Lactic acid in the body which will change the pH of the cell. How does that translate into the perception of pain?

I was taught in a Cognitive Science class that pain is cause my a neurotransmitter (Substance P) which is released due to bodily damage. This might not be entirely true, please correct this if wrong.

So my thinking is:

Lack of Oxygen => Lactic Acid production => ... (?) => Substance P => Pain

Thus possibilities for the "..." would be, bodily damage => Substance P, or Lactic Acid signals Substance P release without a need for bodily damage.

My question is then, how do I get from Lactic Acid to Pain?

  • $\begingroup$ I am assuming that lactic acidosis does cause muscle soreness (at least some of it!). The following supports my assumption, medicinenet.com/script/main/art.asp?articlekey=78966 , but ncbi.nlm.nih.gov/pubmed/6392811 seems to reject lactic acidosis as a cause of DOMS. NOW, this DOESN'T imply that lactate acid does not cause pain. This would be an extension to my original question. $\endgroup$ – aidan.plenert.macdonald Jan 1 '16 at 21:16
  • $\begingroup$ First, acidosis during hypoxia is not due to production of lactic acid, as commonly believed; it is due to loss of respiration, which normally acts as a proton sink. See en.wikipedia.org/wiki/Lactic_acidosis, section "Pathophysiology". Second, if you're considering exercise-related acidosis, I think you must separate acidosis itself from muscle fatigue / damage, which is painful for other reasons. I don't think acidosis itself is particularly painful, although headaches are reported. See ncbi.nlm.nih.gov/pubmed/19932372 . So I'm not sure your premise is correct. $\endgroup$ – Roland Jan 2 '16 at 9:36
  • $\begingroup$ Thank you. My premise is based upon questionable sources. Wikipedia, "This is incorrect, as cells do not produce lactic acid; pyruvate is converted directly into lactate, the anionic form of lactic acid." If so, then what happens to the proton? It seems that there would be a charge imbalance. Is there any local change in pH if there is a lack of oxygen in the system? Is this pH change significant? $\endgroup$ – aidan.plenert.macdonald Jan 3 '16 at 3:51
  • $\begingroup$ Never mind. I found it, Wikipedia for Lactic Acid - "The production of H+ during exercise,[13] has been claimed to be a "construct" or myth, pointing out that part of the H+ comes from ATP hydrolysis (ATP−4 + H2O → ADP−3 + HPO4−2 + H+), and that reducing pyruvate to lactate (pyruvate− + NADH + H+ → lactate− + NAD+) actually consumes H+." $\endgroup$ – aidan.plenert.macdonald Jan 3 '16 at 4:00

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