3 ms·

Could tears be there for some kind of moderating effect on cellular action potentials? Tears are notoriously salty; "salty" has even become synonymous with bit

Could tears be there for some kind of moderating effect on cellular action potentials? Tears are notoriously salty; "salty" has even become synonymous with bitterness, angst, or remorse. I think their production does seem to have a noticeable effect on cognition, but for all I know it's just a coincidence. Certainly they would extract sodium from the body to some degree, I just wonder how much of an effect that would have. EDIT: here is a vaguely-relevant(?) study from 2014 about an infant presenting with inconsolable crying allegedly because of "an alteration of sodium channels inducing neuropathy in small-caliber fibers" (1) EDIT 2: As for why this is something that happens from the eyes specifically, I wonder if it is for a particular local effect on ion channels in the frontal lobes. [1] https://www.ncbi.nlm.nih.gov/pubmed/24468061 https://www.ncbi.nlm.nih.gov/pubmed/24468061

throwaway5752 10y ago
First: tears are just as "salty" as blood, so they have no net effect (120–170 mmol/l sodium for tears vs 140 mmol/l in plasma). Even if that was not the case, it would have hardly any effect (5 liters of blood v. a few ml tears). Finally, any effect would be dwarfed by the robust homeostatic system regulated by the hypothalamus. Second: from your study: "Treatment with analgesics and ice packs was established" the infant was crying because the peripheral vascular disease caused them substantial pain. Edit: I guess your "edit 2" is aimed at me? I mean, maybe? It seems like a huge piece of speculation on your part without a particularly well thought out mechanism, so I'd love to see a bit more work on your part before discussing further. I assume you mean that it would be a change in Na+ concentrations propagated by osmotic difference via the optic nerve, because otherwise I don't know how you'd propose that would work (meninges, blood-brain-barrier, and all). Some kind of quantification of the osmotic difference and the rate that would propagate..
- dwringer 10y ago
  I am not clear on what you mean by "no net effect", and not entirely sure how tears are comparable with blood since tears are expelled from the body; blood (and any sodium it carries away from the brain) hopefully remains in the body (not to mention that blood contains a lot more than just NaCl, compared with tears). I am also curious as to how quickly the hypothalamus would respond to a local drop in sodium ions. What you say is plausible; I do tend to agree with you about the study interpretation. Like you, I really want to hear from an expert in this field. EDIT to reply to your edit: No, my edit 2 above was made before I saw any replies, sorry about that. Yes, it is certainly nothing but speculation. (Thank you for pointing me in the direction of further study - I'm afraid this type of thing is a bit beyond my education when it comes to biology)
  - throwaway5752 10y ago
    "No net effect" = tears are produced by the lacrimal glad. I am more of an interested hack, compared to someone that actually knows something (medical professional like a neurologist or ophthalmologist), but my educated guess is that cells in the lacrimal gland produce tears entirely from inputs from the circulatory system, and because of the circulatory system structure in your brain v. face, it's actually rather not as local as the physiological proximity suggests. So the change would be indirect, mediated by blood, and since the concentration of sodium is the same in each crying wouldn't effect blood sodium concentration in the first place. You may under-appreciate how rapid blood moves through the body, too, and quickly any local change would move to equilibrium. ps: you're welcome, happy studying! (it's daunting, keep in mind https://news.ycombinator.com/item?id=1390027 https://news.ycombinator.com/item?id=1390027, and remember that some of the smartest people out there become doctors, and they spend a dozen+ years training in just single areas of study and indefinite continuing education)

throwaway5752 10y ago

First: tears are just as "salty" as blood, so they have no net effect (120–170 mmol/l sodium for tears vs 140 mmol/l in plasma). Even if that was not the case, it would have hardly any effect (5 liters of blood v. a few ml tears). Finally, any effect would be dwarfed by the robust homeostatic system regulated by the hypothalamus. Second: from your study: "Treatment with analgesics and ice packs was established" the infant was crying because the peripheral vascular disease caused them substantial pain. Edit: I guess your "edit 2" is aimed at me? I mean, maybe? It seems like a huge piece of speculation on your part without a particularly well thought out mechanism, so I'd love to see a bit more work on your part before discussing further. I assume you mean that it would be a change in Na+ concentrations propagated by osmotic difference via the optic nerve, because otherwise I don't know how you'd propose that would work (meninges, blood-brain-barrier, and all). Some kind of quantification of the osmotic difference and the rate that would propagate..

dwringer 10y ago
I am not clear on what you mean by "no net effect", and not entirely sure how tears are comparable with blood since tears are expelled from the body; blood (and any sodium it carries away from the brain) hopefully remains in the body (not to mention that blood contains a lot more than just NaCl, compared with tears). I am also curious as to how quickly the hypothalamus would respond to a local drop in sodium ions. What you say is plausible; I do tend to agree with you about the study interpretation. Like you, I really want to hear from an expert in this field. EDIT to reply to your edit: No, my edit 2 above was made before I saw any replies, sorry about that. Yes, it is certainly nothing but speculation. (Thank you for pointing me in the direction of further study - I'm afraid this type of thing is a bit beyond my education when it comes to biology)
- throwaway5752 10y ago
  "No net effect" = tears are produced by the lacrimal glad. I am more of an interested hack, compared to someone that actually knows something (medical professional like a neurologist or ophthalmologist), but my educated guess is that cells in the lacrimal gland produce tears entirely from inputs from the circulatory system, and because of the circulatory system structure in your brain v. face, it's actually rather not as local as the physiological proximity suggests. So the change would be indirect, mediated by blood, and since the concentration of sodium is the same in each crying wouldn't effect blood sodium concentration in the first place. You may under-appreciate how rapid blood moves through the body, too, and quickly any local change would move to equilibrium. ps: you're welcome, happy studying! (it's daunting, keep in mind https://news.ycombinator.com/item?id=1390027 https://news.ycombinator.com/item?id=1390027, and remember that some of the smartest people out there become doctors, and they spend a dozen+ years training in just single areas of study and indefinite continuing education)