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Background

I know our bodies have a handful of ways to threshold our awareness of sensory stimuli:

  • Neural density

    • Sensory acuity
    • I assume really tiny stimuli could fit between receptors
    • Impacts summative abilities to surpass postsynaptic impulse thresholds
  • Varying thresholds of individual receptors.

    • Dubin & Patapoutian (2010), for example, show variations in classes of neurons based on their temperature (°C) and mechanical force (milli-newtons) thresholds (see thresholds listed in Tables 1 and 2).
  • Reticular formation involvement in regulating what we are consciously aware of

My question:

Which of the above physiological mechanism(s) are primarily responsible for our lack of sensation (or lack of conscious awareness) regarding microbes on our skin?


Update: My question is explicitly about physiology. Regardless of the impacts, benefits, evolution ,etc. of not feeling them, they're there. But even though we're covered in microbes, we don't sense them -- how not? I want to know by what physiological mechanism(s) we don't feel microbes on our skin.

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    $\begingroup$ I don't think any metazoan has evolved mechanisms to 'sense' microbes in the way you are describing. Microbes are everywhere, in fact, more microbe cells than human cells in adults. So, it is not that 'we don't feel them', quite but the opposite: we don't know how it feels to be 'sterile' of microbes. From the time we are born until we die, we are constantly hosting microbes, just as we don't 'feel' the atmosphere pressure, or gravity: these all are environmental constants to us. For such environmental constants, why would we be conscious of its presence/absence if it has no adaptive value? $\endgroup$ – TumbiSapichu Apr 6 at 16:34
  • $\begingroup$ I'd argue that we do have awareness of such organisms, only via smell: medicalnewstoday.com/articles/173478#what-is-body-odor, healthline.com/health/sudden-change-in-body-odor, microbiomejournal.biomedcentral.com/articles/10.1186/… $\endgroup$ – Maximilian Press Apr 6 at 17:08
  • $\begingroup$ @TumbiSapichu sure. I allude to some of your comment in my post -- perhaps some part of our brain (e.g., the RF) prevents us from "feeling" them. Even if we don't know what it feels like to be sterile, you're suggesting that we feel something but we don't know how to interpret it (or that we haven't had reason to evolve a way to be conscious of it). That would be suggestive of CNS level of regulation (vs the PNS options I also propose in my question). I understand that no metazoans (including us) feel microbes, but my question is why (in a physiological sense) that is the case. $\endgroup$ – theforestecologist Apr 6 at 17:38
  • $\begingroup$ Nociceptors probably aren't the most relevant sensory receptors for the question; they aren't really responsible for any feeling of "touch"; you'd expect them to be activated only if a microbe released something chemical like damaged tissue does. $\endgroup$ – Bryan Krause Apr 6 at 17:43
  • $\begingroup$ @Bryan I know they're not. It was just an example paper (the 1st I found) demonstrating density of a type of sensory receptor. I mention it solely for an example of receptor density. I assumed that the density of other sensory receptors are in the same ballpark (though, perhaps this was an incorrect assumption for me to make?). Regardless, I guess it's not really relevant to understanding the main question: which of these reasons would be most responsible for us not feeling microbes? $\endgroup$ – theforestecologist Apr 6 at 22:03
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Ask yourself, what benefit would it have?

Keep in mind you will always bee covered in them, this is the state you evolved in. Thus sensing them would serve no purpose, and sensory organs are expensive in terms of resources so why would we evolve a useless sense?

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  • $\begingroup$ But I'm not asking about the benefit. I realize there is nothing to gain feeling them. But that doesn't make them not there. And so even though we are covered in bacteria, we don't sense it. My question is about physiology. I want to know by what physiological mechanisms we don't feel microbes on our skin. $\endgroup$ – theforestecologist Apr 10 at 5:49
  • $\begingroup$ @theforestecologist because we lack a mechanism to detect them, I don't understand what more you could want, it is like asking why humans can feel beta particle emission. $\endgroup$ – John Apr 10 at 12:59
  • $\begingroup$ but is that true? Are we lacking detection or awareness? You say this very definitively (and it may be true), but I haven't been able to find literature discussing the mechanism. Is this entirely an issue of PNS not sensing or of CNS not making us aware? If the PNS, is it because thresholds aren't met (for mechanoreceptors for example), or some other reason? I get that you want to simplify the answer because the question doesn't seem important to you. However, I have a biology student who asked me quite earnestly for an answer, and I'd like to provide one if such knowledge is known. $\endgroup$ – theforestecologist Apr 10 at 16:51
  • $\begingroup$ your not going to find a paper about a lack of a mechanism, scientists don't usually write papers about non-existent hypothetical structures. There is literally an infinite number of things we can't sense. For explaining it to a biology student the answer is to think of what senses we do have and what purpose they serve, they need to flip their thought process and think of what we can sense and how each mechanism works. we can't sense it because there is no mechanism to sense it because we can only sense things we have mechanisms to generate sensory signals for. $\endgroup$ – John Apr 10 at 17:29

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