Some Observations on Neuropathy

     I've recently finished with chemotherapy for colon cancer, which has afforded me the opportunity to experience some things that, while not necessarily pleasant, have been kind of interesting. In particular, right now I'm thinking about the composite nature of tactile senses in a way it never occurred to me to consider before.

     We all know that the colours we see are really just distinct blends of only three basic colours: red, blue and green. That's because our retinas contain photoreceptor cells that are particularly sensitive to one of these three ranges of frequencies. Primates have better colour vision than most other mammals, which only have two different colour receptors. Most birds have four, giving them a greater sensitivity to distinct colours.
     Likewise, the many flavours we can distinguish are generally made up of composite signals from just five types of receptors on our tastebuds: bitter, salty, sour, sweet and umami. Our brains recognize one particular proportion of these signals as garlic, another pattern as lemon, and so on.
     Indeed, hearing is also a composite sense; tiny hairs are located at a different resonant lengths within the cochlea, and thus are sensitive to different frequency inputs. What we distinguish as a single sound (a violin, a trumpet, a human voice) is a complex blend of the inputs of hundreds of different audio frequencies.

     Now, one of the side effects of oxaliplatin (one of the chemotherapy drugs I was on) is what they call peripheral neuropathy, or damage to the nerves leading to the remote (peripheral) parts of the body: fingers and toes. In short, my fingers and toes are uncomfortably numb. However, it seems that this neuropathy doesn't affect all receptors equally. While I've lost most of the pressure sensors in my fingertips, I can still receive signals from the pain and temperature receptors, which has had some interesting (if annoying) effects.
     First, the pressure receptors have by far the finest resolution, as Braille readers demonstrate. The pain and temperature receptors don't need to be as precise about location; it's enough that the brain knows a particular fingertip is risking damage without worrying about what square millimeter is at risk. This means I can't sense things as precisely with my fingertips as I could before the treatment. I have trouble buttoning my shirt, and I can't play the guitar without visually confirming I'm putting my fingers on the right strings and frets. It also means my fingers constantly feel kind of greasy to me, as if a fine layer of oil is preventing me from detecting the tiny variations in altitude of a surface that would normally show up as small local variations in signals from pressure sensors.
     But I can still sense textures to some extent, and I think this is due in part to the role of what we call the pain sensors. The pain sensors are triggered by extremes, stresses of pressure or temperature that threaten to damage tissues, but they seem to have a wide range of sensitivity, and I think that normally they must play a role in sensing things that are not strictly speaking "painful". For example, when feeling the edge of a knife, one doesn't apply enough pressure to actually suffer any damage, but the relatively extreme stresses on the tissues do trigger a mild pain signal which, combined with the pressure and temperature signals, forms a composite tactile image of a sharp edge. The composite signal doesn't register as painful at all, but the presence of a certain amount of the pain signal is part of what gives the feeling of sharpness.
     I am finding I sense the sharpness of surfaces, like the edge of a fingernail, as sharper than normal. I believe this is due to the "pain" signal making up a bigger portion of the composite input, since the pressure sensors are providing very little signal. It doesn't hurt, exactly; it just feels like edges I touch are sharper than they were before the treatment.
     In a way, it's sort of like the reaction to cold I had at the very first cycle. Touching cold things was startling; it's not that they felt colder than usual, but just more vividly cold. It was sort of like having just cleaned my glasses and then seeing the world much more clearly all of a sudden. (Subsequent rounds of chemo made the effect worse; instead of just feeling vividly cold, handling something right out of the fridge was like grabbing a live wire, a very nasty shock.) I am not sure how to interpret this experience, because the attenuated pressure sensitivity hadn't kicked in yet.

     Anyway, it's been fascinating to be able to take advantage of this experiment, much as I'd prefer to have learned this stuff second-hand.