Not half a bad question. Now that I come to think of it, I had been subliminally aware of the problem, but without thinking clearly about it. I certainly did not know the answer, so the following should be read with reserve; it amounts to thumbsucks and handwaving.

I suspect what it comes down to is that the rubefacient material does not immediately affect the heat-sensing nerve endings, but instead stimulates other cells in the skin to release substances that do so, probably proteins or short chain peptides. The degree to which the particular skin tissue would respond would vary with the particular part of the body.There could be two reasons for such variation, possibly both applying simultaneously. The skin of our palms and soles has specialised and demanding functions, both sensory and resistant to external injury. It simply might not have accommodation for cells that react to rubefacients, or we equally simply could not afford such casual reaction to noxious substances in those areas of skin.

Furthermore, those skin areas are very tightly structured and contoured by dense connective tissue. They have to be to function as required, both for gripping and for resistance to abrasion and similar insult. This reduces their tendency to swell when inflamed. (It also makes it particularly uncomfortable when they do in fact swell.)

Other areas of skin to become inflamed when affected by nettles, superficial infections and minor wounds, and nature's rubefacients, such as poison ivy. It is not that your fingers are immune, but probably that there are less sensitive because they lack the go-between generators of inflammatory compounds that are plentiful in the rest of your body.

Oh, and by the way, your fingers and palms are not truly immune to harmful substances such as rubefacients; just not very sensitive. Be cautious in using and handling such things. If you overdo it you might find yourself very painfully and very inconveniently shedding hardened skin that you neither wanted to harden nor shed.

Trust one who has tried!

That's my guess anyway.

Any ideas anyone?

Jon

I cannot tell you for sure but I do have an idea. What happens at nerve endings is quite complicated and I cannot remember enough of it to go into in detail.

We have nerve endings tha detect heat. What they actually detect is a rise in temperature. There are more than one set of them, I think four though I may be wrong, each set designed to detect a rise in temperature within a certain range. It is only one set of these that is stimulated by capsaicin and so gives the illusion of burning.

There are also nerve endings that detect a fall in temperature, two of them working over different ranges I think. One of these is stinulated by menthol giving the illusion of cold.

We have two sets of nerve endings involved in touch, one set detecting pressure and the other set detecting stretching of the skin.

The density of each type of nerve ending varies over different parts of the skin. We obviously have a very high density of nerve endings involved in touch in our finger tips. I suspect that we have a low density of temperature rise detecting nerve endings in our finger tips and this is why the heat cream doesn't have an effect.

Stewart - if that were true, why do I drop a very hot coffee cup by reflex when I pick it up with my fingertips? I suppose what I am asking is what is the difference between actual heat, and chemically induced heat? Methyl Salicylate and Capsaicin both make certain nerves fire and make us think they are experiencing heat. Does actual heat do the same?

Peter Finan

Deep heat is made to create heat in muscles, Your hands, fingers especially consist of tendons more than muscles, I havent tried, but im sure if you rubbed deep heat into the muscle area palm of your hand beneath your thumb, it would generate heat.

It has little to do with the skin, sure you feel the heat in the skin, but its generated in the muscle.

As to why deep heat specifically creates heat in the muscle, I cant comment.