# What regulates hair growth?

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Hair is technically non-living material, right? And yet it grows to a certain length and then stops (and this length is different on different parts of the body). Not only that, but if it is shaved or trimmed, it will grow out to the same length again. How does the body regulate this? How does the body tell the difference between fully grown hair and hair that has been trimmed, if there are no nerves in the hair itself?

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there are pressure sensors close to the hair in the skin that can tell how long the hair is by how much weight it exerts.

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hairs composed of Keratin IIRC, and yes, its quite "Dead".

but there are more than One hair type on the body, pubic hair is different to that on your head, your head hair will continue to grow ad infinitum (even after youre dead!) pubic type hairs differer again, they have a set geneticaly programed length, sort of works on a "Timer" at the follicle.

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if its genetically programmed then how does it know when its cut?

i'm sure there are sensors under the skin

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it doesnt "know" when its cut.

and also what hair species are you refering to?

neither actualy "Know" anyway, but its still an important point when the effect is to be explained

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For our purposes, let's say I'm talking about human arm hair. It can't merely be a timer, because then the hair would grow once and that would be it. However, this is not the case. It is true that it grows to a certain length and then stops, but if it is shaved or trimmed, it will grow back out to its original length, and stop again. Therefore, the follicle must, in some way, "know" how long the hair is, thus "knowing" when it is cut. And yes, obviously, thought is not involved here, but there must be sensation of some sort. I suppose pressure sensors in the surrounding skin would explain it, although they would have to be astonishingly sensitive. I guess that has to be it, though.

Oh, and I did try the search engine, but I couldn't find anything.

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when I say a "Timer" I use the term loosely, it has growth phases, from full normal growth and then a "Rest" period, during this period a new hair will start to form and the old (original) hair will break off and the new (second) hair will commence growth, so although it may LOOK like it "Knows" what youre actualy seeing is the phase transition, if you were to shave a single hair, depending on its growth/rest cycle youde see one of 2 things.

in mid growth, the hair would grow to half its length with the tip you cut off still missing, in rest phase, youde see a brand new hair grow again (but it wouldnt be the one youde cut).

now if we look at the Gross effect of this, since each hair fillicle is going at a different rate, some at rest some at full growth rate, we simply don`t notice this effect as it Averages out

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