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What is an Arrector Pili Muscle?

By B. Schreiber
Updated Mar 03, 2024
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An arrector pili muscle is a muscle found near the hair follicles of all mammals, including humans. It is the muscle responsible for making hairs "stand on end." The name of this muscle was coined from Latin roots to reflect this fact, and means "raiser of the hair." When many arrector pili muscles contract at the same time, they are the cause of the phenomenon known as "goose bumps." This unconscious reaction reflects a historical adaptation to cold weather and probably frightening situations.

All hairs on the body grow from hair follicles located in the two uppermost layers of skin. The arrector pili muscle is located in the dermis, the middle layer of skin, and extends along the dermal root sheath. The dermal root sheath is the anatomical structure that holds the root of the hair, which is simply that part of the hair that is underneath the skin's surface.

Hair follicles are situated at a slanted angle to the surface of the skin, so the hair shaft, which protrudes above the skin, usually lies flat on places like the arms and legs. When an arrector pili muscle contracts, it pulls the hair follicle into position closer to a right angle to the skin surface. Thus, hair stands nearly straight up, or perpendicular to the skin.

Arrectores pilorum are composed of smooth muscles cells, and is therefore an example of smooth muscle. Unlike the skeletal muscles that direct human movement, for example, smooth muscle is not under conscious or voluntary control. Other examples of smooth muscle can be found in the stomach and intestines, which work unconsciously to process and pass food through the digestive tract.

In furry mammals the contraction of the arrector pili muscle increases the animals' protection from the cold. Raising the fur from the skin results in a layer of warm air being trapped next to the skin. This layer of warm air acts as another layer of insulation in addition to the fur. Raising the hairs also makes the fur thicker.

In some animals, it is thought that the contraction of this muscle in response to fear might make an individual safer. When the furry hair stands on end all over the body, it might make the animal look bigger and thereby deter a possible predator. This leftover evolutionary response isn't very useful for humans in cold weather or in fearful situations. That's because humans have much less hair than most mammals and have gained more control over their environment.

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Discussion Comments
By anon325028 — On Mar 13, 2013

Hairs are part of our sense of touch - brush or vibrate them and we feel it. By standing hairs on end the distance we can feel stuff extends to it's maximum (as much as 30 to 40mm beyond the surface of the skin on my legs). By separating the hair shafts goose bumps reduce the dampening from hairs being laid against each other is reduced, thus increasing fine sensitivity.

As far as I'm aware, this sensory function is universal across all mammals and probably predates mammals as a distinct form and predates hairs as insulation or as a way of warning off competitors or predators. Other functions have come and gone, but, as an element of our sense of touch, this one - with associated goose bumps - has never been lost.

Near my eyes, nose and ears the hairs are so fine that they are difficult to see with the naked eye - yet are so sensitive that the air movements from the wings of tiny insects are easily felt; the insect doesn't even need to actually touch those hairs or the skin.

Protection of vital sensory organs is not an inconsequential function. I've avoided the bites of Australian Paralysis Ticks - very nasty - because they brushed against hairs and I felt them before they could dig in. Very useful.

By lighth0se33 — On Sep 16, 2012

Even if I have just shaved my legs, I can still feel the contraction of the arrector pili muscle when I get a chill. It actually feels like my leg hairs grow when this happens.

If I'm in the tub shaving on a cold night, I sometimes get goose bumps right in the middle of shaving. I run the razor back over these spots, because I can suddenly feel stubble there again.

Am I crazy, or does getting goose bumps really make my hair grow back faster? It's really annoying to get chills while shaving, because it makes me feel like all I've done was for nothing!

By Perdido — On Sep 16, 2012

I get goose bumps often, but I had no idea that muscle contractions were causing them. Who would have thought that our hair follicles even had muscles attached?

If something brushes by or someone whispers in my ear, I get goose bumps on my leg on that same side. If I'm cold, the hair on my arms stands up straight. A few times in my life, I have been really creeped out by a noise, and the hair on the back of my neck has spiked.

By cloudel — On Sep 15, 2012

@shell4life – I've heard that a dog has to be extremely cold for the arrector pili muscle to contract. Since I live in a climate with mild winters, I've never actually seen this happen because of cold.

However, there have been plenty of times when my dog's hair has raised up when he sensed danger. My vet says the muscles respond to a rush of adrenaline, so he totally can't control it.

The hair doesn't stand up all over his body, though. It just erects in one line down his back, all the way to his tail.

By shell4life — On Sep 15, 2012

That's so cool that a dog's muscular tissue can give him an instantly warmer coat! There have been times when I have wished my own body could do this, especially when I'm trapped outdoors unprepared.

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