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.