Arrectores pilorum
From Biocrawler, the free encyclopedia.
A hair follicle, showing its Arrector pili muscle.
Arrectores pilorum (singular Arrector pili) are tiny muscle fibers attached to each hair follicle, which contract to make the hairs stand on end. They are smooth muscle, not skeletal muscle, which explains why you can't voluntarily give yourself goose bumps. In other animals with more hair than humans, they serve an important function — they raise the hairs so air gets trapped between them, providing a layer of insulation to keep the animal warm. Some animals also contract their arrectores pilorum when they are cornered, in order to appear larger and more threatening. Although humans' arrectores pilorum also contract in response to cold or arousal, they are vestigial because humans do not have enough hair to make them effective.
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