Arrector pili are microscopic muscles attached to individual hair follicles. These tiny structures are distributed across the skin where hair is present. Each muscle is a small band of tissue connecting a hair follicle to the upper layers of the skin, and they exist in large numbers across the body.
Anatomy and Location of the Muscle
Each arrector pili muscle is composed of smooth muscle fibers, meaning its contractions are involuntary. These muscles are located in the dermis, the middle layer of skin. The muscle originates from the connective tissue at the hair follicle’s base and extends upwards to attach to the dermis.
This arrangement creates an angled tether between the hair follicle and the skin’s surface. When the muscle is relaxed, the hair typically lies at an angle. The structure is innervated by the sympathetic nervous system, which controls the body’s “fight or flight” responses and allows for rapid reactions to triggers.
The Goosebump Phenomenon
Goosebumps, or piloerection, are the direct result of the arrector pili muscle contracting. This involuntary reflex is triggered by stimuli such as cold temperatures or strong emotions like fear and excitement. The sympathetic nervous system sends a signal that causes the muscle fibers to shorten, pulling on the base of the hair follicle.
As the muscle pulls, it forces the hair to stand up straight, perpendicular to the skin’s surface. Simultaneously, the point where the muscle attaches to the upper dermis is pulled down, creating a small depression. This combination of the erect hair and depressed skin results in the bumps we recognize as goosebumps. The response subsides once the stimulus is removed and the muscle relaxes.
Evolutionary Significance
The function of the arrector pili muscle was more meaningful for our hairier ancestors. A primary proposed purpose was thermoregulation. When the hairs were raised by muscle contraction, they trapped a layer of air close to the skin, which acted as insulation to conserve body heat. This mechanism is still observed in many mammals with dense fur.
A second function was self-defense, as erecting an animal’s fur coat makes it appear larger and more intimidating to predators. In modern humans, with our relatively sparse body hair, neither of these functions is effective. The physiological reflex remains but no longer provides significant thermal insulation or an advantage. For this reason, the arrector pili muscle and the goosebump response are considered vestigial structures.