What are the small muscles at the base of hair follicles called?

The small muscles at the base of hair follicles, known as arrector pili muscles, play a crucial role in the integumentary system. These muscles are smooth muscle fibers that contract in response to stimuli such as cold or fear, leading to what is commonly referred to as "goosebumps." When the arrector pili muscles contract, they pull on the hair follicles, causing the hairs to stand upright. This response not only helps in thermoregulation by trapping a layer of air for insulation but can also serve as a defense mechanism by making an animal appear larger to threats.

The other options do not correctly identify these muscles. Hair follicle receptors are sensory structures associated with hair follicles that detect movement of the hair, dermal papillae are small, nipple-like projections that help supply nutrients to the hair and support hair growth, while keratinocytes are the predominant cell type in the outer layer of the skin responsible for producing keratin. Each has distinct functions unrelated to the contraction of hair muscles.