Answer:
The pili arrector muscle, or erector muscle of the hair, is a muscle composed of smooth muscle fibers which is innervated by the sympathetic nervous system (SNS). When the hair is erect, what is known as "goosebumps" is produced; This action is involuntary and can be caused by fear, cold, among others. The efferent signals to activate this muscle come from the hypothalamus, brain stem and spinal cord.
