Final answer:
The nerve fibers that control sweating are motor fibers, which are part of the sympathetic nervous system's response to increased body temperature for thermoregulation.
Step-by-step explanation:
The nerve fibers that regulate the excretion of perspiration are motor fibers. This is because they control the function of the sweat glands, which are an accessory structure of the skin and a part of the sympathetic nervous system's mechanism to cool the body.
Sympathetic nervous system stimulation causes the sweat glands to produce sweat in response to increases in body temperature, whether from environmental heat or physical activity. This excretion process helps regulate body temperature through thermoregulation, where the evaporation of sweat from the skin surface cools the body and dissipates body heat.
The autonomic nervous system, particularly its sympathetic division, is responsible for this involuntary process, with motor nerves signaling the sweat glands to secrete the sweat, which is a salty fluid that also serves to excrete wastes.