There are many kinds of nerve fibers. Two that are easy to understand are the motor fibers that let us move and the sensory fibers that let us feel. Another important group of fibers that is harder to understand includes the sympathetic nerve fibers.
The sympathetic nerves — also called the “C fiber” or “small fiber” nerves — arise from little collections of nerve clusters called ganglia. These are located outside of the spinal cord. The ganglia can function entirely independently (automatically) from the rest of the nervous system.