Hormones.

Hormones are chemicals that are released into the blood , they are carried in the plasma to varying parts of the body, where they act on target cells. These are particular cells in particular places that need constant adjustment.

Nerves:
Very Fast.
Act for very short time.
Act on a very precise area.

If you have a quick response to something, it is probably nervous.
Information needs to passed to effectors really quickly , I.E. pain signals, from your finger, telling you its in a flame.