As you stub your toe, the impact is sensed by sensory neurons. The impulse created on these neurons, peripheral nervous system, travels then to the central nervous system, where is interpreted as something that's painful and that has to be avoided. Endorphins are released to help soften the pain and then another impulse is released back down, through motor neurons, that will make the effector muscles that will cause to jump and reflex the leg and foot to contract.
