How do internal stimuli affect a positive and a negative feedback loop?

Deviation from homeostasis causes the activation of feedback loops in order to restore equilibrium to the organism. 

An example of a negative feedback loop is the homeostasis of blood glucose; the increase in glucose levels stimulate the production of insulin by the pancreas, which in turn causes the uptake of glucose by tissue cells and the formation of glycogen in the liver. This process is repeated until blood glucose levels return to normal, where there is no excess glucose to stimulate insulin production. 

Contrary, a positive feedback loop causes amplification of its causal stimulus. An example of a positive feedback loop is the propagation of nerve action potential;a small secretion of sodium ions through sodium channels on a nerve fibre causes a change in the membrane potential, which in turns leads to the further opening  of sodium channels and an explosion of sodium ions. When the ammount of internal sodium drops, the membrane action potential reverses and the sodium channels close.