Fig.1.Simple Reflex - Human feet in contact with a tack (external stimulus).
Medula espinhal = spinal cord; Para o Cérebro= to the brain

(Click on the button  "Reinicia" ("Re-start") to see the animation). 

The skin perforation is transduced into signals that travel upwards via sensorial nerves (direction of information flow shown by arrows). This information arrives at the spinal chord and is distributed to inter neurons (neurons that provide intermediate connections with other neuron chains). Some of these neurons send axons to the sensorial region of the brain where the sensation of pain is registered. Other make synapses with motor neurons,which send signals downward to the muscles. The motor junctions command the muscle contraction and withdrawal of the foot. 

This is an example of a neuron chain called reflex arc.

1.Sharp object penetrates the skin;
2.The membranes of nerve fibers in the skin are stretched;
3.Channels permeable to sodium (Na⁺) are opened.

Due to the concentration gradient and negative charge in extracellular fluid,ions enter the fiber by means of these channels.  Sodium entrance(yellow balls) depolarizes the membrane, that is, the side of membrane bathed by extracellular fluid becomes less negative in relation to the cell interior.  If this depolarization, called generator potential, reaches a critical point, the membrane then generates an action potential. The critical point of depolarization that must be overcome in order to fire an action potential is called threshold.The action potentials are caused by depolarization of the membrane above the threshold. 

In the repolarization phase, the sodium channel is deactivated and the conductivity of membrane for potassium increases. These positive ions
follow its chemical and electrical gradient and exit from the cell, thus gradually inverting the membrane potential (green balls) 

In the recovery phase, the active sodium/potassion pump (blue channel in the figure) restablishes the ionic levels which were altered during the action potential: for each sodim ion which is transported from the inside to the outside, a corresponding potassium is transported in the opposite direction.