First lets review some key concepts. When a neuron is polarized a state of opposition exists and it is ready to fire. We call this resting potential. As a neuron’s membrane depolarizes it allows more positive ions in and is close to firing (action potential). Depolarization opens the sodium gates (voltage activated open) and action potential occurs. The neuron then hyperpolarizes (increases the opposition) and becomes harder to fire. This is known as the refractory period. Hyperpolarization can occur when the interior of the neuron becomes more negative (-70 mv to -90 mv), usually due to inhibitory neurotransmitters. Hyperpolarization also occurs when the interior goes from a negative state, passes zero and becomes slightly more positive (+20 mv). This occurs after action potential and is considered the refractory period .
Not every stimulus that is detected by a neuron causes that neuron to fire or reach action potential. A stimulus (or combination of stimuli) must be strong enough to cause a neuron to reach threshold. Threshold can be defined as the minimal voltage necessary to cause the neuron to depolarize to the point of action potential.
Sub-threshold stimuli are referred to as Excitatory Post Synaptic Potentials (EPSPs) when they depolarize the membrane (allowing positive ions in) and are called Inhibitory Post Synaptic Potentials (IPSPs) when they hyperpolarize the membrane (keeping positive ions out or holding positive K+ ions in). The EPSPs and IPSPs summate (add up) to determine whether or not the neuron fires. Once a stimulus (or group of stimuli) reaches threshold, the size and the strength of the resulting action potential are the same. This is known as the all-or-none principle. In other words a neuron either fires all the way or it doesn’t fire at all. Action potentials are the same strength not matter what the strength nor the number of the stimuli that caused them.
When a neuron becomes stimulated, the area of the membrane at the point of stimulation becomes more permeable to Na+ (it is depolarized). This depolarization voltage activates the Na+ gates at the nodes of Ranvier to open. Na+ rushes inside the cell. At the same time the K+ gates, which were partially open allowing K+ to pass freely all along, open wider and K+ rushes out. This rapid exchange of ions reverses the electrical gradient because the influx of positive ions cancels out the negative interior causing the valence to reach zero. Then the influx of Na+ takes the interior to a slightly more positive state (+20 mv). This is the hyperpolarized state. (Remember, hyperpolarization means making it harder to fire, either by increasing the negative interior or going past zero to a slightly positive interior.) This hyperpolarization (known as the refractory period) is only momentary as the rapid outflux of K+ returns the interior to a negative state. The sodium/potassium pump is also working to re-establish the resting potential. In other words the outflux of K+ combined with the action of the sodium/potassium pump restores the electrical and concentration gradients. Within milliseconds the polarity is re-established and the neuron is once again ready to fire.
