Action Potential

When a neuron is not sending a signal, it is "at rest." 

When a neuron is at rest, the inside of the neuron is negative relative to the outside. 

Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels (ion channels). 

At rest, potassium ions (K+) can cross through the membrane easily but, chloride ions (Cl-) and sodium ions (Na+) have a more difficult time crossing. 

The negatively charged protein molecules (Protein-) inside the neuron cannot cross the membrane. 

In addition to these selective ion channels, there is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in. 

Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential. 

The resting membrane potential of a neuron is about -70 mV (mV=millivolt) this means that the inside of the neuron is 70 mV less than the outside. (For a skeletal muscle it is more like -90 mV (mV=millivolt)

At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron.

The resting potential tells about what happens when a neuron is at rest. 

An action potential occurs when a neuron sends information down an axon, away from the cell body. Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential. The action potential is an explosion of electrical activity that is created by a depolarizing current. 

This means that some event (a stimulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. 

This is the THRESHOLD 

If the neuron does not reach this critical threshold level, then no action potential will fire. 

When the threshold level is reached, an action potential of a fixed sized will always fire! 

ALL OR NONE PRINCIPLE

A single nerve fibre will always give a maximum response and producing spikes of the same amplitude when stimulated. 

If the intensity of the stimulus is increased, the height of the spike always remains the same. In short the propagated impulse in a single fibre cannot be graded by grading the intensity or duration of the stimulus. 

The nerve fibre gives a maximum response or none at all

This is called the "all or none" principle. It is also Known as all or nothing law.

Action potentials are caused when different ions cross the neuron membrane

A stimulus causes sodium channels to open FIRST

There are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. 

Sodium (Na+) has a positive charge, so the neuron becomes more positive and becomes depolarized. 

It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. 

Also at about this time, sodium channels start to close. 

This causes the action potential to go back toward -70 mV (a repolarization) or -90 mV in skeletal muscle.

The action potential actually dips below -70 mV/-90mV(a hyperpolarization) because the potassium channels stay open a bit too long. 

Gradually, the ion concentrations go back to resting levels and the cell returns to -70 mV/-90mV

What is the difference between MEMBRANE (RESTING), GRADED and ACTION Potentials?

Graded potential and Action potential are the two types of potential differences that can be generated during depolarization. 

The main difference between graded potential and action potential is that graded potentials are the variable-strength signals that can be transmitted over short distances whereas action potentials are large depolarizations that can be transmitted over long distances. 

Graded potential may lose the strength as they are transmitted through the neuron but, action potentials do not lose their strength during the transmission.

The video below is EXCELLENT!