1. Occur when a neuron’s membrane voltage exceeds its threshold of excitation. This value is approximately negative 55 millivolts.
2. Are all or none signals. This means that action potentials are not “graded” signals, rather, they are binary signals. A neuron will either “fire” and action potential or it will not.
3. Electro-chemical gradients of both potassium and sodium ions, along with the synchronized opening and closing of voltage-gated sodium and potassium ion channels, allow for the generation of an action potential.
Here are some videos that will introduce you to how an action potential is generated in a neuron:
Action Potential: Video #0
Action Potential: Video #1
Action Potential: Video #2
Action Potential: Video #3
Action Potential: Video #4
An action potential is characterizied by 3 phases:
1. Rising Phase
Brief explanation of phases:
1. Rising Phase: Voltage-gated sodium channels open to allow for entry of sodium ions into the cytosol of the neuron. This event increases membrane voltage from approximately -70 millivolts to + 50 millivolts. Activation of voltage-dependent sodium channels triggers opening of voltage-gated potassium channels. As more and more voltaged-gated potassium channels open, more and more voltage-gated sodium channels close. At approximately 1ms, all sodium channels are closed. This state indicates the end of the rising phase and the begining of Repolarization.
2. Repolarization: During repolarization, voltage-gated potassium channels gradually begin to close. Hyperpolarization of the neuron’s membrane is sustained for approximately .25ms. Potassium channel kinetics explain this extended period of hyperpolarization.
3. Hyperpolarization: During hyperpolarization the neuron’s membrane voltage decreases below its resting potential of approximately -70millivolts. This event is called the “undershoot”. During the intial entry into the “undershoot” the neuron’s membrane enters a state called an “absolute refractory period”. During the absolute refractory period the neuron is not capable of generating an action potential. The “relative refractory period” occurs after the absolute refractory period. During the relative refractory period higher that normal levels of electrical stimulation must be applied in order for the neuron to generate another action potential. The conclusion of the relative refractory period begins when stimulation quantity to generate an action potential returns to “baseline” levels.
Reference: Biopsychology, P J Pinel, sixth edition