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.
What are Quantum Computers?
Quantum computers are not that different from normal computers outwardly, but they are in the sense that quantum theory is the basis on which these computers operate. The end result is that they are put together in a completely different way.
A normal computer operates on the basis of units known as bits. Each byte in a normal computer can only be one of 0 or 1 and nothing else. No matter how many bytes you have, each computer at a single point in time can only occupy one combination of these bytes in order for the programming to actually work.
A quantum computer is different from this because of a principle in quantum mechanics known as superposition. If you think back to your high school science courses, you may have learned about superposition when looking at how waves like light and sound waves move from one point to another. Quanta can also be in superposition with respect to each other and the end result is that the quantum bits that make up the computer can actually be 0, 1 and any superposition of the two.
The more quantum bits (also known as qubits) that you have, the more possibilities they are. Because you are dealing with superposition, it also means that the different positions can be occupied simultaneously. Whereas a simple 8-bit computer can only occupy one of the 256 positions generated by those 8 bits at once, the same 8-bit quantum computer could occupy all 256 qubit positions at once.
The end result is that quantum computers can be much more efficient than their conventional computer counterparts. Although quantum computers are still in their infancy, as the technology improves eventually it will become true that these computers will be able to calculate faster than the computers we have today. When that happens, the 3.0 GHz speed of a personal computer that we brag about now will be nothing in comparison to the new quantum computer models that become available on the market.