![]() ![]() So the threshold metaphor entails the all-or-none law of neural activity: there is a spike or there is no spike. Before threshold, you are outside after the threshold, you are inside. ![]() It conveys the notion of a qualitative change. A threshold is a spatial delimitation between two rooms, or between the outside and the inside of a house. When the membrane potential reaches a threshold, the neuron fires. So the discharge metaphor entails a particular model, but one that is now outmoded.įinally, related to the concept of firing is the notion of threshold. But in 1939, Hodkgin and Huxley made the first intracellar recording of an action potential in an animal and they found out that the membrane potential did not go to 0 mV but actually exceeded it quite substantially. This seems to correspond to Bernstein's theory (beginning of the twentieth century), according to which the negative resting potential is due to a gradient of potassium concentration across the membrane and the action potential corresponds to a non-selective increase in membrane permeability, resulting in a decrease of the membrane potential (in absolute value). The metaphor suggests that the membrane is an electrically charged capacitor, and it gets discharged during the action potential. “Discharge” is an interesting term because it relates to a former theory of action potential. So perhaps a mylienated axon fires (repeatedly), but an unmyelinated axon ignites! On the other hand, in myelinated axons, energy is released at discrete locations (Ranvier nodes), so the neuron could be seen as firing in sequence towards the next node: between two nodes, there is a movement that does not use additional energy, as in firing a bullet (dominoes could also be an adequate metaphor). Thus a better metaphor would be that the neuron ignites, where the axon progressively burns. But in neurons, propagation is active and energy is released all along the axon. The metaphor is only partially accurate, because when firing a gun, energy is only released at firing time and then the bullet moves to its target. Firing also conveys a notion of movement: the energy is targeted to some particular place, the axonal terminals. The metaphor is rather accurate as energy is stored in electrochemical gradients across the membrane and the opening of ionic channels releases some of that energy. This is a notion that is strikingly not representational.įiring, impulse and discharge add another aspect: an action potential releases energy. That is, the terms action potential convey the notion that those potentials, unlike other ones (subthreshold potentials), have an effect on other neurons. The action potential is quite a different notion: it is a particular form of potential that allows some form of action. “Spike” seems to refer to the shape of the action potential when looked at on a voltage trace, ie, there is an abrupt rise and fall of the potential. We speak of spikes, impulses, discharges or action potentials. We say that neurons fire, spike, or discharge. Neurons communicate by means of short electrical impulses.
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