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Most accounts I read involving action potentials and synapses and the like tend to focus mostly on the action potential as a mere automatic reaction to another similar event happening upstream.

From a more theoretical point of view, it would be nice to know if (and under what circumstances) a neuron would be able to generate an action potential, in the absence of an upstream neuron firing first.

I suspect that the nervous system, at least in higher animals, is not simply a relay system that processes and transmits information from senses to actuators like the limbs. The relay system picture is fine for short-timescale, reactionary behavior but it is hard to imagine any longer-timescale learning processes happening this way. This is what motivates my question.

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Pacemaker cells in the heart might be an example for you –  Rory M Feb 22 at 11:27
    
You might be interested in this quetions on neurons firing 'randomly'. –  Artem Kaznatcheev Feb 23 at 3:03

5 Answers 5

Neurones fire when they're sufficiently depolarised past their threshold. In the sensory system this is in response to an external stimuli which sets of a chain reaction. Then motor neurones may relay signals back and in this case the signal or initial depolarisation is occurring in the brain. However this is still caused by a stimuli even if indirectly. If we take an action it is generally because we thought which action we should take because of some sort of stimuli e.g. we are cold so we put a coat on.

But anything which depolarises neurones can set of an action potential. However there's always something which causes it to depolarise, perhaps a drug or a stimulus or another neurone. If any of these don't count as a stimuli depending on your definition, there's your answer.

Or perhaps if you consider cardiac muscle which can intrinsically depolarise. Not a neurone but there's an action potential. They just naturally depolarise over time and action potentials are set of regularly with no stimulus required.

As for learning, where the structure of neurones can change so they connect to new neurones or not to old neurones or vary in the strength of their connection.

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There are many spontaneously active neurons. This has been known for many years [1]. There are multiple sources that could make a neuron spontaneously fire. In cortical culture of neurons where there is no external input to the system, neurons are spontaneously active, for example [2]. Of course, there are spontaneous vesicle releases from synapses (miniature EPSC) in this case, so you might say there are still some "input" to each neuron. However, even if you block synaptic transmissions, and some neurons do fire spontaneously [3]. It could be ion channels, or some internal chemical process. If you include the network level spontaneous dynamics, there are even more possible ways to have active neural system without any stimulus.

  1. Alving BO (1968) Spontaneous activity in isolated somata of Aplysia pacemaker neurons. J Gen Physiol 51: 29-45.

  2. Potter, S. M. and Demarse, T. B. (2001). A new approach to neural cell culture for long-term studies. Journal of Neuroscience Methods, 110:17-24.

  3. Häusser, M., Raman, I. M., Otis, T., Smith, S. L., Nelson, A., du Lac, S., Loewenstein, Y., Mahon, S., Pennartz, C., Cohen, I., and Yarom, Y. (2004). The beat goes on: Spontaneous firing in mammalian neuronal microcircuits. The Journal of Neuroscience, 24(42):9215-9219.

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You can read why spontaneous neuronal firing, called retinal waves, are important to structuring the wiring of mammalian retinal representation in dominance columns.

Galli L., Maffei L. (1988). "Spontaneous Impulse Activity of Rat Retinal Ganglion Cells in Prenatal Life." Science, 242

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Memming is right. There are neurons which spontaneously depolarize and are tonically active like Purkinje cells in the Cerebellum

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The well known phenomenon of 'determination', ie. mind's ability to wire-itself without any direct external stimuli,offers adequate proof of neuron's ability to generate action potential. Such pure thought acts has more to do with man's inherent 'drives' than any external stimuli. See details of internal drives from this link.

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Can you add citations to your answer other than just an apparently non-peer reviewed blog? –  kmm Feb 28 at 18:31
    
This answer relates low-level electrophysiological behavior of single neurons by explaining it through cognitive processes. Not the most parsimonious approach. –  Chris Stronks Oct 30 at 22:53

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