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Sometimes a small region of my body starts shaking, without any reason and then stops abruptly. Apparently this is a common phenomenon (source: NIH). I researched its cause for a bit and came to know that it is because of nerve excitation.

Why do those nerves get spontaneously excited in the first place, without any conscious, voluntary, internal stimulus being presented?

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Short answer
Spontaneous muscle twitches are thought to be caused by spontaneous activation of motoneurons controlling the muscles.

Background
Spontaneous muscle activity (twitching) is referred to as muscle fasciculations. Most commonly, they are benign, but they can be associated with certain pathologies too, such as ALS.

Fasciculations are thought to originate due to spontaneous activation of muscle fibers due to processes in the motoneurons. At the axonal endings there are synapses that junction onto the muscle. The axonal tips of motoneurons are thought to be highly sensitive to depolarization (electrical firing), which is what triggers acetylcholine release and muscle contraction. The complicated process itself takes a small fraction of a second. If any of this happens involuntarily, then the muscle fiber contracts spontaneously (source: ALS Association).

Specifically, it is thought that spontaneous depolarization of motoneurons are the cause of fasciculations. This decreased firing threshold can be the cause of disease, disrupted ion homeostasis in the body, fatigue, drugs and other causes (Blum & Rutkove, 2007).

Note that spontaneous / random activity in neurons is not only possible, it is quite common. Especially in neurons close to firing threshold, such as the motoneurons responsible for muscle activity, slight perturbations in ion concentrations inside or outside the cell can trigger an action potential. Further, channel opening is a stochastic process, meaning that even voltage-gated Na+ channels can open without a depolarization being present; although the chance is slim that will happen, it can statistically still happen. Stochastic processes like this can cause neurons to fire spontaneously, without a stimulus being present.

Reference
- Blum & Rutkove, The Clinical Neurophysiology Primer, Springer (2007)

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  • $\begingroup$ So how does it happen involuntarily? Where do they get that stimulus from? $\endgroup$ – Polisetty Mar 11 '16 at 14:33
  • $\begingroup$ @Polisetty - There is no verifiable stimulus, inherent to spontaneous phenomena in general $\endgroup$ – AliceD Mar 11 '16 at 15:04
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    $\begingroup$ @Polisetty - Some neurons happen to cross threshold for non-apparent reasons (local irritation causing surrounding tissue fluid electrolyte imbalance, for example.) Neuronal membranes do not always work predictably (unlike, say, gravity.) It happens all the time, all over the mody, and mostly it goes unnoticed because we become habituated to these minor glitches. $\endgroup$ – anongoodnurse Mar 11 '16 at 16:11
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    $\begingroup$ @Polisetty - I totally agree with anongoodnurse and I have added a paragraph to my answer. I hope this helps. $\endgroup$ – AliceD Mar 11 '16 at 20:56

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