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Since it is not so easy to imagine and visualize the behaviour of the muscles in the fingers, hand, and arm of a violinist performing a fast and accentuated trill vs. a slow and soft vibrato, I try to imagine and visualize how corresponding motor neurons do fire.

I assume there are some motor neurons that are immediately responsible for the speed and amplitude of the movement of a finger performing a fast trill, and — partly the same, partly others — that are immediately responsible for the speed and amplitude of the movement of the same finger performing a slow vibrato.

The behaviour of these motor neurons — during the trill or the vibrato — can be recorded as corresponding sequences/trains/bursts/chats of spikes.

I wonder:

How may these spiking and/or bursting patterns look like?


Maybe like this?

enter image description here fast and accentuated (= large amplitude) TRILL, each group corresponding to one movement (up-down) of the finger

enter image description here slow and soft (= small amplitude) VIBRATO, each group corresponding to one movement (to-fro) of the finger

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I can't answer the whole question, but what I do know: An actionpotential can't have a small amplitude, the amplitude is always the same. If a neuron gets excited enough there will be an action potential, otherwise there won't.

"When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold. If the neuron does not reach this critical threshold level, then no action potential will fire. Also, when the threshold level is reached, an action potential of a fixed sized will always fire...for any given neuron, the size of the action potential is always the same. " quoted from https://faculty.washington.edu/chudler/ap.html

Neurons have a maximum firing rate, a point where they cannot fire faster. (http://blog.eyewire.org/why-do-neurons-fire-at-a-maximum-of-about-200-hz/) So to move faster your body can't fire faster. But it can fire more: more neurons fire if you want to move faster, and less neurons fire if you want to move slower: https://blogs.scientificamerican.com/scicurious-brain/when-you-run-fast-your-brain-runs-faster/

I do think your first row of spikes is kinda correct. Neurons fire as long as you move your hand/finger(s) in a certain direction, and as soon as these neurons stop firing and their complementairy neurons start firing you move your hand to the opposite direction.

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  • $\begingroup$ "An actionpotential can't have a small amplitude, the amplitude is always the same" - this is not true in a series of action potentials. Many cells show amplitude reductions in their action potentials when firing several spikes in close proximity. Though I'm not sure why you brought this up since it has little to do with the question. $\endgroup$ – Bryan Krause Mar 30 '18 at 16:58

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