How is reaction time/reflexes affected by inertia?
That would probably depend upon how you define 'reaction time'. It has no effect upon the neural impulses necessary to initiate a reaction. There will, however, be an increased delay in the physical action resultant of that as the mass/inertia exceeds the speed/power of the muscular response. (ie: if you're arm weighs a lot, or you're holding something like a baseball glove, the reaction would be slower than if you exerted the same muscle effort with less mass.)
On a molecular level, the inertia of the ions that travel across the synapses, and the rotational inertia on the membrane macromolecule radicals which propagate the "action potential",
(along with the resistive force of the ions drifting, essentially Pressure x Area)
determine the speed of the signal along the chain of neurons.
But those are constant, aren't they? I thought that the question related to changing the mass/inertia and noting the effect.
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