Ken Tough writes:

I'm sure the massive shock straight to your central nervous system
(through the spinal cord) would zap you unconscious straight away.
That's not to say dead, just unconscious, like the karate chop to
the neck thing.

People executed by the guillotine remain conscious for a few seconds. A
French doctor once did some observations of executions to determine
this. But a constant and generous blood supply to the brain is
necessary to allow this, and so the period of remaining consciousness
(if any) is extremely brief.

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