What is "instantly"?
Human reaction time is about a quarter of a second, so everything that happens faster than that is perceived as instantaneous.

Why the drift velocity is not the limit Electrons do not have to flow from one end of the body to the other for electric current to occur. In fact, even in an electric circuit they rarely travel very far. The force exerted by electrons on each other however propagates much faster.

Yet, it is not anywhere close to the speed of light
It would be incorrect to say that it propagates as fast as in a conductor, which is a crystalline solid: human body is largely an amorphous material and most of the conductance occurs via electrolytic reactions, i.e., movement of ions in the body cells: thus, the worst case scenario is that the velocity of ions and electrons need be compared with the cell size.

Furthermore, the effect of the electric field/current needs to be translated into a physiological response - mostly in muscle contraction in case of an electric shock (see here for basic background on the effects of electricity on human body, and here for functioning of muscles).

What is "instantly"?
Human reaction time is about a quarter of a second, so everything that happens faster than that is perceived as instantaneous.

Why the drift velocity is not the limit Electrons do not have to flow from one end of the body to the other for electric current to occur. In fact, even in an electric circuit they rarely travel very far. The force exerted by electrons on each other however propagates much faster.

Yet, it is not anywhere close to the speed of light
It would be incorrect to say that it propagates as fast as in a conductor, which is a crystalline solid: human body is largely an amorphous material and most of the conductance occurs via electrolytic reactions, i.e., movement of ions in the body cells: thus, the worst case scenario is that the velocity of ions and electrons need be compared with the cell size.

Furthermore, the effect of the electric field/current needs to be translated into a physiological response - mostly in muscle contraction in case of an electric shock.

Additional reading:

• see here for basic background on the effects of electricity on human body
• here for the basics about functioning of muscles
• here and here for the effect of electricity on a muscle

What is "instantly"?
Human reaction time is about a quarter of a second, so everything that happens faster than that is perceived as instantaneous.

Why the drift velocity is not the limit Electrons do not have to flow from one end of the body to the other for electric current to occur. In fact, even in an electric circuit they rarely travel very far. The force exerted by electrons on each other however propagates much faster.

Yet, it is not anywhere close to the speed of light
It would be incorrect to say that it propagates as fast as in a conductor, which is a crystalline solid: human body is largely an amorphous material and most of the conductance occurs via electrolytic reactions, i.e., movement of ions in the body cells: thus, the worst case scenario is that the velocity of ions and electrons need be compared with the cell size.

What is "instantly"?
Human reaction time is about a quarter of a second, so everything that happens faster than that is perceived as instantaneous.

Why the drift velocity is not the limit Electrons do not have to flow from one end of the body to the other for electric current to occur. In fact, even in an electric circuit they rarely travel very far. The force exerted by electrons on each other however propagates much faster.

Yet, it is not anywhere close to the speed of light
It would be incorrect to say that it propagates as fast as in a conductor, which is a crystalline solid: human body is largely an amorphous material and most of the conductance occurs via electrolytic reactions, i.e., movement of ions in the body cells: thus, the worst case scenario is that the velocity of ions and electrons need be compared with the cell size.

Furthermore, the effect of the electric field/current needs to be translated into a physiological response - mostly in muscle contraction in case of an electric shock (see here for basic background on the effects of electricity on human body, and here for functioning of muscles).