The human physiological and psychological response to mortal dangeis well documented.
When fear explodes inside of you, your sympathetic nervous system instantly dumps a variety of natural drugs and hormones into your body to cause a high arousal state known as fear. You are literally under the influence of these natural chemicals, so your body operates differently, just as it would under the influence of a chemical you deliberately ingested.
These chemically induced changes take effect immediately and last for a "significant" period of time. They have specific implications for one's ability to effectively use a handgun for self-defense without needlessly endangering the lives of innocent persons.
One common effect is distortion of perceived time, called tachypsychia. "An event that takes milliseconds may seem like minutes as everyone and everything appears to move in slow motion." Other physical changes typically include pounding heart, muscle tension, trembling, dizziness, nausea, dry mouth, tingling sensations, the urge to urinate and defecate, and hyperventilation and fainting in some cases. Several of these effects specifically, directly, and dramatically degrade the handgun owner's ability to use his weapon. For example, temporary paralysis—"momentarily freezing as your body is desperately trying to catch up to the sudden awareness that your life is in danger"—is an obvious inconvenience.
Loss of Fine Motor Control. Among the temporary consequences of the adrenaline dump are sudden surge in gross muscle strength, increase in speed associated with increased muscle strength, and insensitivity to pain.
These changes enhance basic animal fighting skills, so they may be useful in a hand-to-hand brawl. "The fight or flight response has not changed since caveman days, when people fought with their bare hands or with clubs and rocks," writes Chris Bird. But, expert Ayoob advises, "there is a downside to this....you will experience gross, severe, dramatic, cataclysmic loss of fine motor coordination. Dexterity falls through your ass....The hands will begin to tremble."
This is a serious problem because "the firing of the gun is dexterity intensive. You can't change that." In short, the use of fine motor skills for tasks like firing handguns are not part of the body's survival design: "Our bodies have not yet adapted to the possibility that fighting may involve a delicate trigger squeeze." Loss of fine motor control also means that reloading, also a high-dexterity skill, especially in revolvers, becomes much more difficult.