In a normal functioning human, motor response to pain is natural reflex.

对于正常人而言，对痛觉产生运动反射是一种自然反应。

Lifting your foot when you step on a tack is an innate, or intrinsic, reflex action — a super fast motor response to a startling stimulus.

当你踩在图钉上时抬起脚是一种先天的或内在的反射动作，是对吓人刺激的超快运动反应。