The Expression "Knee-Jerk Reaction" Refers to Someone Doing Something Automatically

The expression "knee-jerk reaction" refers to someone doing something automatically without thinking. What does this expression mean in biological terms?

A) In the nervous system, this expression implies the involvement of higher brain regions before executing a spinal reflex.
B) In the nervous system, this expression implies the lack of brain input when executing a spinal reflex.
C) This expression implies that the knee-jerk reaction will be shortened in duration following input from higher brain regions.
D) This expression implies that only children exhibit this type of unthinking behavior.