Final answer:
The flexor reflex, which causes the automatic withdrawal of the threatened body part, is initiated by a painful stimulus. For example, stepping on a tack activates nociceptors that activate motor neurons responsible for the contraction of the tibialis anterior muscle, causing dorsiflexion of the foot.
Step-by-step explanation:
The flexor reflex, which causes the automatic withdrawal of the threatened body part, is initiated by a painful stimulus. When a painful stimulus is detected by sensory neurons called nociceptors, they activate motor neurons responsible for the contraction of specific muscles. For example, stepping on a tack activates nociceptors that activate motor neurons responsible for the contraction of the tibialis anterior muscle, causing dorsiflexion of the foot.