Final answer:
The flexor reflex is initiated by a painful stimulus leading to an automatic withdrawal of the threatened body part, involving the contraction of muscles that facilitate this motion while inhibiting opposing muscles to prevent further injury.
Step-by-step explanation:
The flexor reflex is initiated by a painful stimulus, which triggers an automatic withdrawal of the threatened body part. This occurs when nociceptors are activated, leading to the contraction of muscles that move the body part away from the stimulus. Simultaneously, an inhibitory interneuron prevents the antagonist muscles from contracting, which would otherwise oppose this withdrawal motion. An example of this can be seen when you step on something sharp, like a tack, where the nociceptors activate dorsiflexion of the foot while inhibiting plantar flexion to prevent further injury.
Reflexes are the simplest circuits within the somatic nervous system and involve direct pathways between sensory inputs and motor outputs without conscious control.