Final answer:
In response to stepping on a nail, the crossed-extensor reflex causes flexion of the affected limb and simultaneously, contraction of the opposing limb to maintain balance and posture.
Step-by-step explanation:
When stepping on a nail, the body's reaction is a complex reflex action that involves both an immediate withdrawal of the affected limb and a balancing adjustment from the contralateral (opposite side) limb. This coordinated response is known as the crossed-extensor reflex. The nociceptors, which are pain receptors, get activated due to the painful stimulus and send signals to the spinal cord, prompting a withdrawal reflex. This involves the flexion of the affected limb to prevent further damage.
In addition to this withdrawal reflex, there is a simultaneous action in the opposing limb. The collateral branches of the activated sensory axons also facilitate an activation of the extensor muscles in the contralateral limb, helping with balance. Hence, the crossed-extensor reflex produces contraction of the opposing limb, helping maintain stability as the affected foot is withdrawn. This is important to prevent a fall, securing the body's posture during the sudden withdrawal of the injured limb.