Final answer:
When Santi steps on a sharp object, sensory receptors in his foot detect pain and send the message through sensory neurons to the spinal cord, which uses an interneuron to relay a signal to motor neurons that cause the foot to reflexively pull away.
Step-by-step explanation:
When Santi steps on a sharp rock, the sensory receptors known as nociceptors in the skin of his foot detect the painful stimulus. This information is then sent through a sensory neuron to his spinal cord. An interneuron within the spinal cord processes this information and sends a signal via a motor neuron to the muscles in his foot, causing a reflexive withdrawal action. Specifically, the motor neurons trigger contraction of the tibialis anterior muscle, leading to dorsiflexion of the foot away from the rock, and an inhibitory interneuron suppresses the muscles responsible for plantar flexion, namely the gastrocnemius and soleus muscles, to prevent further injury. This all constitutes a reflex arc, which is an automatic and rapid response to a stimulus that does not require conscious thought and helps prevent further damage to the body.