Reflex actions are quick, automatic responses to stimuli that help protect the body from harm. The role of the brain in reflex action is to modify and fine-tune reflex actions. Reflex arcs are neural pathways that control reflexes, and they consist of an afferent (sensory) nerve, usually one or more interneurons within the central nervous system, and an efferent (motor) nerve. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. During a reflex action, nerve signals travel along the following pathway:

• Somatic receptors in the skin, muscles, and tendons
• Afferent nerve fibers carry signals from the somatic receptors to the posterior horn of the spinal cord or to the brainstem
• An integrating center, the point at which the neurons that compose the gray matter of the spinal cord or brainstem synapse
• Efferent nerve fibers carry motor nerve signals from the anterior horn to the muscles
• Effector muscle innervated by the efferent nerve fiber carries out the response

The brain receives the input while the reflex is being carried out, and the analysis of the signal takes place after the reflex action. Therefore, the brain does not play a direct role in reflex actions, but it does receive information about the reflex and can modify the response if necessary.