During the reflex arc, sensory neurons carry signals from receptors to interneurons in the spinal cord, which send signals to activate muscles via motor neurons without first signalling the brain. This sequence provides much quicker responses to certain stimuli and is totally involuntary.
While certain responses, such as jerking a hand away from a hot stove, are relatively simple, jerking a foot away from a nail could result in a fall. The reflex arc is capable of causing more complex responses, such as flexing the muscles in the other leg, to mitigate the consequences of the main reflex action. The interneurons in the spinal cord also send the received information to the brain for voluntary responses afterward.