The reflex arc, the neural substrate of a reflex, is fundamental to the physiology of posture and locomotion, as well as to the clinical examination of the nervous system.

A reflex can be defined as an involuntary, qualitatively unvarying response of the nervous system to a stimulus. Reflexes are the simplest behavioral example of the general function of the nervous system: collection of sensory input, integration, and motor output. Reflexes are often both critical for survival and components of more complex behaviors. The anatomy and function of a reflex arc are often fully developed at birth.