Movements or involuntary reponses initiated by an external stimulus which do not require input from the brain.
In a simple reflex, a sensory receptor initiates a nerve impulse in an afferent sensory nerve fiber which conducts it to the spinal cord. In the gray matter of the spinal cord, the afferent nerve impulse is fired over the synaptic gap to an efferent motor fiber which passes along the impulse to the appropriate muscle, producing the reflex.
There are other reflexes which involve neural pathways connected to the brain. When an ice cube is touched, cold receptors in the skin are stimulated and that afferent information is transmitted to the gray matter of the spinal cord, where it then travels via axons in the white matter to the brain. There, the sensory information is analyzed and movement such as dropping the ice cube (or keeping hold of it) may be initiated. This message is sent down the axons of the white matter to the appropriate motor nerves in the gray matter. This efferent motor information travels to the muscles which initiate the reflex.