A reflex is an automatic, involuntary response to a specific stimulus. The knee-jerk that follows a tap on the patellar tendon is a classic example. So is the rapid withdrawal of a hand from a hot surface. These responses are mediated by relatively simple neural circuits that do not require conscious decision.

Many reflexes are present at birth and serve protective or regulatory functions. The pupillary light reflex adjusts pupil size. The sucking reflex in newborns supports feeding. Spinal reflexes can operate even when the brain is disconnected, demonstrating that the basic circuitry resides in the spinal cord or brainstem.

Although reflexes are stereotyped, they can be modulated by higher centers and by learning. Habituation reduces the response to repeated harmless stimuli. Sensitization increases it after strong stimulation. Classical conditioning can attach new stimuli to existing reflex pathways.

• Automatic and involuntary
• Specific stimulus-response pairing
• Often mediated by spinal or brainstem circuits
• Modifiable by experience

The simplicity of reflex arcs made them an early model for understanding more complex neural control of behavior.