Reflex actions are involuntary and nearly instantaneous responses to a stimulus. They are found in organisms with a nervous system and can range in complexity from simple, short-latency reflexes to more complex reflexes that require multiple synapses in different nuclei in the central nervous system. Reflexes can be fine-tuned to increase organism survival and self-defense, such as the startle reflex or the feline righting reflex. Some processes such as breathing, digestion, and the maintenance of the heartbeat can also be regarded as reflex actions.

Reflexes are built naturally into the body and exist at birth, changing as we grow older. They are automatic and do not require the brain to create the action. Reflexes allow the body to react in ways that help us to be safe, stand upright, and be active. They can act to protect us in many ways, including removing our hand from a hot or sharp object, or ducking when a loud and sudden sound occurs.

The reflex that the doctor checks by tapping your knee is called the patellar, or knee-jerk, reflex. It is also known as a deep tendon reflex (DTR) because the doctor is actually tapping on a tendon called the patellar tendon. This tap stretches the tendon and the muscle in the thigh that connects to it. A message then gets sent to the spinal cord that the muscle has been stretched, and the spinal cord quickly sends a message back to the muscle telling it to contract.