What is defined as an involuntary response to a stimulus?

A reflex is defined as an involuntary response to a stimulus, meaning it occurs automatically without conscious thought. Reflex actions are rapid, predictable, and typically serve protective functions. For example, when you touch something hot, the immediate withdrawal of your hand is a reflex reaction aimed at preventing injury.

Reflexes operate through a complex pathway known as the reflex arc, which involves sensory neurons, interneurons, and motor neurons, but the term "reflex" itself refers specifically to the involuntary action that results from this pathway. Other terms in the choices, like synapse and neurotransmitter, relate to the communication between neurons and the signaling process within the nervous system, but they do not specifically identify the involuntary nature of the response to a stimulus. Thus, defining a reflex purely focuses on the involuntary nature of the response, making it the most appropriate choice.