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In [[biology]], a '''reflex''', or '''reflex action''', is an involuntary, unplanned sequence or action<ref>{{cite web |url=https://www.crackyourtarget.com/reflex-action.html |title=Reflex action |work=Definition, Types and Mechanism and Important solved questions |publisher=Crack Your Target |date=November 11, 2020 |author=parveen |access-date=3 April 2021}}</ref> and nearly instantaneous response to a [[Stimulus (physiology)|stimulus]].<ref>Purves (2004). ''Neuroscience: Third Edition''. Massachusetts, Sinauer Associates, Inc.</ref><ref name="M-W">{{cite web|title=Definition of REFLEX|url=https://www.merriam-webster.com/dictionary/reflex|website=www.merriam-webster.com|language=en}}</ref>
[[File:Improved General Reflex Diagram.png|thumb|429x429px|The simplest reflex is initiated by a stimulus, which activates an afferent nerve. The signal is then passed to a response neuron which generates a response.]]
Reflexes are found with varying levels of complexity in organisms with a [[nervous system]]. A reflex occurs via [[Neural pathway|neural pathways]] in the nervous system called [[Reflex arc|reflex arcs]]. A stimulus initiates a neural signal, which is carried to a synapse. The signal is then transferred across the synapse to a separate neuron which evokes a target response.{{cn|date=November 2022}} These neural signals do not always travel to the brain,<ref name=":02">{{Cite journal |date=2006-02-01 |title=Spinal reflexes, mechanisms and concepts: From Eccles to Lundberg and beyond |url=https://www.sciencedirect.com/science/article/pii/S0301008206000335 |journal=Progress in Neurobiology |language=en |volume=78 |issue=3–5 |pages=215–232 |doi=10.1016/j.pneurobio.2006.04.001 |issn=0301-0082|last1=Hultborn |first1=Hans |pmid=16716488 |s2cid=25904937 }}</ref> so many reflexes are an automatic response to a stimulus that does not receive or need conscious thought.<ref name="FD">{{cite web|title=tendon reflex|url=http://medical-dictionary.thefreedictionary.com/tendon+reflex|website=TheFreeDictionary.com}}</ref>
Many reflexes are fine-tuned to increase organism survival and self-defense.<ref>{{Cite journal |last=Price |first=Joseph L. |date=2005-12-05 |title=Free will versus survival: Brain systems that underlie intrinsic constraints on behavior |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.20750 |journal=The Journal of Comparative Neurology |language=en |volume=493 |issue=1 |pages=132–139 |doi=10.1002/cne.20750 |pmid=16255003 |s2cid=18455906 |issn=0021-9967}}</ref> This is observed in reflexes such as the [[Startle response|startle reflex]], which provides an automatic response to an unexpected stimuli, and the feline [[righting reflex]], which reorients a cat's body when falling to ensure safe landing. The simplest type of reflex, a short-latency reflex, has a single synapse, or junction, in the signaling pathway.<ref>{{Cite book |last=Pierrot-Deseilligny |first=Emmanuel |title=The Circuitry of the Human Spinal Cord: Its Role in Motor Control and Movement Disorders |publisher=Cambridge University Press |year=2005 |isbn=9780511545047}}</ref> Long-latency reflexes produce nerve signals that are transduced across multiple synapses before generating the reflex response.
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