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In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action[1] and nearly instantaneous response to a stimulus.[2][3]

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 pathways in the nervous system called reflex arcs. A stimulus initiates a neural signal, which is carried to a synapse. The signal is then transferred across the synapse to a motor neuron, which evokes a target response. These neural signals do not always travel to the brain,[4] so many reflexes are an automatic response to a stimulus that does not receive or need conscious thought.[5]

Many reflexes are fine-tuned to increase organism survival and self-defense.[6] This is observed in reflexes such as the startle reflex, which provides an automatic response to an unexpected stimulus, 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.[7] Long-latency reflexes produce nerve signals that are transduced across multiple synapses before generating the reflex response.

Types of human reflexes

Myotatic reflexes

The myotatic or muscle stretch reflexes (sometimes known as deep tendon reflexes) provide information on the integrity of the central nervous system and peripheral nervous system. This information can be detected using electromyography (EMG).[8] Generally, decreased reflexes indicate a peripheral problem, and lively or exaggerated reflexes a central one.[8] A stretch reflex is the contraction of a muscle in response to its lengthwise stretch.

While the reflexes above are stimulated mechanically, the term H-reflex refers to the analogous reflex stimulated electrically, and tonic vibration reflex for those stimulated to vibration.

Tendon reflex

A tendon reflex is the contraction of a muscle in response to striking its tendon. The Golgi tendon reflex is the inverse of a stretch reflex.

Reflexes involving cranial nerves

Name Sensory Motor
Pupillary light reflex II III
Accommodation reflex II III
Jaw jerk reflex V V
Corneal reflex, also known as the blink reflex V VII
Glabellar reflex V VII
Vestibulo-ocular reflex VIII III, IV, VI +
Gag reflex IX X

Reflexes usually only observed in human infants

Main article: Primitive reflexes

Grasp reflex

Newborn babies have a number of other reflexes which are not seen in adults, referred to as primitive reflexes. These automatic reactions to stimuli enable infants to respond to the environment before any learning has taken place. They include:

Other kinds of reflexes

Other reflexes found in the central nervous system include:

Many of these reflexes are quite complex, requiring a number of synapses in a number of different nuclei in the central nervous system (e.g., the escape reflex). Others of these involve just a couple of synapses to function (e.g., the withdrawal reflex). Processes such as breathing, digestion, and the maintenance of the heartbeat can also be regarded as reflex actions, according to some definitions of the term.


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In medicine, reflexes are often used to assess the health of the nervous system. Doctors will typically grade the activity of a reflex on a scale from 0 to 4. While 2+ is considered normal, some healthy individuals are hypo-reflexive and register all reflexes at 1+, while others are hyper-reflexive and register all reflexes at 3+.

Grade Description
0 Absent ("mute")
1+ or + Hypoactive
2+ or ++ "Normal"
3+ or +++ Hyperactive without clonus, with spread to adjacent muscle groups
4+ or ++++ Hyperactive with clonus

Depending on where you are, another way of grading is from –4 (absent) to +4 (clonus), where 0 is "normal".

Reflex modulation

An example of reflex reversal is depicted. Activating the same spinal reflex pathway can cause limb flexion while standing, and extension while walking.

Some might imagine that reflexes are immutable. In reality, however, most reflexes are flexible and can be substantially modified to match the requirements of the behavior in both vertebrates and invertebrates.[9][10][11]

A good example of reflex modulation is the stretch reflex.[12][13][14][15] When a muscle is stretched at rest, the stretch reflex leads to contraction of the muscle, thereby opposing stretch (resistance reflex). This helps to stabilize posture. During voluntary movements, however, the intensity (gain) of the reflex is reduced or its sign is even reversed. This prevents resistance reflexes from impeding movements.

The underlying sites and mechanisms of reflex modulation are not fully understood. There is evidence that the output of sensory neurons is directly modulated during behavior—for example, through presynaptic inhibition.[16][17] The effect of sensory input upon motor neurons is also influenced by interneurons in the spinal cord or ventral nerve cord[15] and by descending signals from the brain.[18][19][20]

Other reflexes

Breathing can also be considered both involuntary and voluntary, since breath can be held through internal intercostal muscles.[21][22][23]

See also


  1. ^ parveen (November 11, 2020). "Reflex action | Definition, Types and Mechanism and Important solved questions". Crack Your Target. Retrieved 3 April 2021.
  2. ^ Purves (2004). Neuroscience: Third Edition. Massachusetts, Sinauer Associates, Inc. ISBN 0-87893-725-0
  3. ^ "Definition of reflex". Dictionary by Merriam-Webster. 25 December 2023.
  4. ^ Hultborn H (2006-02-01). "Spinal reflexes, mechanisms and concepts: From Eccles to Lundberg and beyond". Progress in Neurobiology. 78 (3–5): 215–232. doi:10.1016/j.pneurobio.2006.04.001. ISSN 0301-0082. PMID 16716488. S2CID 25904937.
  5. ^ "tendon reflex". The Free Dictionary.
  6. ^ Price JL (2005-12-05). "Free will versus survival: Brain systems that underlie intrinsic constraints on behavior". The Journal of Comparative Neurology. 493 (1): 132–139. doi:10.1002/cne.20750. ISSN 0021-9967. PMID 16255003. S2CID 18455906.
  7. ^ Pierrot-Deseilligny E (2005). The Circuitry of the Human Spinal Cord: Its Role in Motor Control and Movement Disorders. Cambridge University Press. ISBN 9780511545047.
  8. ^ a b Tsuji H, Misawa H, Takigawa T, Tetsunaga T, Yamane K, Oda Y, Ozaki T (2021-01-27). "Quantification of patellar tendon reflex using portable mechanomyography and electromyography devices". Scientific Reports. 11 (1): 2284. Bibcode:2021NatSR..11.2284T. doi:10.1038/s41598-021-81874-5. ISSN 2045-2322. PMC 7840930. PMID 33504836.
  9. ^ Pearson KG (1993). "Common principles of motor control in vertebrates and invertebrates". Annual Review of Neuroscience. 16: 265–97. doi:10.1146/ PMID 8460894.
  10. ^ Büschges A, Manira AE (December 1998). "Sensory pathways and their modulation in the control of locomotion". Current Opinion in Neurobiology. 8 (6): 733–9. doi:10.1016/S0959-4388(98)80115-3. PMID 9914236. S2CID 18521928.
  11. ^ Tuthill JC, Azim E (March 2018). "Proprioception". Current Biology. 28 (5): R194–R203. doi:10.1016/j.cub.2018.01.064. PMID 29510103. S2CID 235330764.
  12. ^ Bässler U (March 1976). "Reversal of a reflex to a single motoneuron in the stick insect Çarausius morosus". Biological Cybernetics. 24 (1): 47–49. doi:10.1007/BF00365594. ISSN 1432-0770. S2CID 12007820.
  13. ^ Forssberg H, Grillner S, Rossignol S (August 1977). "Phasic gain control of reflexes from the dorsum of the paw during spinal locomotion". Brain Research. 132 (1): 121–39. doi:10.1016/0006-8993(77)90710-7. PMID 890471. S2CID 32578292.
  14. ^ Capaday C, Stein RB (May 1986). "Amplitude modulation of the soleus H-reflex in the human during walking and standing". The Journal of Neuroscience. 6 (5): 1308–13. doi:10.1523/JNEUROSCI.06-05-01308.1986. PMC 6568550. PMID 3711981.
  15. ^ a b Clarac F, Cattaert D, Le Ray D (May 2000). "Central control components of a 'simple' stretch reflex" (PDF). Trends in Neurosciences. 23 (5): 199–208. doi:10.1016/s0166-2236(99)01535-0. PMID 10782125. S2CID 10113723.
  16. ^ Wolf H, Burrows M (August 1995). "Proprioceptive sensory neurons of a locust leg receive rhythmic presynpatic inhibition during walking". The Journal of Neuroscience. 15 (8): 5623–36. doi:10.1523/JNEUROSCI.15-08-05623.1995. PMC 6577635. PMID 7643206.
  17. ^ Sauer AE, Büschges A, Stein W (April 1997). "Role of presynaptic inputs to proprioceptive afferents in tuning sensorimotor pathways of an insect joint control network". Journal of Neurobiology. 32 (4): 359–76. doi:10.1002/(SICI)1097-4695(199704)32:4<359::AID-NEU1>3.0.CO;2-5. PMID 9087889.
  18. ^ Mu L, Ritzmann RE (December 20, 2007). "Interaction between descending input and thoracic reflexes for joint coordination in cockroach: I. descending influence on thoracic sensory reflexes". Journal of Comparative Physiology A. 194 (3): 283–98. doi:10.1007/s00359-007-0307-x. PMID 18094976. S2CID 25167774.
  19. ^ Martin JP, Guo P, Mu L, Harley CM, Ritzmann RE (November 2015). "Central-complex control of movement in the freely walking cockroach". Current Biology. 25 (21): 2795–2803. doi:10.1016/j.cub.2015.09.044. PMID 26592340.
  20. ^ Hsu LJ, Zelenin PV, Orlovsky GN, Deliagina TG (February 2017). "Supraspinal control of spinal reflex responses to body bending during different behaviours in lampreys". The Journal of Physiology. 595 (3): 883–900. doi:10.1113/JP272714. PMC 5285725. PMID 27589479.
  21. ^ Mitchell RA, Berger AJ (February 1975). "Neural regulation of respiration". The American Review of Respiratory Disease. American Thoracic Society. 111 (2): 206–224. doi:10.1164/arrd.1975.111.2.206 (inactive 2024-01-19). ISSN 0003-0805. PMID 1089375.((cite journal)): CS1 maint: DOI inactive as of January 2024 (link)
  22. ^ Park H, Barnoud C, Trang H, Kannape OA, Schaller K, Blanke O (February 6, 2020). "Breathing is coupled with voluntary action and the cortical readiness potential". Nature Communications. Nature Portfolio. 11 (1): 289. Bibcode:2020NatCo..11..289P. doi:10.1038/s41467-019-13967-9. ISSN 2041-1723. PMC 7005287. PMID 32029711.
  23. ^ "21.10B: Neural Mechanisms (Cortex)". Medicine LibreTexts. 2018-07-22. Retrieved 2022-09-10.