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I'm interested in how fast the human body can respond to a stimulus. I know the fastest reflex, the blink reflex, operates around 100ms from stimulus to reaction. I also know that the blink reflex is known as the fastest reflex in the human body. My interest is in the fastest responses to stimuli I can find in the body.

Are there any faster responses to stimuli within the human body which use neurons but are not categorized as a reflex (due to some technicality), meaning they could be faster than the fastest reflex? To the best of my understanding a reflex is defined by the use of neurons to convey the information, I'm just wondering if there are any grey areas which don't qualify as a reflex but may be faster. I don't want to potentially write off an entire class of neurological behavior in my research simply because I stopped at the blink reflex.

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  • $\begingroup$ I think the vestibulo-ocular reflex is the fastest. It typically operates in 7-15 milliseconds. $\endgroup$ – Brad Walters Apr 6 '18 at 16:04
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A reflex as fast as the blink in a neural circuit:

I would consider suppression of outer hair cells in the cochlea to be a reflex; the faster component of this reflex is about the same as the blink reflex, around 100 ms. The hair cells themselves aren't considered neurons, but the pathway that suppresses their motility certainly is.

A much much faster non-neuronal "reflex":

That said, the outer hair cells themselves also dance along quite fast in response to sensory input, even faster than the typical hearing range for humans, faster than 20kHz! In some ways, this is a reflex because you are taking sensory (specifically, auditory) information and turning it into a motor response, but all the "action" is taking place within one cell, and it isn't a neuron.

A more classical reflex that is substantially faster than 100 ms

Reflexes in the periphery can be much faster than 100 ms. The myotatic reflex, or stretch reflex, can be as fast as 30 ms in the knee - this is the reflex that is tested when a physician smacks you on the knee with a hammer (used as a test of spinal and peripheral nerve function, not as a punishment). It's likely there are other stretch reflexes that are faster just because distances to the spinal cord are shorter, but these might be more difficult to test (in this paper they report latencies as fast as 20 ms).

Eyeblink reflexes can also be faster than 100 ms!

For example, see this paper: strong auditory stimuli evoked blinks within about 20ms!

tl;dr: I'd check your source that 100 ms is the fastest reflex! I stuck with mostly human examples here (some of the hair cell work is in other mammals), but you will find even faster reflexes if you look at smaller organisms, such as insects.

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  • $\begingroup$ I was thinking of human reflexes when I wrote the question, so your examples are spot on! I've edited the question to add "human" to the wording, but that shouldn't invalidate anything in your answer. Thanks for the answer! $\endgroup$ – Cort Ammon Jan 11 '17 at 21:36
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    $\begingroup$ @CortAmmon No worries, you did indeed have "human" elsewhere in the body of your question, but I certainly prefer it from biological, evolutionary, and philosophical perspectives when people refer to "the human body" rather than "the body" - as if all bodies are human or that the human is the archetype or exemplar animal, so thank you for the edit! $\endgroup$ – Bryan Krause♦ Jan 11 '17 at 21:59
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In a paper studying Rhesus macaques, Huterer and Cullen showed that latency of the vestibulo-ocular reflex (measured from head movement to eye muscle contraction) can be as little as 5 milliseconds1.

Reference:

1: Huterer, M., & Cullen, K. E. (2002). Vestibuloocular reflex dynamics during high-frequency and high-acceleration rotations of the head on body in rhesus monkey. Journal of neurophysiology, 88(1), 13-28.

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