Synapses, Impulses, & Reflexes.

[Woolygimp]

I remember reading that the impulse sent between nerves in the human body has a maximum speed of something like 160m/hr.

Am I correct in assuming that this is why small creatures such as mosquitoes, flies, squirrels, mice, and such have much faster reflexes than larger creatures such as humans, elephants, and whales?

Think about it. If you feel something hit your arm, the neurons fire a pain impulse over your synapses which take time to hit your brain and then rebound on the return trip. This results in A pause in reflexes, since a reflex is never instantaneous.

Your brain communicating within itself, your eyes, mouth, are almost always the first things to react to a certain stimulus. So assuming this, giants (if they did exist) would be a much slower creature than us due to the larger nervous system and this can explain why spiders have a 'spider-sense' or incredibly fast reaction to stimuli.

I do know there are probably small differences in the nervous systems of different creatures allowing for faster or slower travel over the synapses, but I'm generalizing here.

Can someone well-versed in biology please explain whether this is a correct assumption?

[vhawk01]

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I remember reading that the impulse sent between nerves in the human body has a maximum speed of something like 160m/hr.

Am I correct in assuming that this is why small creatures such as mosquitoes, flies, squirrels, mice, and such have much faster reflexes than larger creatures such as humans, elephants, and whales?

Think about it. If you feel something hit your arm, the neurons fire a pain impulse over your synapses which take time to hit your brain and then rebound on the return trip. This results in A pause in reflexes, since a reflex is never instantaneous.

Your brain communicating within itself, your eyes, mouth, are almost always the first things to react to a certain stimulus. So assuming this, giants (if they did exist) would be a much slower creature than us due to the larger nervous system and this can explain why spiders have a 'spider-sense' or incredibly fast reaction to stimuli.

I do know there are probably small differences in the nervous systems of different creatures allowing for faster or slower travel over the synapses, but I'm generalizing here.

Can someone well-versed in biology please explain whether this is a correct assumption?

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It sounds about right, I guess, but bear in mind, the type of reflexes you are talking about do not travel to the brain. Lets take the patellar reflex, for example, the one we're all familiar with. When you hit the tendon with a reflex hammer, a cluster of neurons called the Golgi Tendon Organ contract, sending afferent fibers to the spine, localized around the level of the leg extensor/flexor muscles. These synapse on interneurons IN THE SPINAL CORD which then synapse on efferent motor fibers to the leg, exciting the extensors and inhibiting the flexors. This all happens pretty fast because the distance traveled isn't that great, plus these are myelinated fibers for the most part, which conduct impulses much faster than unmyelinated fibers.

I think your general point is valid. There may be other modalities of nerve fibers that conduct faster than human nerves (since myelinization is a big factor in conduction velocity) and I'm really most familiar with human physiology.

Some other reflexes do go to the brain (usually the brainstem) and so do take longer, although these are usually upper-body reflexes (such as the vestibulo-occular reflex, which keeps your eyes steady when you tilt your head).

[m_the0ry]

Your reflex to a baseball pitch when standing over the plate will always go through some part of your brain and undergo some processing before you react. Touching a hot stove results in a reflex - exactly like vhawk said - that goes to the spinal cord which reports back a hardwired response to increase the speed of the reflex.

Since the fly is not a complex animal, most of its stimulus/response actions are reflexive and involuntary, meaning they do not undergo processing. For example both a swing from a flyswatter and a figure 8 with a laser pointer will make a fly leap off the wall almost instantaneously. One is a real threat to the fly, the other is harmless. It can't process the difference because its response is reflexive. It's 'hard wired'.

This is purely speculative but I think if you compared the response time of purely reflexive stimulus response in a human compared to a fly, there wouldn't be a significant difference.

[Phil153]

I imagine the reflex time would be mostly based on muscle contraction and not nerve impulses. Nerves are pretty fast, at 100m/s for the fastest nerves you're looking at a reflex in ~ 2/100th of a second from the nerve signals. Take a look at this rabbit vs snake. Pretty damn fast reaction time even though brain processing is probably going on as well.

http://www.youtube.com/watch?v=_Ez5QPW-ku4

[Rduke55]

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I imagine the reflex time would be mostly based on muscle contraction and not nerve impulses.

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This and the processing point m_theory said were the first things I was going to say when I saw the OP.

Also, consider the reactions you are comparing. Escape reactions are often highly specialized, dedicated circuits that have MASSIVE selection pressures on them. Because of this, these circuits have evolved as the fastest ones known - look at the giant axons in invertebrates. The most famous is the giant squid axon (which is more the length of our axons than, say, a fly's) that was used in Hodgkin and Huxely's work that they received the Nobel prize for. This axon can reach a diameter of 1mm - which is ridiculously huge when you think about it. The story goes that people thought it was a vessel of some kind before J.Z. Young identified it as an axon.
Anyway, the reason it is so big is to make it fast for escape. Many invertebrates have some giant(although usually not this giant) axons in their nervous systems. These axons are invariably dedicated to escape circuitry. Even in smaller invertebrates where the distances - and therefore the speed increases they confer - are much, much smaller you still see these, indicating the importance of speed in these circuits. Add the lack of processing here to make it even faster and you really cannot compare those circuits to our reflexes that have evolved for things like gait correction, etc. which have a much different selection pressure in addition to different speed requirements.
Same thing goes for the small mammals you mentioned. They are delicious prey items for a number of animals so their reactions are geared towards escape (although they don't have the giant axons that invertebrates have - it's more strength of connections and processing differences)

[mjkidd]

Jesus. I would not want to run into that rabbit in a dark alley.