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While having a dental surgery, I've got this question that why I can still talk, open/close my mouth, move my lips and so on while I can't feel anything at all in my mouth?

If the local anesthesia interrupts the alert that goes through brain, how is the muscle still functional? Is it some sort of one-way interruption which only disables the alert that go "from" the nerve "to" the brain? Or are there different nerves for this task?

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  • $\begingroup$ Why isn't anyone just saying that local anesthetics block sensory neural pathways, not motor neurons? There are two differing neurons, motor and sensory. Is not that the bottom line? This is what I assumed, but ended up confused after reading the answers provided. $\endgroup$ – michelle b Aug 21 '18 at 23:55
  • $\begingroup$ @michelleb this is because local anesthetics block both sensory neurons and motor neurons. They block a specific receptor site inside $Na^+$ channels found in all axons, preventing propagation of an action potential. All neurons need this channel in order to send signals, so it's not a matter of a difference in the physiology of a sensory vs. motor neuron, it's about the anatomy of the specific nerves involved. $\endgroup$ – De Novo Aug 22 '18 at 15:42
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@Nicolai's answer is not entirely correct.

Background

The most common local anesthetics are all the "-caine" drugs - like novocaine, lidocaine, - and even cocaine has some of the same pharmacology, as well as other effects (in movies this is why cops might rub some white powder on their lips - if it's cocaine, it will make their lips tingle; this is not standard training for non-movie police; also not advised for drug sellers or purchasers, because for exactly this reason cocaine is often cut with other -caines or other compounds that give a tingly feeling).

What all of these drugs do is to block voltage-gated sodium channels. These are the channels that propagate action potentials through axons and excitable dendrites. Other local anesthetics that aren't used in the clinic, like tetrodotoxin from pufferfish, block the same types of channels. At sufficient concentrations, local anesthetics block all nervous activity: unlike @Nicolai's answer, they do shut down nerves (as a brief note for terminology: nerves are bundles of axons containing many many individual fibers).

So back to your original question: how come the muscle isn't impaired during dental surgery?

Geometry of innervation of the face

Take a look at this picture of the nerves innervating the facial muscles and the muscles themselves: https://bmc.med.utoronto.ca/cranialnerves/wp-content/images/c_07/Facial-vii12_labelled768.jpg (note: I wasn't able to find a good image that has a license allowing me to reproduce it here; if someone finds one and wants to edit it in, please do - thanks!)

They mostly come in from the sides of the face, and so they are further away from the site of local anesthetic injection. The muscles themselves also extend further away. It may be that the muscle is partly anesthetized, but you don't realize it.

Limited diffusion and location of injections

Diffusion of local anesthetics is limited because they actually act inside cells, and only the unionized form can diffuse across cell membranes. Once inside the cell, the tend to be ionized and therefore get "stuck". Therefore, they don't typically diffuse nearly as far as other compounds of similar size, especially lipophilic compounds.

The nerve endings that sense pain, on the other hand, have to be at the surface of the skin/mucous membranes. That is, they are exactly where the local anesthetic is being injected. The objective of local anesthesia is often to inject enough to affect only those surface tissues, trying to spare the deeper tissues.

Effects on nerves

However, it isn't too uncommon to overdo it and to paralyze the underlying muscles or nerves. Even nerves far away can be affected, for example see this paper discussing cases where the local anesthetic travels near the eyes and impairs eye movement. There are also other non-dental procedures where block of a particular nerve is the intended result: this can be seen as an alternative to general anesthesia. One example is the use of an epidural (essentially into the space around the spinal cord) injection during childbirth or surgery. The extent of paralysis depends on the dose, however, and usually the intent is to avoid major paralysis and use other drugs like opiates to assist in preventing pain.

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    $\begingroup$ Sorry to say, but this isn't exactly correct either. In dental procedures, nerve bundles are usually the target of the anesthetic block. Similar to digital blocks in laceration repair. We block either side of the base of the finger, yet the finger can still move quite easily. $\endgroup$ – anongoodnurse Jul 22 '17 at 23:32
  • $\begingroup$ So to be clear, the neuron itself is not paralyzed? One quick question, If we do a LA on inner ear, is it still able to send crucial feedbacks to the brain? By the way, thanks for the long and nice answer :) $\endgroup$ – Jack Johansson Jul 23 '17 at 3:48
  • $\begingroup$ @anongoodnurse Thanks! Today I learned... Feel free to edit my answer or add your own. Is that always how things are done for dental procedures? Presumably those nerves are separate from the ones that control the muscles. Or is it just because the different fibers have different thresholds? My experience is in rodent surgeries, where we just inject under the skin. I thought the nerve endings were particularly vulnerable. For the finger, isn't part of that because the actual muscles that move the finger aren't in the finger itself? $\endgroup$ – Bryan Krause Jul 23 '17 at 18:45
  • $\begingroup$ For dental procedures, yes. Topical (mucosal) anesthesia is just to pierce the mucosa and get to the deeper nerve bundle. And yes, (that was a stupid example, sorry) the muscles that control finger movement are in the forearm, but other sensations can still be felt (e.g. pressure.) I think it's the type of nerve fiber. IV lidocaine can decrease pain (or, e.g., the cough reflex during a crash intubation) but doesn't cause paralysis because of differing nerve types. Interesting, too, is the effect on cardiac muscle. Btw, been reading about tinnitus; very interesting. Thanks for that. $\endgroup$ – anongoodnurse Jul 23 '17 at 21:26
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    $\begingroup$ @anongoodnurse Started reading some papers where people studied dose-dependent effects of lidocaine so I can improve my answer. Ran into some authors I know personally...oops. $\endgroup$ – Bryan Krause Jul 23 '17 at 22:48
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The answer to this question boils down to 'because local anesthesia does not interrupt the nerves themselves'.

From the wiki article on local anesthetics (LA):

All LAs are membrane-stabilizing drugs; they reversibly decrease the rate of depolarization and repolarization of excitable membranes (like nociceptors). Though many other drugs also have membrane-stabilizing properties, not all are used as LAs (propranolol, for example). LA drugs act mainly by inhibiting sodium influx through sodium-specific ion channels in the neuronal cell membrane, in particular the so-called voltage-gated sodium channels. When the influx of sodium is interrupted, an action potential cannot arise and signal conduction is inhibited.

This means that the LAs don't actually shut down nerves. What they shut down is the ability of nerves to sense mechanical movement. Because mechanical movement on the cellular level is a bit different from what we usually associate with it, this means that in an area affected by LAs the nerves can not report feelings of pain, touch or temperature [actually also spicy taste] to the brain. All other nerve functions (including muscle control) are not affected

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  • $\begingroup$ Thanks for the answer. It's also interesting that how we can talk under LA effects just like normal situation. Was wondering if we can walk normally if our feet or head is under LA effect too? $\endgroup$ – Jack Johansson Jul 22 '17 at 9:07
  • $\begingroup$ @JackJohansson You should still be able to walk in theory, but it will feel very 'wierd' and you will have higher chance of stumbling, because all sensory feedback from your moving legs is missing. E.g. you will not feel the moment your foot hits the floor, so you're brain may not automatically adapt the position of your foot to 'roll' it over the floor. $\endgroup$ – Nicolai Jul 22 '17 at 9:11
  • $\begingroup$ This is also why people under LA do more damage when they bite their tongue, they can't feel it and can't feel the position of the tongue well. $\endgroup$ – John Jul 22 '17 at 16:05
  • $\begingroup$ This answer is incorrect for the most part. $\endgroup$ – anongoodnurse Jul 22 '17 at 23:33
  • $\begingroup$ the quoted text and your assessment of it are incompatible, voltage-gated sodium channels are not for sensing mechanical movement. $\endgroup$ – John Aug 22 '18 at 2:28

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