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Enamel has no nerves, so theoretically a tooth should not feel any pain when a dentist bores into it. Yet still we use anesthetics...

Maybe it’s dentin's fault? It apparently has some tubes filled with liquid and the tooth's nerves can feel this liquid's movement. So, maybe, the closer the dentist gets to dentin (especially if he/she touches it), the more pain is felt? By the way of vibrations from the boring tool?

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    $\begingroup$ I was wondering about this from the opposite direction, actually. I always refuse local anesthetic for simple dental work, because I never feel anything and I hate having my mouth numb. So I'm curious if other people's teeth are somehow more sensitive than mine! $\endgroup$
    – Glenn Willen
    Commented Aug 30, 2020 at 3:45
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    $\begingroup$ @Glenn Willen Definitely! How a local anaesthesia is going to work, how much time it will take to numb the area does depend on an individual. There are people who are resistant to a type of local anaesthesia, eg lignocaine. In those, other types are used. Whereas many people are allergic to one of the components of local anaesthesia. Also if there is lot of inflammation then it takes time plus more dosage of local anaesthesia is required to come into action. So it depends on lots of factors. $\endgroup$
    – Ojasvi
    Commented Aug 30, 2020 at 4:08
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    $\begingroup$ Whether an individual wants to have an anaesthetic injection or he/may will resist that pain, purely depends on him/her. $\endgroup$
    – Ojasvi
    Commented Aug 30, 2020 at 4:08
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    $\begingroup$ @cbeleitesunhappywithSX Agreed! That maybe because of toxic effects of local anaesthesia and some patients are scared of the 'injection'! $\endgroup$
    – Ojasvi
    Commented Aug 31, 2020 at 9:54
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    $\begingroup$ @Mast if it was a root canal then, below the dentin, pulp must have been exposed and severely infected. Pulp has neurovascular supply. Now generally giving local anaesthesia, the pain doesn't happen, but in your case infection would be so much that the effect of anaesthesia would be very less or your tooth might be less sensitive. $\endgroup$
    – Ojasvi
    Commented Aug 31, 2020 at 9:57

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Yes you are right. It is the dentin. Enamel has got no nerves. So when the dentist use his/her instrument, initially there is no pain (if the enamel is intact). The next layer is dentin. As you said, dentin has got dentinal tubules containing dentinal fluid. Whenever there is any stimulus which has not yet reached the pulp, but may have reached the dentin, eg of stimulus in the form of pressure, temperature change, sweet food, sour food etc, cause the fluid to move or get displaced. The displacement of dentinal fluid stimulates nerve endings in the pulp and hence generates a pain/ sensitivity response to your brain. enter image description here

Reference

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Reference

There are three main theories of dentine hypersensitivity: Direct Innervation (DI) Theory Odontoblast Receptor (OR) Theory Fluid Movement/Hydrodynamic Theory

The Hydrodynamic or Fluid Movement theory is one of the main theories in dentistry to explain the mechanism by which a tooth perceives the sensation of pain. It is currently the most widely accepted theory used to explain tooth sensitivity.

Reference

Now the drilling causes heat generation, even though little, due to the coolant, plus pressure and hence causes the fluid to displace and hence pain. This pain usually ends after the treatment.

Hence anaesthesia is required. Hope I have answered your question :)

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    $\begingroup$ The heat generated by grinding is critical. Being very old ,my first ( painful) dental experiences were with the old belt drive drills = hot=painful . When the water turbine drills came along it was a whole new world ; Novocaine shots were unnecessary for drilling.. $\endgroup$
    – blacksmith37
    Commented Aug 29, 2020 at 20:02
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    $\begingroup$ @blacksmith37 Oh! I didn't know about the belt drive drills. With their low speeds and without water coolant they must be very painful plus dangerous for the tooth also! In the current scenario yes, the increased speed handpieces with a water coolant are used, but if the tooth is very infected then they do cause quite significant pain as the dentin thickness in those cases is very less. And hence anaesthesia is required. Interestingly, these days lasers too are used which are not at all traumatic for the patient as well as the tooth. $\endgroup$
    – Ojasvi
    Commented Aug 29, 2020 at 20:47
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    $\begingroup$ I was always told that tooth structure was like a stack of straws on their side. Now, finally, I'm getting the picture. Great answer, thanks! $\endgroup$
    – uhoh
    Commented Aug 30, 2020 at 6:42
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    $\begingroup$ @uhoh well, enamel does have a kinda stack of straws like structure. It has got enamel rods which are arranged vertically and obliquely! $\endgroup$
    – Ojasvi
    Commented Aug 30, 2020 at 7:15
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    $\begingroup$ More generally, the same can be said of sensory nerves anywhere in the body. You don't have to physically touch a living nerve cell in order to stimulate it: Touching/stressing/heating/cooling nearby tissue causes the tissue to release chemical signals that stimulate nerve endings which can be, in some cases, up to several mm away from the site. $\endgroup$
    – Solomon Slow
    Commented Aug 30, 2020 at 18:58

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