I got this question recently, I hope this explanation helps.
First of all, it’s misleading to call it “touching” a tooth.
What we are doing when we “touch” a tooth is providing a proprioceptive input through sensory receptors.
While many of our sensory nerves are very sensitive (we can feel one hair move), generally we are using pressure or stretch to change afferent input, particularly to influence the withdrawal reflex, which is what happens when you are tickled or touch a hot iron.
So if we suspect a problem with a tooth, we press firmly on it, or have the patient bite firmly on the tooth (through a cotton roll or rolled-up tissue) and then retest the muscle. If the patient can’t feel the pressure on the tooth (and teeth can withstand many pounds of pressure) then the brain might dismiss the input as irrelevant.
To understand how pressure on a tooth changes muscle strengh, we need to better understand the periodontal ligament.
This ligament covers each root of a tooth and anchors the tooth to the bone except it’s not really like any other ligament because it’s highly innervated. The sensors within the ligament function in the same way that muscle spindles do – they send constant afferent input to the brain.
The sensors within the ligament detect position, compression, traction and direction of force on the teeth. This is why it’s so uncomfortable to get something stuck in your teeth.
In 1994, researchers in Adelaide published an extraordinary study in the journal of Experimental Brain Research. Dr Kemal Turker and colleagues inserted small electrodes into the biting muscles of volunteers which measured the facilitation and inhibition of those muscles. (1)
The researchers then applied slow pressure to a front tooth, simulating what would happen if the volunteer was biting through something hard. The measurements showed that increasing the pressure on the tooth facilitated the biting muscles, making them stronger.
Each time we bite, we can’t be sure how hard we will need to bite to break through the apple or nut or marshmallow. The sensors found in ligaments surrounding the roots of the teeth are stimulated by pressure and they cause the bite muscles to fire. If the apple or nut turns out to be harder than we anticipated, the sensors detect more pressure and create more output from the muscles.
In another phase of the experiment, the researchers gave the tooth a brisk tap, simulating the situation where we suddenly bite something hard like a seed or a stone. The biting muscles were immediately inhibited or weakened.
Chewing and biting are relatively subconscious activities.
If we are chewing away and suddenly bite something hard, the same sensors which can increase bite muscle strength can also inhibit or switch off the bite muscles.
Remember the last time you unexpectedly bit on something hard? Was it just your jaw that dropped open?
Depending on the amount of pain involved, you probably flinched most of your body, activating the stomach muscles, back muscles, neck muscles, and even arm muscles (as you brought your hand up to your face). Whatever contortion you performed, it was a reflex reaction to the proprioceptive insult to your tooth.
Your teeth are intimately wired into your brain and provide the brain with an huge amount of afferent input.
Mess up that afferent input with foreign bodies and you mess up muscle function in ways that could be very remote from the teeth and jaw.
Sensory information from the teeth, mouth and lips supply an area of the brain called the sensory homunculus which has a direct connection with the motor centres.
In ways that I don’t fully understand, amalgam fillings have the power to alter the afferent signals from the tooth to the brain. Metal crowns have an even stronger effect because they are in contact with the very sensitive gum tissues.
I believe both of these bad sensory inputs simulate the inhibitory tooth inputs which would be associated with a sudden tap on the tooth or biting on something hard.
When we apply pressure to the tooth, either by pushing it or asking the patient to bite on it, we stimulate the facilitatory reflexes, making the weaknesses stronger (more normal). Removing the offending metal allows the normalisation of afferent input from the tooth and subsequent normalisation of efferent output to the muscles.
I don’t believe we are making normal stronger, we are just restoring normal by removing inhibition.
(1) Turker KS; Brodin P; Miles TS Reflex responses of motor units in human masseter muscle to mechanical stimulation of a tooth. Exp Brain Res 1994;100(2):307-15