Modern Neurophysiological Theories on the Effects of Acupuncture for the Treatment of Dental Pain

Thalia Kritidou, DDS,Dip.Ac.

ACUPUNCTURE is a very old Chinese method for treating disease, without the use of drugs, addressed to the energy level of the human organism.

Acupuncture is based on the use of specially made needles, which are inserted at absolutely specific points of the trunk, the head and the limbs called "acupuncture points"

The advantage of this method is in the treatment of disease without side effects caused by drugs.

Apart from its healing potential, an unquestionable and well-known property of acupuncture, is the treatment of pain and the analgesic effect it has on all body organs and, of course, those of the head and the mouth, which is of great interest for dentists.

Starting many years ago, Western and Chinese colleagues were driven to extensive research into the effects of acupuncture based on medical knowledge as it existed at the time.

More recent research all over the world, based on modern neurophysiology, showed that acupuncture could achieve relief of pain through at least 3 mechanisms.

The first one is based on

(1) The "gate control" Theory.

Proposed by Melzac and Wall in 1964.

Pain is a sensory stimulation and consequently it is transmitted from the periphery to the centre through 3 successive neurones.

1st Sensory Neurone Stem:

In this case, periphery is the tooth pulp which is predominantly a pain organ and especially suitable for experimentation in the study of acupuncture anaesthesia.

From the tooth pulp, the stimulus is taken over by afferent neural fibres and reaches the Gasserian ganglion. Therefrom, through the posterior sensory neural fibres, it enters the nuclei of the posterior horns of the spinal cord (at the same height, or 1-2 myelotomia anterior to the entrance point of the spinal nerves).

2nd Sensory Neurone Stem:

From the nuclei of the posterior horns it follows the thalamospinal bundle and reaches the thalamus (in the lateral ventricle nucleus).

3rd Sensory Neurone Stem:

From the thalamus through the thalamocortical fibres it reaches the somatosensory level of the cortex (rear central helix of the parietal lobe) where the highest level of pain perception is situated.

This is where the "pain" stimulus is perceived leading to subsequent reactions.

The theory states that each neurone with its synapses constitutes a "station", an entrance gate the stimulus reaches and through which the message may or may not be transmitted should a variety of factors inhibit its passage.


One such factor is Acupuncture.

Acupuncture points are rich in Ab neural fibres (thick, medullated and fast) which are responsible for transmitting the touch and tactile signals.

At the same time they are poor in Ad and c neural fibres [thinner, slower (Ad are medullated, c non medullated)], which are responsible for transmitting the sensation of pain.

Specifically, Ad fibres are responsible for acute localised pain, while c fibres transmit diffused blunt pain.

This is why acupuncture is painless, because acupuncture points do not have many neural fibre endings that transmit pain signals.

Consequently, when an acupuncture point is punctured, the endings of all neural fibres are stimulated.

But since the Ab pressure and touch neural fibres are faster, they forward their stimulation signal 20-50 times faster to the 1st sensory neurone stem.

Therefore, when the pain signal, a little delayed through the slower Ad and c neural fibres, reaches the same sensory neurone stem, it finds the synapses "engaged", we could say "blocked". It is as if it has found the gate closed, and so the signal transmission to the metasynaptic fibres, to the thalamus and, of course, to the cortex is interrupted and, therefore, the brain cannot perceive the pain stimulus.

According to the same theory, there is a central mechanism located on the reticulum of the medulla oblongata in the substantia gelatinosa (of Rolando), which controls the procedure of pain repression at all levels of the Central Nervous System; in other words it is a pain control threshold.

Pain is caused by the secretion of substance P on the synapses due to stimulation of the Ad - c fibres of pain.

When the Ab neural fibres (touch-pressure) are stimulated, endorphins are secreted at the synapses neutralising the pain action of substance P.

Similar pain control thresholds also exist, in more posterior positions of the pain channel, for example in the thalamus.

These pain control thresholds can be influenced positively or negatively by several psycho-affective situations.

For example, during intense psychological stress pain perception is intensified.

When we are distressed or tired, the pain threshold level decreases.

On the contrary:

Strong battle stress causes catecholamine as well as Dynorphine production in soldiers (adrenaline), this is the reason they do not experience intense pain caused by their wounds during battle.

Let me remind you "stroking a sore spot", which is a universal instinctive reaction (also encountered in animals).

It is the "Mummy will kiss it better" effect.

Moreover, the stimulation of the peripheral mechano- or thermo-cryoreceptors affects higher centres of the Central Nervous System, which influence the pain control threshold.

(2) Hormonal theory:

It was advanced ten years later, in 1975, by Simons & Pomeranz.

Scattered all over the nervous system are receptors capable of receiving external analgesic substances, which results in the reversal of the feeling of pain.

Thus, it is reasonable to assume that there should exist some internally released analgesics, which are naturally produced by the body itself, otherwise the existence of those receptors could not be explained.

Research has indeed shown that acupuncture causes the release of endogenous analgesic substances.

These substances are polypeptides, endorphins a, b, c, encephalines: methionine, leucoencephaline and dynorphines.

It also produces a quantitative modification of neurotransmitters Dopamine, Serotonine, ACTH, et al.

These indigenous analgesics are called "indigenous opiates", because they have the same pharmacological action with the external opiates, e.g. morphine.

In other words, they produce analgesia, well being, euphoria, increase of the libido, etc., exactly like morphine, with the only difference that they are not addictive. This is because our body produces exactly the quantity it needs, and, anyway, they are produced through a natural process.

It is worth mentioning that the analgesic action of internal opiates is many times stronger than that of morphine; it is 200 times higher in the case of endorphines and more than 400 times higher in the case of Dynorphines.

When we want to achieve analgesia through acupuncture for surgery, we insert the needles in a specific way with the help of a multitude of electro-acupuncture equipment and we succeed in obtaining secretion of Dynorphine.

Surgical analgesia through acupuncture is based on Dynorphine. In other words, the production of endogenous substances can generally be effected selectively, depending on the points and method of acupuncture.

The presence of these substances has been confirmed and measured in vitro.

Experiments on rabbits have shown that following an arterial anastomosis, analgesia is achieved not only in rabbits where analgesic acupuncture was applied, but also in rabbits that received the blood of the acupunctured ones.

Transfusion of cerebrospinal fluid from an acupunctured cat to another one brought about analgesia in the second cat.

At the N.M.T.S. Hospital similar research was conducted by acupuncturist colleagues on humans. Specifically, measurements were taken of the endorphine level prior to, during and after the application of acupuncture for analgesic purposes. Very high endorphine levels were found in the blood, even 4 hours after the application of acupuncture.

Naloxone is a substance that antagonises the action of morphine. Following a local or intravenous injection of Naloxone, the analgesic result of acupuncture is completely reversed.

(3) The Theory of the Reflex Arc.

The basis for this theory is the "axial reflex arc"

"Periphery - Central Nervous System - periphery"

As already mentioned, pain is transmitted from the periphery via the posterior sensory neural fibres to the posterior horns of the spinal cord.

Therefrom, it may follow two routes:

a) Via afferent neural fibres it may reach the cortex through the thalamus, or

b) It may be transmitted from the posterior to the anterior cornets of the spinal cord (transverse or non-transverse - transverse - non-transverse reflex arc) and through the motor efferent anterior roots it may be directed to various organs, muscles, skin, vessels, glands, etc, which are served by the same or neighbouring neurotomes.

The action of the acupuncture needle may also follow the 2nd route of the reflex arc.

This explains the action of acupuncture from a point in the periphery to an internal organ and, of course, the dental pulp, tongue, cheek, glands and, generally, the skull and face, and, thus, act against dental pain.

It is possible that another, as yet unknown to date mechanism exists that explains the effective action of acupuncture.

The future will reveal and confirm the truths of the past.