How does Sound Analgesic work
The sound relieved the pain of the mice
Anastasia Lyashenko, N+1
Doctors from China and the USA have proven that sound can reduce inflammatory pain in mice and have shown that it is the low ratio of sound to ambient noise that is important, not the type of sound. The authors described the process through the corticothalamic pathways in an article published in Science (Zhou et al., Sound induces analgesia through corticothalamic circuits).
Pain, expressed in unpleasant sensations or experiences, helps to track the threat of damage to body tissues. However, often this protective mechanism has the opposite effect: prolonged severe pain significantly reduces the quality of life. To help such patients, researchers are studying how pain works and how it can be reduced. Studies have shown that not only analgesic drugs can remove it, but also, for example, caffeine, which has shown an analgesic effect on mice, or even the touch of loved ones.
Music and other sounds have also previously shown their effectiveness in reducing pain: Dutch scientists using meta-analysis revealed the ability of music therapy to reduce postoperative pain, and a group of Vietnam and Sweden found its effect in reducing pain in children during lumbar puncture. But the mechanisms of how music relieves pain are not yet fully understood.
Researchers from China and the USA with the participation of Wenjie Wenjie Zhou from the University of Science and Technology of China decided to study the analgesic effect of sound using mice for this. First, the authors identified which sound reduces pain, and then conducted a series of experiments to describe this mechanism.
In the first stage, the authors used a pleasant (harmonious) sound of 50 and 60 decibels, which was delivered to an experimental group of mice with inflammatory pain caused by the introduction of a full Freund adjuvant through closely installed speakers. A group of control mice underwent the same procedure, but for them the sound was 45 decibels (the usual sound of background noise).
Experiment procedure
To measure mechanical sensitivity, the authors used estesiometers that were pressed against the inflamed area. The result showed that a sound of 50 decibels raised the nociceptive threshold of mice compared to the control group, but a sound of 60 decibels did not give such an effect, and there was no difference between consonant, dissonant sound and white noise. Further, the authors of the article checked whether the effect is associated with the level of 50 decibels or it depends on the difference in signal intensity and ambient noise. To do this, they turned on white noise of varying intensity to mice, which at the same time perceived ambient noise of 57 decibels. The authors found that the increase in the nociceptive threshold depends precisely on the difference between noise and this sound. In the experiment, the threshold increased only in response to sound of 62 decibels with ambient noise of 57 decibels: that is, the analgesic effect occurs due to a low noise/signal ratio of 5 decibels.
Tests with a maze and a dark-light chamber did not reveal anxious behavior or stress in mice at 5 decibels and 15 decibels of the noise/signal ratio, which means that the analgesic mechanism is not associated with a decrease in anxiety or stress. After conducting a series of tests, the researchers found that sounds with a low noise/signal ratio inhibit (suppress) glutamatergic signals coming from the auditory cortex to the posterior nuclei of the thalamus and to the posterior ventral nuclei of the thalamus, which interacts with areas associated with pain. To test this hypothesis, the authors artificially inhibited these pathways and found an effect that repeated the effect of sound with a low noise/signal ratio. Activation, on the contrary, canceled this effect.
The authors note that the mechanism of reducing pain with the help of music in humans is definitely more complicated. Their current study is one of the steps towards creating alternative ways to reduce pain.
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