According to the gate control theory of pain, hypothesized decades ago by Melzack and Wall, A-β nerve fibers (which transmit information from touch and vibration receptors in the skin) stimulate inhibitory interneurons in the spinal cord, which in turn act to reduce the amount of pain signal transmitted from A-δ and C fibers across the midline of the spinal cord and from there to the brain.
Two types of afferent fibers involved in mechanical pain have been described: slower but very fine unmyelinated C fibers (0.4-1.1 µm in diameter), which mainly transmit burning pain, and A fibers, which include thinly myelinated A-δ fibers (1-5 µm in diameter), which chiefly transmit pricking pain. The A-δ fibers release several peptide neurotransmitters, of which substance P is the most important in exciting secondary dorsal horn neurons (see the above diagram). Secondary neurons then ascend to the brain mainly via spinothalamic tracts. Touch, like pain, is transmitted as a superficial sensation by A-β fibers; vibration is a deep sensation.Each major system forms a separate sensory tract in the spinal cord. The lemnical (dorsal column) system carries vibration, whereas the other important system, the ventrolateral system, relays nociceptive stimuli such as pain and crude touch.
The proposed mechanisms by which vibration minimizes pain may include both peripheral and central mechanisms. According to the gate control theory of pain, the strength of synaptic transmission at the dorsal horn and, similarly, at the trigeminal ganglia junctions is decreased, probably by presynapatic inhibition, when large, non-pain-signaling axons within the nerve are stimulated (causing the gate to "close").These are touch receptors and vibration receptors (Pacinian corpuscles and Meissner's corpuscles). Transcutaneous electrical nerve stimulation (TENS) may, in some ways, work in a manner similar to vibration. According to Longe et al. in a discussion comparing and contrasting vibration and TENSm "The mechanism of action of these procedures is generally explained by the gate-control theory of pain inhibition in which large diameter sensory fibres (A-β), conducting impulses from the selective activation of low threshold mechanoreceptors, reduce the painful input of the small diameter nociceptive afferents (C-fibres) by triggering local inhibitory circuits in the substantia gelosa of the dorsal horn". In the cat model, vibration inhibits nociceptive receptors in the dorsal horn neuron.Acting in a central location, vibrotactile and pain sensations produced activation in similar regions within the somatosensory cortices of the brain.
The interaction of vibration and pain is complex; under some circumstances chronic exposure to vibration can cause or exacerbate pain. For example, the long-term use of hand-held vibrating tools may cause nerve dysfunction. Prolonged exposure to vibration is a risk factor for lower back pain.
Whole-body vibration has, however, been tested as a treatment for back pain. As Rittweger et al. point out, "There are differences, of course, between industrial and therapeutic whole-body vibration, namely, the method of application, the subject's posture, the frequency of application, and the temporal duration of exposure and the resulting fatigue."
In addition to reducing acute pain by counterstimulation, vibration has been used to treat and reduce other types of pain. In chronic pain syndromes, vibratory stimulation has been shown to be more effective than placebo. Lundberg, in a study of 366 patients, reported that muscle vibration could reduce chronic muscle pain. "The best pain reducing site was found to be either the area of pain, the affected muscle or tendon, the antagonistic muscle or a trigger point outside the painful area. In most patients the best pain reducing effect was obtained when the vibratory stimulation was applied with moderate pressure. To obtain a maximal duration of relief of chronic pain the stimulation had to be applied for about 25-45 minutes".In a similar study, vibration produced relief of chronic muscle pain when applied with moderate pressure for 20 minutes. The pain relief lasted at least 3 hours but often 12 hours or more. In another study, 59 percent of 267 patients with chronic neurogenic or musculoskeletal pain had relief of up to 18 months.
Vibration of an unexercised muscle reduces pain perceived from local pressure. Vibratory stimulation reduced the pain associated with both electrically induced and clinical pain overlying the extensor carpi radialis longus muscle. In this study, moderate pain was produced by low frequency (2 Hz) electrical stimulation with cutaneous fascicles of the median nerve. The authors report, "Vibration within the area of projected pain reduced the sensation of pain more efficiently than vibration outside that area. Moderate pain was sometimes completely inhibited but intense pain was only moderately reduced. Pressure and cooling produced some pain relief, whereas mild warming had an ambiguous effect. Since the painful input was derived from stimulation of fibres in the nerve trunk, and not from peripheral nociceptors, the pain suppressing effects of vibration and cooling are not explicable in terms of lowered excitability of the nociceptive nerve endings in the skin. Instead, the results indicate that activity in low threshold mechanoreceptive and cold sensitive units suppress pain at central (probably segmental) levels."
Kakigi and Shibasaki confirm the role of vibration in alleviating pain.In a later study Kakigi and Watanabe show that both the gate control theory and diffuse noxious inhibitory control accounted for the pain relief following CO2 laser stimulation, rather than simple changes in the subjects' attention. The authors believe that the vibratory pain relief effect is not simply at the level of the dorsal horn but also in the brain, "The responsible sites for this phenomenon are considered to be the dorsal horn of the spinal cord, the brainstem and some parts of the brain such as the second sensory cortex and the cingulate cortex."
The pain-relieving effect of vibratory stimulation on orofacial pain is not affected either by probe size or by vibratory frequency, at least within the 10-200 Hz range. In one study vibration applied distally is more effective than vibration applied proximally to the pain threshold.In another study, whether the vibration is to an area of 12.5 cm2 or 25 cm2 does not affect mean pain threshold. Interestingly, in a study of vibration and thermal pain, vibration applied in the adjacent dermatome as well as the contralateral dermatome in the volar forearm significantly decreases the thermal pain perception.
Sinus pain may be relieved by vibratory stimulation.Vibratory stimulation decreases the intensity of tinnitus in one study. Phantom pain is relieved by vibration in 75 percent as opposed to 44 percent of patients treated with placebo.
TENS, vibratory stimulation, and electroacupuncture are more effective than placebo. TENS and vibratory stimulation are overall more effective than aspirin in relieving pain of myofascial or musculoskeletal origin. TENS plus vibratory stimulation is helpful in the pain relief in a patient with painful legs and moving toes, with vibration alone being more successful than TENS.
Vibration or TENS are not sufficient, however, to relieve the intense orofacial pain associated with pulp surgery, abscess incision, or tooth extraction; all of the patients have to be given conventional local anesthesia.
One study shows that vibration is not helpful in relieving HIV-associated neuropathic pain. The HIV symptoms that are tested, including burning and painful soles of the feet, as well as paresthesia, are associated with sensory findings, so they are likely not applicable to the immediate pain elicited by certain dermatologic surgery or cosmetic procedures.
Pain relief with vibratory stimulation is likely not associated with release of endogenous opioids, as it was not reversed with naloxone. Similarly, naloxone does not influence oral pain after molar removal, followed by TENS or vibratory stimulation. Relief of pain from vibratory stimulation does seem to be correlated with the CSF substance-P-like immunoreactivity levels (SPLI).
The role of vibration in minimizing pain may not simply be a result of counterstimulation. In a study using functional MRI in 2001, attention altered perception of pain.
Consequently, although the gate control theory of pain explains how vibration, either by stimulating touch receptors and or by stimulating vibration receptors, can minimize or alleviate the sensation of pain at the spinal cord level, other central and peripheral mechanisms may also have influence.