Red light irradiation can dilate local blood vessels. When red light penetrates the skin tissue, it can cause the vascular endothelial cells to release nitric oxide (NO). NO is an important vasodilator factor, which can relax the smooth muscle of blood vessels and thus increase the inner diameter of blood vessels. For example, in some animal experiments, after red light therapy, the diameter of local small arteries can be increased by approximately 15% - 20%.
This vasodilation effect can increase blood flow and improve the blood perfusion of local tissues. For pains caused by poor blood circulation, such as the pain in the fingers of patients with Raynaud's disease (local ischemic pain due to vasospasm), red light therapy can effectively relieve the symptoms.
Red light can improve the microcirculation system. It can enhance the autonomous movement of microvessels and accelerate the blood flow velocity of microvessels. Under normal circumstances, the blood flow velocity in the microcirculation is relatively slow and prone to stasis. After red light therapy, the aggregation of red blood cells is reduced, and the hemorheological properties are improved, enabling more effective delivery of oxygen and nutrients to tissue cells. For example, in patients with diabetic peripheral neuropathy, the microcirculation disorder leads to nerve ischemia and hypoxia and thus causes pain. Red light therapy relieves the pain by improving the microcirculation and reducing the ischemic and hypoxic state of the nerves.
Red light can regulate the functions of inflammatory cells. During the inflammatory process, cells will release a variety of inflammatory mediators, such as prostaglandin E2 (PGE2), interleukin-1 (IL-1), and tumor necrosis factor-α (TNF-α). These inflammatory mediators will stimulate nerve endings and lead to pain. After red light irradiation, it can inhibit the release of these inflammatory mediators by inflammatory cells (such as macrophages, neutrophils, etc.). For example, in the synovial tissue of the joints of patients with arthritis, red light therapy can reduce the release amount of PGE2 by approximately 30% - 50%.
Red light can accelerate the absorption of exudates. Inflammation will cause exudates to appear in the tissue spaces, which will compress the surrounding tissues and nerves and cause pain. Red light can promote cell metabolism, enhance the activity of phagocytes in local tissues, and accelerate the phagocytosis and clearance of exudates. For example, in soft tissue inflammation, red light therapy can shorten the absorption time of exudates by approximately 2 - 3 days, thereby reducing swelling and pain.
Red light can act directly on nerve endings. Low-intensity red light irradiation can inhibit the conduction of nerve impulses of nociceptors. From the perspective of neurophysiology, this is based on the "gate control theory". Red light may stimulate non-pain sensory afferent fibers, close the "pain gate" in the dorsal horn of the spinal cord, and prevent the further transmission of pain signals to the central nervous system. For example, in patients with postherpetic neuralgia, red light therapy can reduce the excitability of the damaged nerves and reduce the transmission of pain signals.
Red light therapy may also promote the release of endogenous opioid peptides. The human body can produce opioid peptide substances such as endorphins and enkephalins by itself, and these substances have a strong analgesic effect. After red light irradiation, it may regulate the neuroendocrine system and stimulate the body to release endogenous opioid peptides. In some clinical observations, the levels of endorphins in patients who received red light therapy were increased, and the pain threshold was correspondingly raised, thereby reducing the sensation of pain.
