Pain and pain modulation
1. Basic concepts.
(1) Pain is a painful sensation formed after the brain abstracts and generalizes the injurious afferents caused by acute or chronic tissue injury, and is often accompanied by complex autonomic activity, motor reflexes, mental and emotional reactions.
(2) Involvement pain is the phenomenon of pain or nociceptive hypersensitivity on the surface of the body caused by visceral disorders, which is called involvement pain.
(3) Nociceptive hyperalgesia is a state in which a stimulus that did not originally cause pain causes pain, or an injurious stimulus causes more intense pain than usual, usually as a result of damage to nerves or tissues leading to persistent transmission of injurious sensations.
(4) Clockwork tightening is a phenomenon in which the repetitive discharge of class C fibers causes a progressive increase in the responsiveness of spinal cord posterior horn neurons to afferent impulses in the case of severe and persistent injury, also known as central sensitization.
2. Causes of pain.
Various injurious stimuli acting on the organism can cause pain after reaching a certain intensity, and these stimuli usually include the following three kinds.
(1) temperature stimulation: too high or too low temperature (> 45 or < 5 ℃) can excite the temperature injury receptors.
(2) mechanical stimuli: strong mechanical stimuli such as squeezing, cutting, pulling, rubbing, and strong muscle contraction applied to tissues can excite mechanical injury receptors.
(3) chemical stimulation: exogenous (such as strong acids, strong bases, etc.) and endogenous (such as stomach acid, substance P, etc.) pain-causing substances can excite polysensory injury receptors.
3.The tissues that produce pain and their pain characteristics.
(1) skin: sharp pain with clear localization, sensitive to various pain-causing substances.
(2) Deep tissues: the muscles, tendons, joints and periosteum in the deep part of the body are stimulated to produce dull pain, and the localization is not clear; tendons and tendon membranes are sensitive to needles, extrusion, etc.; synovial membranes in joints are sensitive to chemical stimulation.
(3) Viscera: generally produce dull pain with unclear localization, sensitive to mechanical pulling and spasm and other stimuli, but not sensitive to knife, cut and clamp stimuli.
(4) Vascular: especially the outer membrane of blood vessels is sensitive to mechanical dilatation and chemical stimulation, which can produce intense pain.
(5) Nerve fibers: super-strong stimulation acting directly on the nerve fibers can cause significant pain. Moreover, the peripheral parts of the nerve innervation can also be painful and can be accompanied by circulatory and nutritional disorders.
4.The basic occurrence of pain.
Various kinds of injurious stimuli act on the body to excite specific injurious receptors and issue nerve impulses, which are transmitted to the spinal cord via Aδ and C nerve fibers and terminate in the intermediate neurons in the superficial and deep layers of the posterior horn of the spinal cord. These neurons send out fibers through nociceptive transmission pathways such as the thalamic tract of the spinal cord, which further upload injurious information to the sensory centers of the thalamus and brain, thus producing nociception.
5.Types of injurious receptors.
(1) temperature injury receptors: receptors that feel injurious temperature stimuli and can be excited by high or low temperatures (>45 or <5°C). These receptors consist of Aδ nerve fiber endings that are thin in diameter, thinly myelinated, and have slow conduction velocities.
(2) Mechanical injury receptors: receptors that sense injurious mechanical stimuli and can be excited by strong mechanical stimuli applied to tissues, such as squeezing, cutting, and strong muscle contraction. Mechanical injury receptors are also composed of Aδ nerve fiber endings.
(3) Multi-sensory injury receptors: exogenous and endogenous chemical pain-causing substances can excite these receptors, which can also be activated by high-intensity mechanical and temperature stimuli. Polysensory injury receptors consist of unmyelinated class C nerve fiber endings with slower conduction velocities (<1m/s) and finer diameters.
(4) Silent injury receptors: a recently discovered class of injury receptors that do not belong to the classical C injury receptors and are widely distributed in skin, muscle, joints and viscera. Usual injurious stimuli do not activate these receptors, which are called silent injury receptors, but inflammation and a variety of chemical stimuli can sharply lower their release threshold and can be activated by lighter injurious stimuli to produce nociception. Thus, excitation of such injury receptors plays an important role in nociceptive sensitization and central sensitization.