Etiology and Control Systems
Damaging factors that trigger the pain response are divided into three groups:
- Physical: mechanical trauma, temperature fluctuations, high doses of radiation (UV, laser), electric current.
- Chemical: strong acids and alkalis, as well as the accumulation of calcium salts in tissues.
- Biological: pathogenic microorganisms, their toxins, and high concentrations of mediators (kinins, histamine, serotonin) in the inflammation zone.
Pain sensation occurs when the antinociceptive system, which normally exhibits tonic activity and suppresses pain, is inhibited. It includes the opioidergic, serotonergic, noradrenergic, and GABAergic systems of the brain. If the activity of these systems drops, pain arises (e.g., psychogenic pain during depression). Physical therapy procedures, such as massage or local hypothermia, conversely activate this system.
Types of Pain Sensitivity
Pain is perceived by free nerve endings in all tissues except the brain. There are two main varieties, which in combination allow the body to accurately assess the nature and localization of the pathogenic impact.
| Feature | Epicritic (Fast) Pain | Protopathic (Slow) Pain |
|---|---|---|
| Localization | Skin and mucous membranes | Skin, internal organs, and deep tissues |
| Fiber type | Myelinated (A-fibers) | Unmyelinated (C-fibers) |
| Threshold | Low | High |
| Character | Precisely localized, resolves quickly | Diffuse, persists long after stimulus removal |
Impulse Conduction and Integration
The pathways of epicritic and protopathic pain pass through the spinal cord and brain, synapsing on neurons of the dorsal horns and ascending to the thalamus.
Reticular formation (with an impulse generation frequency of 4–6 Hz) plays a massive role. It activates the cerebral cortex to form an integrated sensation of pain and distributes the signal to the thalamus, hypothalamus, and amygdaloid complex. This exact mechanism underlies the complex systemic response: autonomic reactions, motor activity, emotions, and behavior.
Special Types of Pain
In addition to the division into acute (nociceptor activation) and chronic (CNS sensitization), clinical practice distinguishes special conditions:
- Referred (cutaneo-visceral) pain: arises due to the convergence of somatic and visceral impulses in the spinal cord. Example: pain in myocardial infarction radiates to the left arm, and during a gallstone passage — under the right scapula.
- Phantom pain: the result of irritation of amputation neuromas (clusters of regenerating axons at the ends of cut nerves) due to pressure, inflammation, or muscle contraction.
- Causalgia: paroxysmal burning pain in the zone of damaged nerve trunks. Associated with hypersensitization of nociceptors and the release of mediators (norepinephrine, substance P) by the sympathetic nervous system.
Unlike somatic pain, neuropathic pain is poorly localized, has an unusual, excruciating character (hyperpathia), and responds poorly to narcotic analgesics.