Etiology and control systems
Damaging factors that trigger the pain response are divided into three groups:
- Physical: mechanical trauma, temperature extremes, 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 is suppressed, which normally exhibits tonic activity and inhibits pain. It includes the brain's opioidergic, serotonergic, noradrenergic, and GABAergic systems. If the activity of these systems drops, pain occurs (e.g., psychogenic pain in depression). Physiotherapy 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.
| Sign | Epicritic (fast) pain | Protopathic (slow) pain |
|---|---|---|
| Localization | Skin and mucous membranes | Skin, internal organs, and tissues |
| Fiber type | Myelinated (A-fibers) | Unmyelinated (C-fibers) |
| Perception threshold | Low | High |
| Character | Precisely localized, resolves quickly | Diffuse, persists long after the stimulus is removed |
Impulse conduction and integration
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.
The reticular formation plays a huge role (impulse generation frequency of 4–6 Hz). It activates the cerebral cortex to form an integrated sensation of pain and distributes the signal to the thalamus, hypothalamus, and amygdala. 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) pain, medical practice distinguishes special conditions:
- Referred (cutaneovisceral) pain: occurs 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 passage of a gallstone — under the right scapula.
- Phantom pain: the result of irritation of amputation neuromas (tangles of regenerating axons at the ends of severed nerves) due to pressure, inflammation, or muscle contraction.
- Causalgia: paroxysmal burning pain in the area of damaged nerve trunks. Associated with increased nociceptor sensitivity 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.
Briefly: answers to topical questions
What stages are distinguished in the development of painful shock?
The provided sources do not separately highlight the stages of painful shock per se. The following stages/phases are indicated for shock.
Modern classification:
- Compensated shock.
- Decompensated reversible shock.
- Decompensated irreversible shock.
Obsolete two-phase model:
- Erectile phase — short-term; accompanied by motor and speech agitation, increased or maintained systolic BP.
- Torpids phase — characterized by lethargy and decreased systolic BP.
The criterion for irreversible shock is the absence of a pressor response to intravenous norepinephrine administration or intra-arterial blood/plasma substitute infusion.
How are nociceptors classified depending on the adequate stimulus?
In the provided sources, the following are associated with pain sensitivity:
- Mechanoreceptors — associated with nerve A-fibers.
- Polymodal nociceptors — associated with C-fibers; excited by mechanical, thermal, and chemical stimuli.
Nociceptor stimuli: mechanical, thermal (>45 °C or <15 °C), chemical (K⁺, H⁺, serotonin, histamine, bradykinin, prostaglandins, substance P). For polymodal C-nociceptors, chemical stimuli are also indicated: acid solutions, histamine, acetylcholine.
Sources also present an alternative theory: pain occurs upon excessive irritation of any receptor type.
What is the role of endogenous opioid peptides in the antinociceptive system?
Endogenous opioid peptides are natural ligands of opiate receptors and perform a crucial pain-relief function in the antinociceptive system. They inhibit the release of major mediators during painful excitation and suppress the sensitivity of second-order neuron receptors to these mediators. Their synthesis rate maintains the optimal pain sensitivity threshold.
| Peptide | Receptor | Level of analgesia |
|---|---|---|
| Endorphins | μ | High (brain and spinal cord) |
| Enkephalins | δ | Mean |
| Dynorphins | κ | Low in the brain, high in the spinal cord |
Epicritic pain — the EPIcenter is known (clear localization, fast A-fibers). Protopathic — EXTENsive (diffuse, slow C-fibers).
Class and exam questions
Why does the left arm hurt during a myocardial infarction?
This is the phenomenon of segmental ("referred") pain. It arises due to the convergence of visceral and somatic nerve impulses on the exact same neurons in the spinal cord.
What explains the analgesic effect of massage and physical therapy procedures?
Irritation of tactile or cold receptors activates the body's antinociceptive system, which suppresses pain impulses.
What is the main cause of phantom limb pain after amputation?
Amputation neuromas—tangles of regenerating axons—form at the ends of severed nerves. Their irritation by pressure or muscle contraction triggers a pain attack.
What else to study on this topic
- Inflammatory mediators in pain formation (kinins, histamine, serotonin)
- Ascending spinal cord tracts (decussations and synaptic relays in laminae)
- The role of the hypothalamus and amygdala in the emotional coloring of pain
- Differences between acute and chronic pain at the receptor and CNS levels
- Neuroimpulse and neurotrophic mechanisms of the nervous system's influence on tissues
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