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Nociceptors

Nociceptores

For medical students2 min readUpdated 2026-10-10

Nociceptors are high-threshold pain receptors consisting of free nerve endings of unmyelinated and thinly myelinated fibers. They respond exclusively to intense, potentially damaging stimuli (mechanical, thermal, or chemical) and trigger physiological reactions aimed at protecting the body and maintaining homeostasis.

Excitation thresholdVery high — they respond only to intense, tissue-damaging stimuli.
MorphologyFree nerve endings of unmyelinated or thinly myelinated fibers forming plexiform networks.
Sensory unitThe receptor itself and the peripheral branch of the afferent fiber.
SummationA sensation of pain occurs only with the repeated excitation of multiple receptors.

Causes of Pain Generation

Pain reactions in the body are triggered by two key factors that threaten tissue viability.

First is the breaching of protective integuments. Any damage to bodily barriers threatens homeostasis (internal environment constancy). In this case, pain performs a crucial monitoring function. It acts as a powerful negative biological drive, creating motivation for the individual to remove the pain source as quickly as possible.

Second is the alteration of oxygen metabolism. Pain arises when oxidative processes are disrupted or normal blood supply to tissues ceases (leading to ischemia or hypoxia). The body responds systemically: the autonomic nervous system is activated, predominantly its sympathetic division. This has a pronounced compensatory character, as sympathetic stimulation aims to improve oxygen delivery to the affected organ and restore tissue trophic support.

General Principles of Nociception

According to the "specificity theory" in physiology, pain is perceived by specialized receptors—nociceptors. Structurally, they form plexiform networks in the skin, muscles, and certain internal organs.

A vital feature of pain sensitivity is the discrepancy between the excitation of a single receptor and the actual conscious sensation of pain. A single impulse is insufficient. According to the "intensity theory", for a person to feel pain, multiple receptors must be repeatedly excited. The pain sensation is always the result of spatial and temporal summation of sensory inputs from intense stimulation.

Classification: Mechanonociceptors (Type I)

According to classical neurophysiological classification, the first type of pain receptors is mechanonociceptors. Their membrane depolarizes exclusively as a result of mechanical displacement. They are located in integumentary membranes, deep tissues, visceral organs, and vessel walls. Their primary function is monitoring tissue integrity.

Several groups of mechanonociceptors are distinguished:

  1. Cutaneous with Aδ-fiber afferents. Respond only to mechanical stimuli, ignoring thermal and chemical cues. Characterized by small receptive fields and rapid adaptation. They mediate epicritic (primary, sharp) pain.
  2. Epidermal with C-fiber afferents. Also activated solely by mechanical forces, unresponsive to temperature fluctuations. They possess small receptive fields and adapt easily.
  3. Muscle with Aδ-fiber afferents. Located on muscle surfaces and musculotendinous junctions. Excitable by heavy blunt pressure. Adapt rapidly.
  4. Articular with Aδ-fiber afferents. Located in joint capsules. Activated only under extreme conditions: excessive joint flexion or unnatural twisting.

Classification: Polymodal C-Nociceptors (Type II)

The second type of receptors is excited by a complex of intense stimuli: mechanical, thermal, and chemical. They respond to substances that disrupt normal oxidative tissue processes (histamine, acetylcholine, acid solutions). Signals from them are transmitted predominantly via C-fibers. The physiological role of this group is monitoring tissue respiratory function.

Polymodal receptors are divided into the following subgroups:

Mnemonic

To keep fiber types straight: Aδ-fibers (A-delta) = Attack (mechanical displacement) and Acute (sharp, primary epicritic pain). C-fibers = Complex/Cocktail (polymodal: gather all stimuli — mechanical, thermal, chemical).

Frequently asked questions

What types of pain are distinguished by clinical and pathophysiological classifications?

Clinical and pathophysiological classifications divide pain into several primary types:

  • Somatic pain — subdivided into superficial (skin stimulation) and deep (receptors in muscles, joints, tendons).
  • Visceral pain — associated with pathology of internal organs.
  • Nociceptive pain — arises in response to tissue damage and activation of pain receptors.
  • Neuropathic pain — caused by lesions in structures of the peripheral or central nervous system.
  • Noplastic pain — referenced in complex regional pain syndromes.

Additionally, based on localization and character, clinicians distinguish epicritic, protopathic, phantom, and referred pain, and by onset timing — early and late pain.

Through which pathways are pain impulses transmitted to the brain?

Pain signal transmission to the brain occurs predominantly via spinothalamic tracts:

  • Lateral spinothalamic tract (tractus spinothalamicus) — the main highway in the lateral funiculus of the spinal cord, whose fibers cross to the opposite side and carry impulses to the thalamus.
  • Neospinothalamic tract — conducts fast, well-localized pain to specific thalamic nuclei (VPL).
  • Paleospinothalamic tract — transmits slow, affective-motivational pain to non-specific (intralaminar, reticular) thalamic nuclei.

From thalamic nuclei, signals project to the cerebral cortex (specifically the postcentral gyrus) via thalamocortical tracts for final analysis.

Which neurotransmitters participate in synaptic pain transmission within the spinal cord?

Synaptic transmission and modulation of pain signals in the spinal cord involve several neurotransmitters. The main excitatory ("nociceptive") neurotransmitters released at the dorsal horn level include:

  • Glutamate — mediator of fast pain in the neospinothalamic pathway.
  • Substance P (Substance P) — a tachykinin involved in slow pain transmission (paleospinothalamic pathway) with a modulatory amplifying effect on nociceptive signaling.

Additionally, first-relay station modulators include histamine, neurotensin, cholecystokinin, norepinephrine, and serotonin, alongside inhibitory transmitters like GABA and enkephalin, which suppress pain signals.

What is the antinociceptive system, and which endogenous substances participate in it?

The antinociceptive system is a regulatory network responsible for inhibiting the activity of nociceptive neurons at the levels of the spinal cord dorsal horns, brainstem reticular formation, and thalamus. Its function is to suppress pain impulses and induce analgesia.

  • Endorphins — include $\alpha$-, $\beta$-, and $\gamma$-fractions.
  • Enkephalins — possess a broader distribution throughout the CNS.

These neuropeptides bind to opiate receptors, inhibit second-order neurons' sensitivity to pain mediators, and suppress their release, inducing hypoalgesia or analgesia.

Does the excitation of a single nociceptor cause a feeling of pain?

No, there is a discrepancy between the threshold of an individual receptor and the conscious perception. Pain arises only as a result of the spatial and temporal summation of multiple sensory inputs.

What chemical substances can excite polymodal C-nociceptors?

Such irritants include acid solutions (sulfuric, acetic, hydrochloric), acetylcholine, histamine, and bradykinin, which disrupt oxidative processes in tissues.

Why does tissue ischemia activate the sympathetic nervous system?

This is a compensatory bodily response to oxygen deprivation. Activation of the sympathetic division of the autonomic nervous system helps improve blood supply to the affected organ and restores tissue trophic support.

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