Pain

For medical students2 min readUpdated 2026-10-10

Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage. From a physiological standpoint, it serves as a vital danger signal that forces the organism to seek ways to eliminate the threat.

Main functionDanger signal for the entire organism

Behavioral roleFormation of goal-directed activity

ControlIntegrity of integumentary coverings and tissue respiration level

CauseIntense stimulation of sensory receptors

Nature of the Pain Phenomenon

According to the International Association for the Study of Pain (IASP) framework, pain cannot be viewed solely as a physiological process. It is a complex experience that invariably includes two components:

Sensory — the physical sensation of discomfort.
Emotional — the subjective negative perception.

It is important to note that pain can occur not only during actual (already occurred) cell destruction, but also during potential damage, when the threat is merely escalating. In the scientific community, debates regarding the nature of this phenomenon persist. Some researchers define pain exclusively as an affective state. Other authors lean toward viewing it as a state whose mechanisms resemble motivation, yet which is inevitably accompanied by a pronounced affect.

Causes and Triggers

For a pain sensation to arise, specific factors that disrupt the normal functioning of tissues are required.

Fundamentally, the cause is intense stimulation of various sensory receptors. Under normal conditions, these receptors respond to adequate stimuli (e.g., temperature changes or mechanical pressure), but under excessive stimulus intensity, they initiate a pain signal.

Primary triggers include:

Action of physical damaging agents (extreme temperatures, mechanical trauma).
Exposure to chemical damaging substances.
Hypoxia — oxygen deficiency in tissues or entire organs, leading to metabolic disruption and the accumulation of metabolic waste products.

Pain from the Perspective of Functional Systems Theory

Within the framework of functional systems theory, pain is regarded not merely as a symptom, but as a critical danger signal for the organism.

It plays a key role in human and animal behavior. The onset of a pain sensation predetermines goal-directed activity. This means that the organism does not simply passively perceive damage, but mobilizes all available resources. The ultimate result and main objective of this formed activity is the complete elimination of the root cause that triggered the pain (e.g., withdrawing a hand from a hot object or seeking shelter).

Biological Significance

From a biological perspective, pain acts as a potent negative biological drive. In response to a threat, a specialized perceptual-motivational functional system is formed within the organism.

This system is vitally essential because it strictly monitors two critical constants:

Integrity of the integumentary coverings. This is a baseline condition for ensuring general homeostasis and protecting internal environments from external aggression.
Level of tissue oxygen respiration. Pain signals hypoxia, forcing the organism to react immediately to cellular oxygen deprivation, thereby preventing cell death.

Mnemonic

To remember the two primary constants controlled by pain, use the association "Skin Respiration": tissue oxygen respiration (internal) and the integrity of the integumentary coverings (external).

Frequently asked questions

Which groups of chemical substances act as tissue mediators of pain (algogens)?

Sources classify the following as algogens and pain mediators:

Potassium ions (K+) — alter nerve cell membrane excitability.
Hydrogen ions (H+) — listed among chemical irritants of nociceptors.
Substance P.
Prostaglandins — increase the sensitivity of pain receptors to algogens.
Bradykinin.
Serotonin.
Histamine — listed as a chemical irritant of nociceptors and as a component of the pain mediator complex during inflammation.

During inflammation, pain mediators are described as prostaglandins combined with histamine, serotonin, bradykinin, and substance P. Metabolic factors of hypoxia are considered universal algogens.

Which types of nerve fibers participate in the conduction of pain signals?

Two types of nerve fibers participate in conducting pain signals:

A-fibers (Aδ) — myelinated, fast-conducting fibers. Conduction velocity is 5–30 m/s (6–30 m/s in some reports). Responsible for conducting fast, sharp, epicritic pain that is easily localized.
C-fibers — slow-conducting (0.5–2 m/s), unmyelinated thin fibers. Responsible for transmitting slow, dull, protopathic pain that is poorly localized.

Which CNS structures form the organism's antinociceptive system?

The antinociceptive system has a hierarchical structure and includes the following levels and structures:

Cortical level: Secondary somatosensory cortex.
Hypothalamic level: Hypothalamic nuclei.
Brainstem/Spinal level:
Periaqueductal gray (PAG) of the midbrain.
Raphe nuclei (including nucleus raphe magnus).
Locus coeruleus.
Reticular formation (pontine tegmentum, medulla oblongata).
Spinal cord (dorsal horns).
Additional structures: Amygdala, hippocampus, specific cerebellar nuclei.

Where are the first-order neurons of the spinothalamic pain pathway located?

Reference sources explicitly state the following: impulses from receptors enter the spinal cord via dorsal roots to interneurons of the dorsal horns. For the spinothalamic pathway, the localization of second-order neurons is indicated as the gray matter of the spinal cord; their axons project to the thalamic nuclei.

The direct localization of first-order neurons of the spinothalamic pain pathway is not explicitly detailed in the source texts.

How does sharp epicritic pain differ from dull protopathic pain?

Differences between epicritic and protopathic pain are presented in the table below:

Characteristic	Epicritic Pain	Protopathic Pain
Character	Sharp	Dull, aching
Speed of perception	Perceived rapidly	Perceived more slowly
Localization	Easily, precisely, clearly localized	Poorly localized (diffuse)
Adaptation	Rapid adaptation occurs	Does not undergo adaptation (persists long-term)
Conducting fibers	Myelinated A-fibers (Aδ)	Thin C-fibers
Conduction pathways	Neospinothalamic tract	Paleospinothalamic tract

What definition of pain is provided by international expert committees?

Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage.

What is the primary trigger for the onset of pain?

Triggers include the action of physical or chemical damaging agents, as well as tissue and organ hypoxia.

What is the main behavioral role of pain?

Pain predetermines goal-directed activity, the ultimate result of which is the elimination of the cause of the pain sensation.

Which vital constants does pain monitor?

It monitors the integrity of the integumentary coverings (to ensure homeostasis) and the level of tissue oxygen respiration.

Go deeper

Mechanisms of affective state generation during pain
Role of temperature and pressure receptors in nociception
Similarities and differences between pain and basic biological motivations
Impact of hypoxia on the activation of the perceptual-motivational functional system
Homeostasis maintenance during disruptions of integumentary barrier integrity