Pain Classification

Evolutionary Types: Epicritic and Protopathic Pain

From the standpoint of evolutionary physiology, all pain sensations are conventionally divided into two fundamental types that differ cardinally from each other in their perception properties.

1. Epicritic pain. This represents a younger and more advanced mechanism for signaling damage. It is characterized by an acute nature of sensation. The signal is perceived by the brain very quickly. A crucial property of epicritic pain is the ability for easy and maximally precise localization — the patient can point to the source of discomfort. Furthermore, this type is characterized by rapid adaptation of the receptor apparatus.
2. Protopathic pain. This type is considered more ancient from an evolutionary perspective. Unlike acute epicritic pain, this pain is dull in nature. Signals travel more slowly, causing the sensation to be perceived by the subject with a delay. Such pain is poorly localized — it is perceived as something spread out, diffuse. The most important physiological difference lies in the fact that protopathic pain is not accompanied by the development of adaptation. Because of this, the sensation is able to persist for a long time.

Experimental Evidence: Henry Head's Experiment

The theory of sensory separation received practical confirmation thanks to a bold experiment. English neurologist Henry Head decided to study the recovery of sensitivity by cutting a peripheral nerve in his own arm.

Observing the regeneration, the researcher identified two stages:

• Stage 1 (Onset of Recovery). In the early stages of nerve regeneration, the scientist developed exclusively protopathic sensitivity. It manifested as intense, dull, and poorly localized pain that persisted even after the external stimulus ceased.
• Stage 2 (Final Nerve Consolidation). When complete nerve fusion occurred, the picture changed. Epicritic sensitivity appeared: the pain became sharp, easily localized, and rapidly passing.

Anatomical Classification: Somatic and Visceral

This classification is important for understanding the mechanisms of nociception and clinical diagnostics. Depending on the localization of receptors, the following are distinguished:

1. Somatic pain Occurs upon irritation of body tissues and is subdivided into two subtypes:

• Superficial: occurs upon skin irritation (e.g., during a routine pinprick). Physiologically, it consists of primary and secondary sensations.
• Deep: formed by nociceptors located in the meninges, tendons, muscle tissue, and joints. A classic example of such deep somatic pain is a headache.

2. Visceral pain This type of pain is associated with internal organ diseases. In its physiological properties, it possesses characteristics of protopathic pain (dull, diffuse, without adaptation). It is important to note that visceral pain is very similar to deep somatic pain due to the presence of pronounced autonomic reactions.

Temporal Classification and Autonomic Response

In normal physiology, a classification based on the time of sensation formation is also used. Within this system, pain is subdivided into:

• Early.
• Late.

> Important: Regardless of which type, class, or formation time the pain syndrome belongs to, all types of pain are normally always accompanied by autonomic reactions.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of formation of primary and secondary sensations of superficial pain
• Autonomic reactions in various types of nociception
• Molecular mechanisms of the absence of adaptation in protopathic receptors
• The process of final consolidation of a peripheral nerve
• Early and late pain: temporal mechanisms of formation

Other topics in the "Pain" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.