Evolutionary Types: Epicritic and Protopathic Pain
From the perspective of evolutionary physiology, all pain sensations are divided into two fundamental types that differ cardinally in their perceptual properties.
- Epicritic Pain. Represents a younger and more refined signaling mechanism for tissue damage. It is characterized by an acute sensation. The signal is perceived by the brain very quickly. A key property of epicritic pain is the ability to be easily and precisely localized—the patient can point directly to the source of discomfort. In addition, this type features rapid adaptation of the receptor apparatus.
- Protopathic Pain. This type is considered more ancient from an evolutionary standpoint. Unlike acute epicritic pain, this pain is dull in character. Signals travel more slowly, causing a delay in conscious perception. Such pain localizes poorly—it is perceived as diffuse and spread out. The most important physiological distinction is that protopathic pain does not undergo adaptation. Because of this, the sensation can persist for a long time.
Experimental Evidence: Henry Head's Experiment
The theory of sensory separation received practical confirmation through a daring experiment. British neurologist Henry Head decided to study sensory recovery by cutting a peripheral nerve in his own arm.
Observing regeneration, the researcher identified two stages:
- Stage 1 (Early Recovery). In the early stages of nerve regeneration, the researcher experienced exclusively protopathic sensation. It manifested as intense, dull, and poorly localized pain that persisted even after the external stimulus ceased.
- Stage 2 (Complete Nerve Consolidation). When complete nerve healing occurred, the clinical picture changed. Epicritic sensation appeared: the pain became sharp, easily localized, and transient.
Anatomical Classification: Somatic and Visceral
This classification is essential for understanding nociceptive mechanisms and clinical diagnosis. Depending on the receptor location, pain is divided into:
1. Somatic Pain Arises from the stimulation of body tissues and is subdivided into two subtypes:
- Superficial: occurs upon skin stimulation (e.g., a simple pinprick). Physiologically, it consists of primary and secondary sensations.
- Deep: generated by nociceptors located in the meninges, tendons, muscle tissue, and joints. A classic example of deep somatic pain is a headache.
2. Visceral Pain This type of pain is associated with internal organ pathology. Physiologically, it shares the characteristics of protopathic pain (dull, diffuse, non-adapting). Notably, visceral pain closely resembles deep somatic pain due to the presence of pronounced autonomic reactions.
Temporal Classification and Autonomic Response
Normal physiology also utilizes a classification based on the timing of sensation formation. Within this system, pain is categorized into:
- Early.
- Late.
> Important: Regardless of the type, class, or temporal formation of the pain syndrome, all normal types of pain are invariably accompanied by autonomic reactions.