Afferent pathways of pain sensitivity

Peripheral nociceptive fibers

The transmission of information from pain receptors to the central nervous system is carried out via sensory nerves. They contain two cardinally different types of afferent fibers, each responsible for its own component of the pain sensation:

1. A-δ fibers (A-delta): These are relatively thick nerve fibers covered with a myelin sheath. The presence of myelin ensures a high conduction velocity of the nerve impulse — from 4 to 30 m/s. Primary, acute pain is transmitted via these pathways.
2. C fibers: These are thin fibers lacking myelin insulation. Due to the absence of myelin, the signal transmission velocity along them is extremely low, ranging from 0.5 to 2 m/s. They are responsible for transmitting secondary, slower pain impulses.

Lemniscal system (fast pathway)

At the level of the spinal cord and brainstem, the unified afferent stream splits. The first major system is the lemniscal system (Lemniscal system). Its main function is the rapid conduction of impulses and providing discriminative sensitivity (the ability to clearly distinguish spatial details of a stimulus).

• Anatomical composition: Includes posterior column projections and the spinocervical tract.
• Pathway course: Afferent fibers pass through the medial lemniscus and reach specific thalamic nuclei (optic thalami), where relaying occurs in the ventrobasal complex.
• Cortical projections: Axons of thalamocortical neurons project predominantly to the primary somatosensory cortex ($S_I$), partially to the secondary area ($S_{II}$), and the orbital cortex.

It is the lemniscal system that allows us to instantly and precisely determine the localization of a pain stimulus on the body surface.

Extralemniscal system (slow pathway)

The second ascending pathway system is the extralemniscal system (Extralemniscal system). It runs parallel to the lemniscal system but has a diffuse organization and provides relatively slow signal conduction.

• Anatomical composition: Includes the spinoreticular, spinotectal, and spinobulbothalamic tracts.
• Relay features: Signals enter the non-specific thalamic nuclei — the mediobasal thalamus, which includes intralaminar nuclei.
• Cortical projections: Pathways project diffusely, predominantly to the secondary somatosensory cortex ($S_{II}$) and other brain regions.

Functions of the extralemniscal system: It is responsible for the formation of protopathic pain — a generalized sensation without fine differentiation of the modality and localization of the stimulus (it is difficult to determine the exact location of the pain). This system integrates the pain stream, provides emotional coloring to sensations, forms the phenomenon of slow pain, and maintains the general level of arousal of the central nervous system (arousal).

Electrophysiological markers

The organization of the two systems is clearly demonstrated by recording evoked potentials in the cerebral cortex:

• The lemniscal system generates a clear, high-amplitude evoked potential in the $S_I$ area. This indicates rapid, synchronous signal transmission with minimal dispersion.
• The extralemniscal system elicits polyphasic, time-spread potentials in the $S_{II}$ area. This distinct response form directly reflects the multisynaptic nature of transmission: the signal is relayed multiple times and spreads diffusely before reaching cortical structures.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Anatomy and physiology of the medial lemniscus
• Spinocervical tract: structural features
• Mechanisms of formation of protopathic and epicritic pain
• The role of intralaminar thalamic nuclei in nociception
• Integrative function of the secondary somatosensory cortex ($S_{II}$)

Other topics in the "Pain" section

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