Peripheral Nociceptive Fibers
Information is transmitted from pain receptors to the central nervous system via sensory nerves. Two fundamentally different types of afferent fibers are involved, each subserving a distinct component of the pain experience:
- A-δ fibers (A-delta): These are relatively thick, myelinated nerve fibers. The presence of myelin ensures a high conduction velocity of 4 to 30 m/s. These pathways transmit primary, sharp pain.
- C fibers: These are thin, unmyelinated fibers. Due to the absence of myelin, signal transmission velocity 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 primary function is the rapid conduction of impulses and the provision of discriminative sensitivity (the ability to clearly distinguish spatial details of a stimulus).
- Anatomical composition: Includes projections of the dorsal columns and the spinocervical tract.
- Pathway course: Afferent fibers pass through the medial lemniscus and reach specific thalamic nuclei, synapsing in the ventrobasal complex.
- Cortical projections: Axons of thalamocortical neurons project primarily to the primary somatosensory cortex ($S_I$), partially to the secondary area ($S_{II}$), and the orbital cortex.
The lemniscal system is what allows us to instantly and accurately determine the location of a painful stimulus on the body surface.
Extralemniscal System (Slow Pathway)
The second ascending system is the extralemniscal system (Extralemniscal system). It runs parallel to the lemniscal system but has a diffuse organization and mediates relatively slow signal conduction.
- Anatomical composition: Includes the spinoreticular, spinotectal, and spinobulbothalamic tracts.
- Relay features: Signals arrive at non-specific thalamic nuclei, specifically 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 generating protopathic pain—a generalized sensation without fine differentiation of stimulus modality or exact location (making it difficult to pinpoint the exact site of pain). This system integrates pain input, provides the emotional coloration of sensations, forms the phenomenon of slow pain, and maintains the general level of central nervous system arousal.
Electrophysiological Markers
The organization of these two systems is clearly demonstrated when recording evoked potentials in the cerebral cortex:
- The lemniscal system generates a distinct, high-amplitude evoked potential in the $S_I$ area. This indicates rapid, synchronous signal transmission with minimal dispersion.
- The extralemniscal system elicits polyphasic, temporally dispersed potentials in the $S_{II}$ area. This response shape directly reflects the multisynaptic nature of transmission: the signal undergoes multiple relays and spreads diffusely before reaching cortical structures.