F. Cervero

Neonatal capsaicin does not affect unmyelinated efferent fibers of the autonomic nervous system: functional evidence.

Visceral nociceptive thresholds, viscero-visceral reflexes and the effects of efferent stimulation of autonomic nerves has been studied in adult rats treated at birth with capsaicin. These animals showed visceral analgesia and depressed visceral reflexes. However, stimulation of unmyelinated efferent fibers in autonomic nerves produced normal effects. It is concluded that neonatal treatment with capsaicin destroys selectively unmyelinated afferent fibers.

Neonatal capsaicin and thermal nociception: a paradox

Thermal and mechanical nociceptive thresholds and somato-visceral reflexes have been studied in normal rats and in rats treated neonatally with capsaicin. Mechanical nociceptive thresholds were increased in capsaicin treated rats whereas thermal nociceptive thresholds remained unchanged. In contrast, somato-visceral reflexes evoked by thermal noxious stimulation of the skin could not be elicited in capsaicin treated rats whereas mechanical noxious stimulation was effective in evoking visceral reflexes.

Fine afferent fibers from viscera do not terminate in the substantia gelatinosa of the thoracic spinal cord

Transganglionic transport of horseradish peroxidase (HRP) has been used to study the anatomy of the central projection of somatic and visceral afferent fibers to the thoracic spinal cord of the cat. A dense concentration of somatic afferent fibers and terminals was found in laminae I and II of the dorsal horn and more scattered terminals were present in laminae III, IV and V and in Clarkes column. In contrast, visceral afferent fibers and terminals were found only in lamina I or reaching lamina V via a small bundle of fibers located in the lateral border of the dorsal horn. These results indicate that fine afferent fibers from viscera, unlike those of cutaneous origin, do not project to the substantia gelatinosa (lamina II) of the dorsal horn.

Ascending projections of nociceptor-driven Lamina I neurones in the cat

SummarySingle unit activity has been recorded from nociceptor-driven Lamina I neurones in the lumbar spinal cord of chloralose anaesthetized and gallamine paralysed cats. Ninety-four nociceptor-driven Lamina I neurones were identified by their superficial location in the dorsal horn and their ability to respond only to noxius stimulation of their cutaneous receptive fields. One-third of the Lamina I neurones responded only to noxious mechanical stimulation of the skin (Class 3a) und two-thirds responded to both mechanical and thermal noxious stimulation (Class 3b). Lissauers tract was stimulated electrically two and three segments rostral to the recording sites. Ninety percent of the neurones tested showed a post-synaptic excitation mediated by fibres conducting at a mean velocity of 5.2 m/s (range 0.9–13.3 m/s). It is concluded that Aδ and C afferent fibres running in Lissauers tract excite nociceptor-driven Lamina I neurones. Ninety-six percent of the neurones tested showed a long period of inhibition (100–200 ms) following stimulation of large afferent fibres in the dorsal column. This inhibition was increased when the intensity of stimulation recruited Lissauers tract fibres. Fifteen percent of the neurones tested were antidromically activated by Lissauers tract stimulation from up to 3 segments rostal to their origin. A further 18.5% were antidromically excited by stimulation of deeper tracts. The mean conduction velocity of the axons of these projecting neurones was 8.6 m/s (range 3.8–16.5 m/s) and thus are small myelinated axons. The Class 3b neurones exhibited a significantly lower conduction velocity (7.5±2.8 (S.D.) m/s) than the Class 3a neurones (10.7±3.7 (S.D.) m/s).It is concluded that at least two-thirds of the population of nociceptor-driven Lamina I neurones are segmental interneurones.

Noxious intensities of visceral stimulation are required to activate viscerosomatic multireceptive neurons in the thoracic spinal cord of the cat

Single unit electrical activity has been recorded from neurons in the Th8 and Th9 segments of the spinal cord in chloralose-anesthetized cats. These neurons had cutaneous receptive fields in the right costal region from which they could be driven by noxious and/or innocuous stimulation of the skin. They could also be activated by distensions of the biliary system but only at noxious intensities of visceral stimulation. No viscero-somatic convergent neurons has been found responding to innocuous visceral stimulation.

Supraspinal linkage gelatinosa neurones: effects of descending impulses

It is now well established that sensory transmission through the dorsal horn can be modified by descending influences from supraspinal structures (for review see ref. 15). Most dorsal horn neurones in laminae IV, V and VI of Rexed 23 are under tonic descending inhibition14,18,19, 24, which particularly affects their responses to noxious inputs is. The specific nociceptor-driven neurones in lamina I are also under tonic descending inhibition from supraspinal centres s. Similar inhibitory actions have been described on spinal cord neurones projecting through the spinocervical tract4,5, 9, the spinothalamic tractZ,el, 27, and the spinoreticular tract 16. Several descending pathways, including the pyramidal tract 6, have been proposed as mediators of this descending control. Anatomical and functional studies have shown that most of these descending tracts course in the dorsolateral funiculus (DLF) of the spinal cord1,5,17 and terminate in the centre and base of the dorsal horn, i.e. laminae IV, V and VI. Basbaum et al. 2 have recently described three bulbospinal pathways from the rostral medulla of the cat, one of which originates in the nucleus raphe magnus and projects bilaterally through the DLF to laminae V, VI and VII and in addition to the most superficial dorsal horn, lamina I and the substantia gelatinosa (SG). The functional organization of the small neurones of the SG is now under intensive studyl0-~2,~0, ~5. In this paper we report results from a study on lumbar SG neurones of influences descending from more rostral parts of the central nervous system. Results have been obtained from cats anaesthetized with chloralose (60 mg/kg) and paralyzed with gallamine triethiodide, using standard procedures to ensure full anaesthesia of the animals at all times. SG neurones were recorded in the L6 and L7 segments of the cord using high impedance glass microelectrodes 2e. Tonic descending influences from supraspinal centres were tested by placing a thermode on the L1 segment which permitted reversible spinalization of the animals by cold block s. Balltyped surface electrodes were placed on the dorsolateral funiculus at L2-L3 level and