A quantitative study of the central projection patterns of unmyelinated ventral root afferents in the cat.

Abstract

1. The ventral roots of the spinal cord contain a large number of unmyelinated primary afferent neurones. There is some controversy, however, about the function of these fibres and the route of their central projection. Here we have used electrophysiological techniques to quantify the central projection patterns of these neurones in the segment S2 of adult chloralose-anaesthesized cats. 2. A total of 1185 single unmyelinated units were recorded in small filaments isolated from intact and de-efferented ventral roots or intact dorsal roots of the segment S2 in nineteen cats. The projection patterns of these neurones were tested using supramaximal electrical stimulation of the pelvic and pudendal nerve (the main tributaries of the spinal nerve of this segment) and of the segmental companion root (dorsal or ventral as appropriate). 3. The principal finding of this study is that 85% of unmyelinated afferent axons in the ventral root are direct and exclusive projections. They constitute a separate class of afferents which is only capable of transmitting information from the periphery via the ventral roots. However, the proportion of these fibres that enter the central nervous system is unknown and it seems likely that some of them peter out as they approach the spinal cord and end blindly. The functional role of such afferents remains obscure. 4. For the remaining 15% of unmyelinated ventral root afferents, a projection into the segmental dorsal root was found. The majority of those fibres (about two-thirds) are primary afferent neurones innervating the pia mater. Some of these units had a small spot-like receptive field and responded to mechanical stimuli such as pressure and stretch of the root. They did not have axon projections in a peripheral nerve. 5. A few (5%) unmyelinated ventral root fibres are collateral branches of normal primary afferents projecting through the dorsal root. These trifurcating neurones are a small population which make up only some 0.5% of all dorsal root ganglion cells. The functional significance of this population too is unknown. 6. For none of the fibres that projected into both dorsal and ventral root was there positive evidence for the existence of looping axons that merely make a detour into one of the roots. Although the existence of loops cannot completely be excluded, our evidence suggests that they can constitute at most 5% of the unmyelinated ventral root afferents.

Clarification of the Innervation of the Bladder, External Urethral Sphincter and Clitoris: A Neuronal Tracing Study in Female Mongrel Hound Dogs

“…Loss of these inputs after sacral root transection suggests that they traversed sacral originating pelvic splanchnic nerves to the bladder. Several studies have shown that bladder visceral afferents can travel on hypogastric nerves, thoracic and lumbar splanchnic nerves, and on abdominal arteries to SG located lower thoracic, lumbar and sacral levels (Bahns et al, ; de Groat, ; Habler et al, ; Vera and Nadelhaft, ; Reitz, ). One or more of these pathways may be the mode by which afferents reached T11 and L5 SG.…”

Afferent signaling from the bladder: Species differences evident from extracellular recordings of pelvic and hypogastric nerves

“…Extracellular bladder afferent nerve recordings have been conducted over the last 40 years to evaluate the hypogastric and pelvic afferents in different animal species, such as dogs,2 cats,3–12 rats,13–19 mice,20–24 and guinea pigs 25–27. Different models and setups have been described, both in vivo and in vitro, handling an intact or opened bladder and using a variety of stimuli to evoke afferent activity.…”

“…Animals need to be anesthetized for the afferent recordings, and most anesthetics have an influence on the micturition reflex 45–47. In cats, alfa‐chloralose has been used in nearly all experiments,3–11 whereas in rats, pentobarbital,16–18 and urethane19, 32 have been used. Urethane has been shown to spare the micturition reflex45 and would, therefore, be best suited for afferent nerve recording studies.…”

Re‐utilization of Schwann cells during ingrowth of ventral root afferents in perinatal kittens

“…The presence of nociceptive afferents in the ventral root and pia mater allows the nervous system to respond to potentially harmful mechanical stimuli affecting these structures, such as pressure and torsion (Frykholm et al. 1953; White & Sweet, 1955; Häbler et al. 1990).…”