Convergence onto interneurons subserving primary afferent depolarization of group I afferents

Publisher Website
Google Scholar
Abstract
Whether common or independent neuronal pathways are used to evoke primary afferent depolarization (PAD) from selectively activated group Ia and Ib afferents of different muscles was investigated. The spatial facilitation of effects of various cat afferents, indicating convergence on the same interneurons, was used as a test. Its occurrence was assessed on dorsal root potentials (DRP) evoked in unspecified fibers or using intra-axonal recording from identified group Ia muscle spindle afferents or group Ib tendon organ afferents. Spatial facilitation was found in PAD pathways from various Ia-afferents, whether of flexors or extensors; from various Ib-afferents, whether of flexors or extensors; and from flexor Ib-afferents and flexor or extensor Ia-afferents. No indications were found for common pathways from extensor Ib- and any Ia-afferents under conditions that proved effective in other combinations. Latencies of those components of PAD that appeared as a result of the spatial facilitation ranged from 2 to > 7 ms, indicating that the convergence occurred in the shortest (trisynaptic) as well as longer pathways. The same patterns of convergence were found in PAD pathways to extensor and flexor Ia-afferents (in experiments with intraaxonal recording from these afferents). The possibility might thus be considered that some neuronal pathways are used to modulate transmission via Ia-afferents independently of their muscle origin. The same might hold true for extensor and flexor Ib-afferents. The minimal number of distinct neuronal populations subserving PAD of group I afferents may be 2-6. Actions of cutaneous, joint and interosseous afferents on DRP from Ia-afferents were reexamined for further comparison between neurons mediating PAD and those mediating postsynaptic excitation or inhibition of motoneurons. Only depression of Ia DRP followed stimulation of these afferents at intensities of 1.5-2.0 times threshold and higher; lower threshold afferents were apparently ineffective. Because of the lack of convergence of extensor Ib and Ia muscle afferents and of low-threshold cutaneous afferents, interneurons mediating PAD may be distinguished from the interneurons subserving Ib and Ia-like-Ib postsynaptic actions in motoneurons. The latter are coexcited by these 3 groups of afferents.