Reticulospinal actions on primary afferent depolarization of cutaneous and muscle afferents in the isolated frog neuraxis

The effects of the brainstem reticular formation on the intraspinal excitability of low threshold cutaneous and muscle afferents were studied in the frog neuraxis isolated together with the right hindlimb nerves. Stimulation of low threshold fibers (less than two times threshold) in cutaneous nerves produced short latency, negative field potentials in the ipsilateral dorsal neuropil (200–400 m depth) that reversed to positivity at deeper regions (500–700 m). Stimulation of low threshold fibers (less than two times threshold) in muscle nerves produced, instead, negative responses that acquired their maximum amplitude in the ventral neuropil (700–900 m depth). These electrophysiological findings suggest, in agreement with observations in the cat, that low threshold cutaneous and muscle afferents end at different sites in the spinal cord. Intraspinal microstimulation applied within the dorsal neuropil produced antidromic responses in low threshold cutaneous afferents that were increased in size following stimulation of the dorsal or ventral roots, as well as of the brainstem reticular formation. This increase in excitability is interpreted as being due to primary afferent depolarization (PAD) of the intraspinal terminals of cutaneous fibers. Antidromic responses recorded in muscle nerves following intraspinal stimulation within the ventral neuropil were also increased following conditioning stimulation of adjacent dorsal or ventral roots. However, stimulation of the bulbar reticular formation produced practically no changes in the antidromic responses, but was able to inhibit the PAD of low threshold muscle afferents elicited by stimulation of the dorsal or ventral roots. It is suggested that the PAD of low threshold cutaneous and muscle afferents is mediated by independent sets of interneurons. Reticulospinal fibers would have excitatory connections with the interneurons mediating the PAD of cutaneous fibers and inhibitory connections with the interneurons mediating the PAD of muscle afferents. Although our results provide no direct information on whether the reticulospinal depression of the PAD elicited in low threshold muscle afferents is due to inhibition along the pathways producing PAD of muscle spindle or of tendon organ afferents, it seems likely — by analogy with what has been seen in the cat spinal cord — that these inhibitory actions are mostly restricted to the pathways producing PAD in the terminal arborizations of muscle spindle afferents. These results emphasize the specificity of the descending control of the synaptic efficacy of low threshold cutaneous and muscle afferents which could be of importance for motor performance.