[Distribution of the terminals of primary afferent fibers polysynaptically connected to motoneurons in the spinal cord in the frog].

In experiments carried out on an isolated perfused frog spinal cord connected with the hindlimb nerves primary afferent fibres and motoneurons were studied by intracellular electrical stimulation and recording with subsequent injection of HRP. It is found that collaterals of fast-conducting fibres belonging to the cutaneous or muscle nerves and establishing polysynaptic connections with motoneurons terminate within the upper and middle portions of the dorsal horn. Terminal branching of these fibres are characterized by numerous collaterals originating at short distances from each other. In some cases HRP-positive interneurons with axons terminating in immediate proximity to injected fibres were found. It is suggested that HRP-positive interneurons make axo-axonal contacts with terminals of primary afferent fibres.