[Structure and synapses of recurrent collateral axons of motor neurons in the vertebrate spinal cord (facts and hypotheses)].

Original data on structural and functional organization of spinal motoneuron axon collaterals in fish, amphibians and reptiles are presented. It is stated for the first time that synapses formed by recurrent collaterals on motoneuron dendrites should be referred to as excitatory chemical ones according to their ultrastructure. Comparison of present findings and reports on motor axon collaterals and their synapses in mammals allows to demonstrate characteristic features of the structure of neuropil plexus interneurons and synaptic connections that determine the action of recurrent inhibition and facilitation mechanisms in spinal cord of the vertebrates.