[Electrotonic synapses in the mammalian spinal cord]

By means of light and electron microscopy methods structural peculiarities of motor nuclei have been studied in the rat spinal cord (17 animals) on the 1st-3d and on the 10th-18th days of postnatal ontogenesis. Synaptic junctions of the gap type are revealed; they are considered as electrotonic synapses. Dendro-somatic and dendrodendritic synaptic junctions of the gap type are found. Together with the electrotonic synapses, morphologically mixed synapses of axo-somatic and axo-axonal types are disclosed; they contain, besides organells, specific for chemical synapses, close opposition areas of pre- and postsynaptic membranes of the gap junction type. Morphologically mixed synapses occur in neuropil of the motor nuclei of the spinal cord in young rats of all age groups studied. Homologous synapses are detected in the motor nuclei of the white mouse spinal cord. Synaptic junctions of the gap type in the mammalian spinal cord could be a substrate of electrical interaction between its motor neurons.