Ultrastructure of the Mauthner axon collateral and its synapses in the goldfish spinal cord

The ultrastructure of the synapses formed by the Mauthner axon collateral (MAC) was examined in the goldfish spinal cord. All such collaterals form axo-dendritic chemical synapses with dendrites of motor and interneurons. Besides these chemical synapses, contacts have been found between some MAC and axons of interneurons, which can be characterized as gap junctions with presumed electrical transmission. All cellular processes contacted by the MAC contain 'dense bands' of material in their cytoplasm. These 'dense bands' are absent from other parts of the fish C.N.S. These observations and the peculiar synaptic pattern are discussed and correlated with electrophysiological results.