Physiology and ultrastructure of electrotonic junctions. 3. Giant electromotor neurons of Malapterurus electricus.

Abstract

The 2 giant electro-motor neurons show no direct contact with each other. There are many axodendritic and axosomatic endings which show regions of membrane fusion with the giant neurons. At other endings there are no regions of fusion but only desmosome-like complexes. These endings in general contain more vesicles than those where there are regions of fusion, and vesicles are usually more closely clustered to the desmosome-like complexes. The postjunctional potentials evoked in the giant cells by stimulation of the medulla appear to be electrotonically mediated. The latency may be as short as 0.2 msec, faster than at known chemically transmitting junctions. Large hyperpolarizations in a giant cell reduce the postjunctional potentials in it. Small hyperpolarizations may increase the postjunctional potentials, presumably by increasing the height of the prejunctional spikes. A direct spike in a giant neuron blocks postjunctional potentials with time relations indicating that the impulse invades the prejunctional fibers. When the giant neurons are iontophoretically injected with chloride ions a spike is followed by a depolarizing potential, apparently an inverted inhibitory postsynaptic potential. The pathway mediating this potential probably involves collaterals of afferents to the giant neurons rather than recurrent collaterals of the giant neurons themselves. Electrotonic coupling between the giant neurons is probably by way of the prejunctional fibers. Calculations from observed coupling and junctional area show that the junctional membranes are of lowered resistivity. Excitatory transmission to the giant neurons is probably purely electrotonic and does not have a chemically mediated component. The morphological correlates of chemical transmission at junctions where there are regions of membrane fusion appear to play no role in excitation. Endings on the giant neurons without regions of fusion and with numerous vesicles may be inhibitory synapses. The electrotonic junctions on the giant cells act integratively under experimental conditions and probably also in normal impulse initiation.