Synaptic transmission. An analysis of the electrical activity of the lateral geniculate nucleus in the cat after optic nerve stimulation

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Abstract
The morphology of the optic nerve, the optic tract and the region of the dorsal nucleus of the lateral geniculate body (L.G.D.) in the cat is discussed from the point of view of the interpretation of electrical records obtained by means of microelectrodes inserted into them. The L.G.D. offers a very favourable morphological situation for studying the electrical correlates of synaptic transmission. All the fibres in the cat's optic nerve are myelinated. By stimulating the nerve electrically at its exit from the eyeball and adjusting the stimulus strength it may be arranged that of the two groups of fibres in the nerve, only those of larger diameter make a significant contribution to the records obtained from the contralateral geniculate. In this paper only large fibre synaptic activity has been investigated. The conduction velocities of the bulk of the fibres in the two groups are 39 and 16 m/s respectively. The large-fibre impulse reaches the L.G.D. at about 0.9 ms after the stimulus artifact. It is concluded that the impulse is propagated to the extreme tip of the axon terminals. The synaptic delay averages 0.4 ms. The post-synaptic activity within the L.G.D. consists of a slow negative potential similar to the synaptic potential of Brooks & Eccles (1947) and a propagated axonal spike. A positive potential similar in time course to the negative synaptic potential is recorded from the proximal portion of the optic radiation. The negative synaptic potential has a rise time of 0.3 to 0.6 ms, a half-time of 2.0 ms and falls to a low value after 5 ms. It is concluded that the presence of a positive synaptic potential indicates that the proximal portions of the axons are acting as sources of current for sinks in the region of the dendrites and cell bodies. It is suggested that the flows of current associated with the synaptic potential lead to the generation of the post-synaptic propagated spike in the region of the proximal portion of the axon. Conduction along dendrites probably does not occur in the L.G.D. The post-synaptic spike can be followed up the optic radiation towards the striate cortex. The duration of the radiation spike is significantly longer than that of the tract spike.

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