ELECTRICAL ACTIVITY OF THE LATERAL GENICULATE OF CATS FOLLOWING OPTIC NERVE STIMULI

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Abstract
4 groups of optic nerve fibers, recognisable by their potentials following a single electric shock, were traced oscillographically to their destinations by recording the post-synaptic responses. The 1st group, of largest size, fastest conduction and lowest threshold activates the dorsal nucleus and cortex. The 2d and 3d groups activate regions along the tract to the collieulus, and the 4th, corresponding to a C group in peripheral nerves, activates mainly the collieulus. No effect on the cortex of any but the 1st group is detected. In general the smaller fibers activate the more posterior structures, with some overlap. Facilitation of a 2d response by a first occurs at intervals of 2 to 15 msec in geniculate and cortex, depression occurs at longer intervals up to several secs. Slow potentials of the order of afterpotentials but of greater amplitude than in peripheral nerve are recorded at geniculate cell regions, and correspond to the facilitation interval. They may be assigned tentatively to cell bodies or their dendrites. Transitory after- potentials of another type are produced by injury. Interaction between facilitation of the type referred to above, and a fluctuation of excitability, previously described, associated with the [alpha] rhythm, results in responses which at frequencies around 100 per sec. still show the [alpha] period effect on their amplitudes. This effect disappears after a few cycles as temporal dispersion occurs, and is reminiscent of the [alpha] wave disappearance following stimulation by light. The spontaneous rhythmic activity of the cortex apparently involves regulation of cortical excitability to extrinsic impulses.

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