Midbrain Response to Electrical Stimulation of the Optic Nerve

Abstract

Evoked potentials to electrical stimulation of the optic nerve were recorded by a wick electrode and also with a KCl-filled microelectrode from the optic tectum of the carp. 1. The potential consisted of two positive spikes of about 1 msec. in duration, a positive deflection of a few msec. and a negative one of about 10 msec. The latter was followed by a second negative deflection when the stimulus was sufficiently strong. 2. The negative deflections, especially the second, were facilitated at first, but depressed later by topical application of d-tubocurarine. The second negative deflection was augmented by strychnine. All the positive components were not affected by these drugs. 3. The negative deflection could not follow repetitive stimulation at a rate higher than about 60 cps., but the positive components could. 4. When a microelectrode was inserted into the optic tectum, the negative deflections reversed their sign at a certain depth. The depth of phase reversal was a little deeper with the second deflection. The positive components showed no phase reversal. 5. By simultaneous recording of the action potential of the opti nerve and the evoked potential it was confirmed that the two precedin; positive spikes of the evoked potential are of presynaptic origin. 6. Based on the findings stated above it was inferred that the firs negative deflection represented synaptic potentials at receiving cells, an_??_ that polysynaptic excitation produced the second negative deflection.