Pathways mediating metrazol induced irradiation of visual impulses An experimental study in the cat

Abstract

Studies of the irradiation pathways for photic-Metrazol induced impulses have been made in the cat. 1. 1. It has been confirmed that Metrazol increases the amplitude of the evoked potentials to photic stimulation in the visual cortex. 2. 2. Two pathways for the irradiation of responses to the frontal lobe have been established and isolated anatomically: one is transcortical and one is subcortical. There is evidence to suggest the existence of a third pathway (cortico-subcortico-cortical). 3. 3. The subcortical pathway demonstrated after ablation of the visual areas has been identified with the “unspecific” reticulothalamic pathway for the visual modality (Magoun et coll., Dell et coll.) because identical regions in the brain stem appear to be involved. The latency of frontal irradiation responses elicited along this sub-cortical pathway has been found to be more than 40 msec. (42–50 msec.), i.e., “long latency” irradiation. The underlying mechanisms was sensitive to the experimental procedure itself and ceased to function if the experiment was prolonged. 4. 4. It has been confirmed that frontal irradiation potentials in the intact preparation have a latency of 21–38 msec. from the light flash (“short latency irradiation”). 5. 5. The transcortical pathway was demonstrated after the removal of mesencephalic and diencephalic structures (except the lateral geniculate body and its immediate surroundings). Frontal irradiation responses mediated via this pathway had a latency of the same order as in the intact preparation (21–38 msec., “short latency irradiation”). This type of irradiation was unaffected by the experimental procedure. 6. 6. From the timing of visual events in the central nervous system in non-anesthetized conditions preparations without Metrazol (Ingvar and Hunter 1955a), as well as in preparations with Metrazol, it is concluded that the primary ascending visual pathways and the visual cortical projection areas play a leading role during irradiation in the intact brain. 7. 7. Transcallosal connections may transmit irradiation potentials between the hemispheres. 8. 8. At the Metrazol concentrations used, no evidence has been found of an extraneuronal irradiation of visual impulses. 9. 9. At a certain Metrazol concentration when light flashes of frequencies of 2.5-4 flashes per sec. are used for stimulation, the irradiation responses will either attenuate and disappear or become driven into a 3/sec. spike and wave pattern which usually leads to a seizure. Inherent cortical factors have been considered responsible for bringing about the attenuation. 10. 10. It has been confirmed that 3 per second spike and wave activity may occur in a self-sustained form during photo-Metrazol activation. 11. 11. Transcortical irradiation is blocked by the anticonvulsant Tridione. This finding does not exclude an action of the drug upon subcortical structures.