Acta Physiologica 48. (1976)
2-3. szám - PHYSIOLOGIA - Pór I.-Fehér O.: The Frequency Dependence of Evoked and Postsynaptic Potentials in the Somatosensory Cortex of the Cat
Acta Physiologica Academiae Scientiarum Hungaricae, Tomus 48 (2— 3), pp. 105—116 (1976) THE FREQUENCY DEPENDENCE OF EVOKED AND POSTSYNAPTIC POTENTIALS IN THE SOMATOSENSORY CORTEX OF THE CAT By I. Pór and 0. Fehér DEPARTMENT OF COMPARATIVE PHYSIOLOGY, ATTILA JÓZSEF UNIVERSITY OF SCIENCES, SZEGED (Received June 8, 1975) Intracellular records were made from neurones of the somatosensory cortex of cats, during stimulation of the ventrobasal complex of the thalamus. The aim of the experiments was to detect correlations between frequency dependence of surface evoked potentials and that of unit discharges. The amplitude of the surface evoked potential showed a strong diminution when the frequency of the thalamic stimulation was raised from 1 cps to 15 cps. In spite of this, frequency dependence in amplitude of unit discharges was never seen. As regards their frequency of occurrence the unit responses (full spikes, dendritic, postsynaptic potentials) behaved differently: a part of them showed increasing, another part gave decreasing occurrence, and the remaining portion did not change it. The authors conclude that temporal dispersion fails to give account for the frequency dependence, therefore further possibilities have to be examined. Evoked potentials are regarded as highly informative signals of central nervous activity. Their mechanism is complex and still awaits final clarification to raise the informative value of records taken from human and animal brains. Basing upon the works of Eccles (1951), Purpura (1959), Kandel et al. (1961) and Creutzfeldt and Kuhnt (1967) it is widely accepted that the evoked potentials can be derived from summation and synchronization of excitatory and inhibitory postsynaptic potentials (PSP-s) taking place in the soma-dendritic membrane of the cortical neurones. This view seems to be supported by the theoretical considerations of Humphrey (1968). At the same time the works of Fox and O’Brien (1975) and Frost and Gol (1966) demonstrated a close correlation between the time course of the evoked potential and the probability of occurrence of unit discharges in the cerebral cortex. In the present work we have made an attempt to examine (1) to what extent evoked potentials can be traced back to a summation of PSP-s and (2) whether PSP-s are in possession of the properties necessary for generation of evoked potentials. As a tool we have made use of the frequency dependence in order to clear up the relation between evoked and postsynaptic potentials. Methods The experiments were performed on 48 adult cats under intraperitoneal pentobarbitalsodium anaesthesia (40 mg/kg). After venous and tracheal cannulation and, in some cases immobilization with gallamine triethyiodide, the cerebral cortex was exposed around the primary somatosensory area (Sj) and a macroelectrode was placed on the surface, covered Acta Physiologica Academiae Scientiarum Hungaricae 48, 1976