Intracellul�re Reizung corticaler Nervenzellen

Advertising of firing during long pulses seems to be a process which appears only if the firing level is reached or surpassed, and accommodation to linearly rising transmembranous currents is absent or negligible in cortical cells.

Abstract

1. Intracellular records were taken from pyramidal cells of the pericruciate gyrus of anesthetized cats. Action potentials of cortical neurons do not always show an IS/SD break, but are followed regulary by a primary polarization and a delayed depolarization. The firing level is 6.0±1.6 mV (mean and S.D.). 2. The time constant of cortical pyramidal cells as determined with the strengthlatency method is 8.5±2.2 msec. Almost the same value has been obtained by measuring the potential decay after break of a depolarizing and make of a hyperpolarizing rectangular pulse. Depolarizing make and hyperpolarizing break give slightly higher values. 3. The mean rheobase was 2.6±1.2×10−10 A. From the individual values for rheobase and firing level of the different cells a mean neuron resistance of 28±12×106 Ω has been calculated. This value corresponds well to the bridge unbalance after penetration of a cell membrane. 4. With long depolarizing pulses adaptation of firing frequency appears during the first 70–100 msec and is followed by a steady state frequency which is linearly dependent from the current strength. Only little further drop of firing frequency can be seen during the steady state. The initial frequency decrease is accompanied by a diminution of the action potential measured from the firing level. This diminution appears eventhough the spike interval increases, and its time course is much longer than the transient change of the membrane potential. It suggests a change of firing level during adaptation possibly due to a delayed Na+-inactivation. 5. Accommodation to linearly rising transmembranous currents is absent or negligible in cortical cells. Adaption of firing during long pulses therefore seems to be a process which appears only if the firing level is reached or surpassed. 6. Some possible implications on functional aspects of the findings are discussed.