ADAPTATION IN INVERTEBRATE NEURONS

Injection of constant depolarizing current pulses into nervous cells induce repetitive firing. Some neurons show adaptation; the firing declines in frequency throughout the duration of the stimulus. Two types of behaviour of repetitive discharges can be observed: 1) the cells adapt slowly to a steady-state frequency and the stimulating currents induce sustained spike dicharges as long as the currents are maintained, 2) the cells adapt rapidly and stop to fire during the stimulus. This phenomenon, as a function of depolarizing current strength, has been studied in neurons of Helix pomatia and Arion subfuscus subesophageal ganglia. Arion subfuscus neurons mainly adapt rapidly, while in Helix pomatia the two types of adaptation were observed. In the slowly adapting neurons there is a linear relation between initial firing frequency, as well as steady state firing frequency, and current strength, with a tendency for a decrease at higher currents. A given nerve cell was characterized by means of four parameters obtained following a behavioural model accounting also for cells spontaneously discharging at rest. In Helix pomatia neurons, in the current frequency linearity range, the parameter mean values varied in different cells: the proportionality constant between the initial firing frequency and the stimulating current between 0.18 and 4.98 Hz/nA, the decay constant of the frequency between 1.02 and 3.85 s, the inhibitory current from a single nerve impulse in the range 0.05 ¸ 0.95 nA and the decay time constant of the inhibitory current between 1.74 and 22.33 s. Experimental results have been analyzed considering inhibitory current, electrogenic sodium pump and other proposed adaptation parameters.