Quantitative aspects of repetitive firing of Helix aspersa neurones, caused by injected currents (long lasting)

Suboesophageal ganglion neurones of Helix aspersa were stimulated by different long lasting (1-20 s) current strengths (0-30 nA). The electrical response has been studied. Repetitive firing to intracellular injected current can be obtained in a large number of snail neurones, but some cells are not stable for a long enough time. This work is limited to the neurones repetitive discharging for all the stimulation time. Both silent and rhythmic cells were used. As in many other preparations the instantaneous firing frequency (defined as the inverse of the time interval between successive nerve impulses) decreases with respect to the time (t), to reach a steady state value. The frequency of discharge (f) is related to current strength (with a linearity range). Arrhythmias were obtained at high current intensities (I). Experimental (f,t) (f,I) curves have been analyzed with the Sokolove model (Biophys. J. 12: 1429). The model exponential decay well describes the experimental characteristics, also for autorhythmical neurones. Some experiments were also performed changing the external potassium concentration.