MAPPING OF HELIX POMATIA NEURONS BY VOLTAGE-CLAMP MEASUREMENTS

Neurons in the abdominal ganglia of land snail Helix pomatia were identified and mapped by means of data from voltage-clamp measurements. The cells were impaled with two microelectrodes and voltage-clamped by means of a conventional type electronic circuit. The twelve examined cells had a diameter between 45 and 215 microm and a membrane resistance between 150 and 3000 ohm cm2. During prolonged voltage-clamp steps delayed outward current rises to a peak and then declines to non-zero steady-state. Outward currents associated wíth depolarizing voltage step from dífferent holding potentials were analyzed and the time decay studied. The outward currents minus the current values at the steady-state were plotted on a logarithmic scale against time. In a great number of cells the studied currents appeared to reach a steady value with a single exponential. In these neurons the decay time constant of the delayed outward current ranged between 50 and 750 ms. The pattern of delayed outward current varies considerably between cells. Time constants, for inactivation were in some cells voltage-dependent. In some cases where the decline of the delayed outward current was more complicated, a simple exponential time course was obtained when extracellular Ca++ was replaced by Ba++.