Intercellular potentials of CNS neurons in the pteropod molluskClione limacina

Abstract

Intracellular potentials were studied in giant central nervous system (CNS) neurons of the pteropod molluskClione limacina which lives in the White Sea. The mean amplitude of the action potential (AP) and the resting potential (RP) of 30 neurons from different ganglia was 56. 9±3. 3 mv (35–94 mv) and 46. 6±1. 3 mv (33–60 mv) respectively. The maximal rate of rise of the AP was 20–50 v/sec. Many APs had well-marked after-hyperpolarization (mean amplitude 9.8±1. 2 mv, duration 134. 7±20. 4 msec). In some neurons spontaneous excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) were recorded constantly. Mainly IPSPs were recorded from giant neurons of the cerebral ganglia. The mean amplitude of single IPSPs was 4. 9±0. 6 mv and their duration varied from 100 to 500 msec. The reversal potential was about 60 mv. EPSPs recorded from other CNS neurons were of similar amplitude and duration.

The AP amplitude showed no significant change during fluctuations of the external Na ion concentration from 250 to 500 mM, After complete removal of Na⁺ excitability and ability to generate APs disappeared in the course of 3–7 min. By the end of this time APs of some neurons were sharply reduced. This process was reversible. In some giant neurons kept in running salt solution with a reduced Ca⁺⁺ concentration (mM) the ordinary single APs were converted into double APs, then into burst discharges, and finally into prolonged APs. This abnormal unit response was abolished by artificial hyperpolarization of the cell.

Complete replacement of CaCl₂ by iso-osmotic concentrations of MgCl₂ evoked a rapid decrease in AP amplitude. Addition of 8–10 mM MnCl₂ to the normal salt solution reversibly suppressed AP generation by the cell.