Passive and active electrical properties of two giant neurones in Helix aspersa

Abstract

1. 1. Recordings of the response to prolonged depolarizing stimuli were made in two identified giant neurones in the brain of Helix aspersa, E13 and E16. 2. 2. AP frequency adaptation took place to the same degree in both cells, but in E13. the adapted frequency was reached more quickly. The amplitude of after-hyperpolarization of El3 action potentials was larger than that of E16. 3. 3. Small but prominent increases in duration of action potentials and maximum rate of change of membrane potential accompanied adaptation. In E13. these were more prominent in the falling phase, and in E16. in the rising phase. The changes were almost complete 3 sec after the start of the stimulus. 4. 4. Over the small ranges of stimulus intensities employed, the current/voltage relations of the two cells were similar. The time constant (τ) somal/axonal conductance ratio (ρ)and whole soma capacitance (C s) in E13 were somewhat larger than in E16. 5. 5. The action potential frequency of E13 bore an anomalous relationship with the degree of depolarization applied in comparison with E16. This was due to a delayed rectification in the 1/ V characteristic in E13, compared to a linear characteristic in E16. 6. 6. Different ionic mechanisms may underly the conductance changes which regulate adaptation in the two cells.