The passive electrical properties of the membrane of a molluscan neurone.

1. The passive electrical properties of the membrane of the gastrooesophageal giant neurone (G cell) of the marine mollusc, Anisodoris nobilis were studied with small current steps.2. The membrane transient response can be fitted with a theoretical curve assuming as a model for the cell a sphere (soma) connected to a cable (axon). The axo-somatic conductance ratio (rho), determined by applying this model, is large (approximately 5) and the membrane time constant (tau) is long (approximately 1 sec).3. When the actual surface area of the cell, corrected for surface infoldings, and the spread of current along its axon is taken into account, the electrical measurements imply a specific resistance of the membrane of approximately 1.0 MOmega.cm(2).4. Estimates of specific membrane capacity, either from measurements of the initial portion of the membrane transient or from the ratio of the time constant to the specific membrane resistance are close to the value of 1 muF/cm(2) expected for biological membranes.5. Thus, our measurements of specific capacitance, time constant, length constant and axo-somatic conductance ratio all indicate that the value found for the specific membrane resistance of the G cell, while unexpectedly large, is valid.6. The magnitude of this value suggests that the conductance (permeability) of its membrane to ions is much smaller than that previously assumed for nerve membranes; this small conductance may be related to the larger surface-to-volume ratio of the G cell.