Threshold stimulation and accommodation of the Hodgkin-Huxley axon.

The charge-duration and strength-duration relations for just threshold rectangular stimuli were numerically investigated for the Hodgkin-Huxley axons of different lengths and different membrane capacitances under normal conditions and blockage of the development of accommodative processes. Two linear portions could be distinguished on the charge-duration curve. One of them followed the Weiss law. The other one represented a portion of a straight line passing through the zero point of the coordinates. The slope of the second portion was determined by the charge for very short stimuli (Q0), the slope of the first portion, and the maximum time to excitation (tau max). The rheobase reflected the slope of the second portion. Upon varying the fibre length the slope of the first and the second linear portions and the rheobase changed. The membrane capacitance substantially affected both the value of Q0 (as in the case of myelinated fibres) and the rheobase. The accommodative processes affected the Q0, the slope of the first line, tau max, and, consequently, the rheobase. The effect of potassium activation was stronger than that of sodium inactivation. The slope of the first line, tau max, and the rheobase might be considered more comprehensive indicators of the accommodative processes than the usually used indicators.