Accommodation to hyperpolarization of human axons assessed in the frequency domain

Human axons in vivo were subjected to subthreshold currents with a threshold-"ZAP" profile (Impedance [Z] Amplitude Profile) to allow the use of frequency domain techniques to determine the propensity for resonant behavior, and to clarify the relative contributions of different ion channels to their low-frequency responsiveness. Twenty-four studies were performed on the motor and sensory axons in 6 subjects. The response to oscillatory currents was tested between 'DC' and 16 Hz. A resonant peak at ~2 to 2.5 Hz was found in the response of hyperpolarized axons, but there was only a small broad response in axons at resting membrane potential (RMP). A mathematical model of axonal excitability developed using DC pulses provided a good fit to the frequency response for human axons, and indicated that the hyperpolarization-activated current I_h, and the slow potassium current I_Ks are principally responsible for the resonance. However the results indicate that if axons are hyperpolarized more than -60% of resting threshold, the only conductances that are appreciably active are I_h and the leak conductance - i.e., that the activity of these conductances can be studied in vivo virtually in isolation at hyperpolarized membrane potentials. Given that the leak conductance dampens resonance it is suggested that the -60% hyperpolarization used here is optimal for I_h. As expected differences between the frequency responses of motor and sensory axons were present and best explained by reduced G_Ks, up-modulation of I_h and increased persistent Na⁺ current, I_NaP (due to depolarization of RMP) in sensory axons.

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