The admittance of the squid giant axon at radio frequencies and its relation to membrane structure.

Abstract

SUMMARY1. The admittance of the squid giant axon membrane has been measured, using an intracellular electrode, at frequencies up to 40 MHz. The existence of a radio frequency dispersion, previously detected with extracellular electrodes (Cole, 1976) and attributed to the Schwann cell layer, has been confirmed and followed to higher frequencies.2. For a comparable method of analysis, membrane parameters similar to those given by Cole (1976) have been calculated. The radio frequency dispersion has a centre frequency at approximately 1-8 MHz, and the properties of a parallel combination of a 28 nF cm-2 capacity and a 3-3 Q cm2 resistance.3. When the axon membrane capacity is calculated, taking into account the radio frequency dispersion, as described above, the capacity remains frequency dependent throughout the range studied.4. If it is assumed that at high frequencies the axolemma capacity becomes constant at approximately the value for a lipid bilayer, a radio frequency dispersion is found which cannot be accounted for in terms of a simple equivalent circuit with two passive components, but appears to arise from a network with a distribution of relaxation times. This result could be consistent with the morphology of the Schwann cell layer.5. The radio frequency dispersion referred to in (4) can be described reasonably well by a circuit with two dispersions having centre frequencies of 250 kHz and 3-2 MHz respectively. The corresponding axolemma capacity (100-500 kHz) would be approximately 0-6 ,#F cm2.6. It is argued that between 50 and 100 kHz the geometrical capacity arising from the non-polar regions of the membrane is a major contributor to the axon membrane capacity, and that capacity variations arising from compositional changes in the lipid bilayer are best monitored in this frequency range.

Non-monotonic kilohertz frequency neural block thresholds arise from amplitude- and frequency-dependent charge imbalance

“…Monotonic increases in block threshold with frequency are expected with charge-balanced sinusoidal or rectangular KHF waveforms due to the low pass filtering property of the axonal membrane 19,48 . While frequency-dependent membrane capacitance reduces low pass filtering in computational models 49,50 , previous measurements showed only a factor of 2 reduction in capacitance between 10 and 100 kHz 51,52 , rather than the factor of > 10 reduction needed to reverse the low pass filtering and generate non-monotonic thresholds within this frequency range. Taken together with our results, the implication is that non-monotonic frequency dependent block thresholds arise from charge imbalances due to DC offsets or due to charge-imbalanced waveform asymmetries.…”

Effects of frequency-dependent membrane capacitance on neural excitability

“…The parameterization of c(f) in this study was based on data from a limited number of studies on squid giant axons (Haydon et al, 1980, Haydon and Urban, 1985, Takashima and Schwan, 1974). These studies all indicate that the membrane capacitance is approximately constant from 100 Hz to 1 kHz and declines by approximately 50 % between 1 kHz and 100 kHz.…”

High-Frequency Stimulation Selectively Blocks Different Types of Fibers in Frog Sciatic Nerve

“…Simulation work in our lab has attempted to understand the decrease in block thresholds at higher frequencies and a modified Hodgkin-Huxley (HH) model with a frequency-dependent capacitance, based on data from squid axons, showed a deviation from the linear threshold behavior [25, 26]. The HH model assumes that capacitance is constant even at higher frequencies when actually, experimental data from the giant squid axon shows that capacitance decreases as frequencies increase [27]. Incorporating this frequency-dependent capacitance into a HH model showed a non-linear block threshold response, but the nonmonotonic block threshold response observed experimentally could not be replicated.…”