Squid axons, membrane currents

Fig. 14.14. Unit for potential transient measurement during excitation of a squid axon by current pulses from electrodes 1 and 1 2 and 2 are micropipettes. (Reprinted from A. L. Hodgkin and A. F. Huxley, J. Physiol. 116 497, 1952. Reprinted from J. Koryta, Ions, Electrodes and Membranes, Fig. 93. Copyright J. Wiley Sons, Ltd. 1991. Reproduced with permission of J. Wiley Sons, Ltd.)...

Table 5B summarises the data for comparison [43,44]. In order to calculate current densities for the experiments on the nodal membrane, the area was assumed to be 50 jum [43,44]. It may be seen that (1) in the nodal membrane is about ten times larger than in the squid axon membrane (2) the temperature coefficients for and for are very similar for both membranes (3) Vq in the node is 7.7 mV more negative (4) the effective charge of the dipoles, a, is somewhat larger in the nodal membrane, and (5) the least change in membrane potential to produce an e-fold change in the steady-state distribution of charges (kT/a) is 14.9 mV in the node and 18.5 mV in the squid axon [58]. 

An approach somewhat analogous to that of Barnes and Hu was adopted by Pickard and Rosenbaum.They examined the effects of an impressed radiofrequency field on the resting potential across the plasma membrane and the effects of such fields with regard to ion transit time within an ion channel spanning the membrane. Of interest is their conclusion that the frequency range at which transit time effects are no longer important lies well below the microwave portion of the spectrum. This estimate was based upon data for the sodium ion current in the squid axon membrane. 

Excitation of the squid axon membrane in iso-osmotic potassium chloride solution has been investigated [46 8] and is found to be particularly interesting. In this context, current-voltage curves were obtained by keeping (i) the current constant and (ii) the voltage constant. A typical curve [48] is shown in Fig. 8.7 which displays hysteresis. 

Figure 8.7. Voltage-current curve for squid axon membrane in 0.5 M KCl control,-------current control [3b].

A less direct estimate of and is obtained from the amplitude of the power spectra of current fluctuations. Basically, power spectra contain more information than the simple variance, as shown by eqn.(7). The additional information is very useful to ascertain to what extent the measured current fluctuations can be attributed to the flickering of ion specific channels between open and closed states rather than to other noise sources. This control was particularly desirable in the early studies of nerve membrane noise, which revealed the presence of large 1/f spectral components of still unclear origin (16). Indeed, the first unequivocal characterizations of sodium and potassium channel noise in the squid axon membrane (13) and of sodium channel noise in frog nodes (14,15) were obtained from the fitting of measured spectra with the superposition of Lorentzian-like spectra plus 1/f components. From the low frequency asymptote and the cut-off frequency of the Lorentzian components estimates of Y =12 pS and were obtained for the ionic channels of quid... 

Baker, P. F. (Ed.), The Squid Axon. Current Topics in Membranes and Transport, Vol. 22, Academic Press, Orlando, 1984. 

Figure II. Membrane currents In voltage-clamped squid axons.

TABLE I. The Hodgkin-Huxley equations which describe the relationship between total membrane current 1 and transmembrane potential V in the squid giant axon [see (1)]. 

Takenaka T, Horie H, Hori H. Effects of fatty acids on membrane currents in the squid giant axon. J Membr Biol 1987 95 113-120. 

Type II pyrethroids also modify the sodium channel kinetics (20-24) In a squid axon internally perfused with 10 pH deltamethrin a step depolarization from a holding membrane potential of -80 mV to -20 mV produced a peak transient sodium current which was followed by a slow current (Figure 3). With a prolonged, 510 msec depolarization the slow component of sodium current was hardly inactivated. The tail current associated with step repolarization of the membrane decayed very slowly with a dual time constant of 33 msec and 1074 msec. Like the peak... 

In crayfish giant axons most veratridine effects persist, but the maintained current at the holding potential between pulses during a train vanishes on washout (Warashina 1985). Only partial recovery is reported for externally applied alkaloid to squid giant axons (Ohta et al. 1973), but when this preparation is perfused internally, full reversibility is achieved, possibly because washing on either side of the membrane was feasible (Meves 1966). In this connection it is interesting to note that externally applied veratridine (100 M at 5°C) slowly but markedly depolarizes non-perfused squid axons whereas depolarization is much less if at the same time the axons are internally perfused and thus washed with toxin-free solution (Seyama et al. 1988). 

Hodgkin and Huxley studied the membranes of squid axons and found that the instantaneous current-voltage relations of both Na+ and K+ channels are linear when external concentrations are normal (property 4 above). Instantaneous means that measurements are made at times short compared with the kinetics of the M process. The Ohm s law statement of this relation for Na+ channels is simply... 

FIG. 16 Families of membrane currents associated with step depolarization (10-mV steps) in a squid giant axon before and during external applications of 3 x 10 M TTX and after washing with toxin-free medium. Note that TTX blocks transient sodium currents without any effect on steady-state potassium currents (Narahashi, 2001). 

Fig. 7, Experimental protocol to measure selectivity ratios in the giant axon of squid. (A) Set of superimposed membrane current records in response to depolarising pulses to -60, —50, —40, -30, - 18, —8, 2, 10, 20, 30 and 40 mV with the fibre in 230 mM Na -sea water. (B) Set of superimposed membrane current records in response to depolarising pulses to —40, -30, — 18, —8, 2, 10. 20, 30, 40 and 50 mV with the fibre in 103 mM hydrazinium -sea water. (C) Set of current records after the addition of 50 mM TTX to the hydrazinium -sea water. (D) Superimposed membrane action potentials and passive membrane response for depolarising and hyperpolarising current pulses of increasing size, with the fibre in hydrazinium -sea water after the removal of TTX. Calibrations Vertical represents 2.75 mA/cm for A, B, C and 100 mV for D. The horizontal calibration represents 10 ms for A. B, C and D. Henderson and Hasselbach s equation for a proton acceptor of the type R-NH2,...

The first experimental results which suggest the existence of two different membrane structures controlling the movements of sodium and potassium during a nervous impulse was produced by Hodgkin et al. [26]. They observed that if a squid giant axon is subjected to sudden displacements of the membrane potential in the positive direction, the resulting membrane current i could be analysed as the sum of three components,... 

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