Squid axon, current

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

Conti, F., and E. Neher, Single channel recordings of K+ currents in squid axons, Nature, 285, 140 (1980). 

The ion-channel blocking mechanism has been widely tested and found to be important in both pharmacology and physiology. Examples are the block of nerve and cardiac sodium channels by local anesthetics, or block of NMDA receptor channels by Mg2+ and the anesthetic ketamine. The channel-block mechanism was first used quantitatively to describe block of the squid axon K+ current by tetraethylammonium (TEA) ions. The effects of channel blockers on synaptic potentials and synaptic currents were investigated, particularly at the neuromuscular junction, and the development of the single-channel recording technique allowed channel blockages to be observed directly for the first time. 

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

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.)...

Measurements have been reported by Keynes Meves (1993) of the probability functions both for the initial opening of the Na channels in squid axons, and for the reopenings that generate the late current in the inactivated steady state. Plots... 

Cummins An important factor in understanding inactivation and the generation of persistent currents may be subtle differences between the skeletal muscle and neuronal channels or isoforms. Patlak Ortiz (1986) found that noninactivating or persistent Na currents in skeletal muscle cells were very small, whereas in the squid axons persistent Na+ currents are much larger. There seem to be subtle but important differences between the different isoforms. There may be differences in cooperativity. All of this has to be worked out very carefully. 

Bean We think of the resurgent current as being connected with a second inactivated state that recovers through the open state. The open state would therefore be the same as the usual open state, which would be in contrast to the model for the squid axon, where the idea is that there is a second open state kinetically. 

Chandler WK, Meves H 1970a Sodium and potassium currents in squid axons perfused with fluoride solutions. J Physiol 211 623-652... 

Keynes RD, Elinder F 1998a On the slowly rising phase of the sodium gating current in the squid axon. Proc R Soc Lond B Biol Sci 265 255—262... 

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... 

CiAY JR and Shlesingee MF (1983) Efifiects ofi external caesium and rubidium on outward potassium currents in squid axons. Biophys J 42 43-53. 

---