. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. ION ACCESS TO THE GIANT SYNAPSE 475 Figure 5. Effect of Ca^^-free infusion on synaptic transmission at the giant synapse, a. Infusion of Ca^'^-free ASW (onset at arrow head) resulted in a failure of the action potential component of the intracellularly recorded evoked response in the postsynaptic giant axon followed by the gradual disappearance of the EPSP. Calibration: 50 m V, 5 s. b. Same preparation as in (a) at a fast trace sweep sjjeed. The superimposition of EPSPs that are gradually declining amplitude obscures the
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. ION ACCESS TO THE GIANT SYNAPSE 475 Figure 5. Effect of Ca^^-free infusion on synaptic transmission at the giant synapse, a. Infusion of Ca^'^-free ASW (onset at arrow head) resulted in a failure of the action potential component of the intracellularly recorded evoked response in the postsynaptic giant axon followed by the gradual disappearance of the EPSP. Calibration: 50 m V, 5 s. b. Same preparation as in (a) at a fast trace sweep sjjeed. The superimposition of EPSPs that are gradually declining amplitude obscures the visualization of individual traces. Calibration 20-mV, time = 1 ms. Both La^^ and Cd^^ also had a limited depolarizing effect on the resting potential of the postsynaptic axon. The basis for this effect is not well established. Cd^^ has been reported to reversibly slow the activation kinetics of voltage sensitive K^ channels (Gilley and Armstrong, 1982). If some of the K^ channels were open at rest, their closure would result in a decline in the resting potential. Cd^^ and La^^ may also reduce the resting potential by direct effects on membrane ion permeability, for example La^^ increases the flux of Na^ ions (P. DeWeer, pers. comm.). The results of this study show that ion penetration into the stellate ganglion is greatly enhanced by arterial perfusion. Clearly substances can gain quick access to a small fraction of the giant synapse from the bathing medium; for example Ca^^ iontophoresed on to the presynaptic terminal in Ca^^-free medium is capable of triggering limited transmitter release (Miledi and Slater, 1966). However, a substantial fraction of the presynaptic terminal must be inaccessible to bath application, as discussed above. The usefulness of the perfusion technique is that the ions must gain access to the entire synapse or it would not be possible to completely eliminate transmitter release. Similar arguments can be made for the postsynaptic giant axon where bath
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