Transformation of signals by interneurones in the barnacle's visual pathway

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Abstract
The photoreceptors of the median eye of the giant barnacle drive decrementally-conducting neurons in the supraesophageal ganglion termed inverting cells (I-cells) which in turn drive impulse-producing neurons termed amplifying cells (A-cells). Using intracellular recording techniques the role of I-cells was studied in visual processing. Horseradish peroxidase injections show that I-cells are interneurons whose processes are confined to the regions of the photoreceptor terminals on both sides of the bilaterally symmetrical ganglion. In the dark, I-cell membrane potentials (-45 mV) are considerably less negative than those of other ganglion cells (-60 to -70 mV). At the onset of a maintained light, I-cells undergo a transient peak hyperpolarization which declines to a steady-state response. Both response components are graded with light intensity. The reversal potential of the peak of the I-cell light response depends on the external K+ concentration more strongly than does the dark resting potential (3-30 mM-K+). The hyperpolarization may result from an increase in the cell''s permeability to K+. At the offset of light an I-cell undergoes a transient depolarization that overshoots the dark membrane potential. Dimming of a background light also can cause the I-cell membrane potential to overshoot its dark resting value. This overshoot is associated with a large depolarizing synaptic potential in A-cells. An overshoot of the dark resting potential also can be elicited by the break of a hyperpolarizing pulse of current injected into an I-cell. The amplitude of this overshoot increases with pulse duration over a time course of seconds. In the presence of external tetraethylammonium ion (TEA) and tetrodotoxin, (TTX), the break of a hyperpolarizing pulse or the onset of a depolarizing pulse can evoke in an I-cell an action potential whose rate of rise and amplitude depend on the external Ca concentration. This action potential can be maintained by replacement of external Ca with Ba, or blocked by addition of 15 mM-Co to the saline. Depolarizing potential changes in this cell may activate a voltage-sensitive Ca conductance. When hyperpolarizing current pulses are injected into an I-cell, the voltage during the pulse sags back slowly towards the dark resting potential. During hyperpolarization with light or current an I-cell''s membrane properties change over a time course or seconds. The onset of a depolarizing pulse or the offset of a hyperpolarizing pulse of current injected into an I-cell leads to a transient depolarization of a simultaneously impaled A-cell. Synaptic transmission occurs when the I-cell is depolarized to the vicinity of the dark resting potential. The amplitude of the response in an A-cell depends on the rate of change of the I-cell voltage.