Adaptation in the input‐output relation of the synapse made by the barnacle's photoreceptor.

Abstract

SUMMARY1. A study was made of synaptic transmission between the four median photoreceptors ofthe giant barnacle (Balanus nubilus) and their post-synaptic cells (I-cells). Simultaneous intracellular recordings were made from the presynaptic terminal region of a photoreceptor and from the soma of an I-cell.2. The photoreceptor's membrane potential provided feed-back to bath electrodes that passed current into the receptors' axons, permitting the voltage to be controlled at the point of arborization of their presynaptic terminals. Simultaneous recordings from a second photoreceptor showed that its voltage tracked the first.3.Step depolarizations of the receptors from their dark resting potential (about -60 mV) caused hyperpolarizations of the I-cell that reached a peak, then decayed to a plateau value. The amplitude of the I-cell's response grew with presynaptic depolarizations, saturating at presynaptic values 10-20 mV depolarized from dark rest.

Adaptation and responses to changes in illumination by second- and third-order neurones of locust ocelli

“…For an incremental change of fixed contrast, the change in L-neurone membrane potential is the same, independent of background intensity, over a range of at least 4 log units. This kind of adaptation has been reported before in a variety of neurones involved in early visual processing, including second-order neurones of the compound eyes of flies and dragonflies (Laughlin and Hardie 1978), second-order neurones of barnacle ocelli (Hayashi et al 1985); and cells postsynaptic to photoreceptors in the vertebrate retina (Normann and Perlman 1979). In image-forming eyes, such as arthropod compound eyes, an important function for adaptation is to aid registering objects of particular reflectances against a background over a variety of environmental lighting conditions (review, Laughlin 1989).…”

“…Adaptation is more complete, and also more rapid, in large monopolar cells of the compound eyes of flies and dragonflies (Laughlin and Hardie 1978), and in the secondorder "I" cells of barnacle oceUi (Hayashi et al 1985).…”

“…This contrasts with synapses between photoreceptors and the postsynaptic large monopolar cells in the fly compound eye (Laughlin et al 1987) in which the curve that described the relation between the potentials in the two cells shifts with changes in background illumination, so that the synapse operates at high gain under a variety of lighting conditions. Similarly, the curve describing the relationship between potentials in a photoreceptor and second-order "I-cells" of barnacle ocelli shifts along the photoreceptor potential axis when the photoreceptor holding potential is altered (Hayashi et al 1985).…”

The Performance of Synapses That Convey Discrete Graded Potentials in an Insect Visual Pathway

“…Synaptic gain here is usually Ͻ0.5, in comparison with 20 at output synapses from ocellar photoreceptors (Simmons, 1995), which makes L -neurons extremely sensitive to changes in light (Wilson, 1978a). A high gain is a important for improving signal-to-noise ratio (Laughlin et al, 1987) and is characteristic of synapses from photoreceptors: it is 6 in the fly compound eye (Laughlin et al, 1987), 2.5 for barnacle ocelli (Hayashi et al, 1985), and perhaps as great as 100 for outputs from photoreceptors in the vertebrate retina (Shiells, 1995). Elsewhere, synaptic gain is lower: for example, ϳ1 at outputs from nonspiking neurons in locust thoracic ganglia (Siegler, 1985), between motion-sensitive neurons in the locust brain (Rind, 1984), and between a proprioceptor and a motor neuron in the crab (Blight and Llinás, 1980).…”

“…Elsewhere, synaptic gain is lower: for example, ϳ1 at outputs from nonspiking neurons in locust thoracic ganglia (Siegler, 1985), between motion-sensitive neurons in the locust brain (Rind, 1984), and between a proprioceptor and a motor neuron in the crab (Blight and Llinás, 1980). A consequence of high gain is that a synapse only operates over a narrow range of presynaptic potentials, so that adaptation is necessary to remove any sustained signal proportional to mean intensity that the photoreceptor produces (Laughlin and Hardie, 1978;Hayashi et al, 1985, Simmons, 1995.…”