Summary
Photoreceptors and large second-order neurones (L-neurones) of dragonfly ocelli have been penetrated simultaneously with microelectrodes to study the operation of the synapse between them. The responses of L-neurones to changes in illumination are of opposite polarity and are more phasic than those of photoreceptors.
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1.
When pulses of light are superimposed on a constant background illumination, the phasic nature of L-neurone responses is often enhanced. Sometimes, during a light stimulus, an L-neurone is depolarised relative to its resting potential. With rapidly repeated short pulses of light, responses of L-neurones decrement markedly, although the full response of photoreceptors is maintained.
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2.
Passive membrane properties of L-neurones cannot contribute significantly to the cutback in the hyperpolarising response of these neurones to light on.
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3.
When depolarising or hyperpolarising currents are injected into a photoreceptor, responses in an L-neurone it synapses with are of opposite polarity and markedly phasic. The voltage gain of the connexion between one photoreceptor and an L-neurone can be as great as nine.
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4.
No evidence for feedback connexions from L-neurones to photoreceptors has been found.
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5.
Conductance measurements on some L-neurones indicate that synaptically-induced currents may make a small contribution to the cutback in the hyperpolarising response to light on.
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6.
Some L-neurones make rapidly decrementing inhibitory connexions with other L-neurones.
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7.
A variety of neurones which respond to changes in illumination by alterations in spike rate have been found. Some of these connect with L-neurones.
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8.
Possible mechanisms for the cutback in the hyperpolarising response of an L-neurone at light on include intrinsic properties of the pre- or postsynaptic terminals, and excitatory synapses made by small second-order neurones on L-neurones.
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Simmons, P.J. The operation of connexions between photoreceptors and large second-order neurones in dragonfly ocelli. J. Comp. Physiol. 149, 389–398 (1982). https://doi.org/10.1007/BF00619154
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DOI: https://doi.org/10.1007/BF00619154