The operation of connexions between photoreceptors and large second-order neurones in dragonfly ocelli

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.

• 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.

• 2. Passive membrane properties of L-neurones cannot contribute significantly to the cutback in the hyperpolarising response of these neurones to light on.

• 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.

• 4. No evidence for feedback connexions from L-neurones to photoreceptors has been found.

• 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.

• 6. Some L-neurones make rapidly decrementing inhibitory connexions with other L-neurones.

• 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.

• 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.