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The Journal of Neuroscience, December 1, 1999, 19(23):10584-10594

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

Peter J. Simmons

Department of Neurobiology, University of Newcastle upon Tyne, Newcastle upon Tyne NE2 4HH, United Kingdom

Synapses from nonspiking neurons transmit small graded changes in potential, but variability in their postsynaptic potential amplitudes has not been extensively studied. At synapses where the presynaptic signal is an all-or-none spike, the probabilistic manner of neurotransmitter release causes variation in the amplitudes of postsynaptic potentials. I have measured the reliability of the operation of synapses that convey small graded potentials between pairs of identified large, second-order neurons in the locust ocellar system. IPSPs are mediated by small rebound spikes, which are graded in amplitude, in the presynaptic neuron. A transfer curve plotting amplitudes of spikes against amplitudes of IPSPs has a characteristic S shape with a linear central portion where IPSP amplitude is between 0.2 and 0.6 as large as spike amplitude but shows appreciable scatter. Approximately half of the scatter is attributable to background noise, most of which originates in photoreceptors and persists in darkness. The remaining noise is intrinsic to the synapse itself and is usually 0.3-0.7 mV in amplitude. It limits the resolution with which two spike amplitudes can be distinguished from one another to ~2 mV and, because the linear part of the transfer curve occupies ~10 mV in spike amplitudes, limits the number of discrete signal levels that can be conveyed across the synapse to approximately five. The amplitude of the noise is constant throughout the synaptic operating range, which means it is unlikely that presynaptic membrane potential controls transmitter release by setting a single probability level for quantal release.

Key words: synapse; resolution; noise; locust; ocellus; transfer curve

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Copyright © 1999 Society for Neuroscience  0270-6474/99/192310584-11$05.00/0

This article has been cited by other articles:

				U. Beckers, M. Egelhaaf, and R. Kurtz Synapses in the Fly Motion-Vision Pathway: Evidence for a Broad Range of Signal Amplitudes and Dynamics J Neurophysiol, March 1, 2007; 97(3): 2032 - 2041. [Abstract] [Full Text] [PDF]

				M. M. Parsons, H. G. Krapp, and S. B. Laughlin A motion-sensitive neurone responds to signals from the two visual systems of the blowfly, the compound eyes and ocelli J. Exp. Biol., November 15, 2006; 209(22): 4464 - 4474. [Abstract] [Full Text] [PDF]

				P. J. Simmons and R. de Ruyter van Steveninck Reliability of Signal Transfer at a Tonically Transmitting, Graded Potential Synapse of the Locust Ocellar Pathway J. Neurosci., August 17, 2005; 25(33): 7529 - 7537. [Abstract] [Full Text] [PDF]

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				P. Simmons Intrinsic noise at synapses between a wing hinge stretch receptor and flight motor neurons in the locust J. Exp. Biol., January 1, 2001; 204(1): 127 - 138. [Abstract] [PDF]

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