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The performance of synapses that convey discrete graded potentials in an insect visual pathway

Lookup NU author(s): Dr Peter Simmons

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Abstract

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 back- ground 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.


Publication metadata

Author(s): Simmons PJ

Publication type: Article

Publication status: Published

Journal: Journal of Neuroscience

Year: 1999

Volume: 19

Issue: 23

Pages: 10584-10594

Print publication date: 01/12/1999

ISSN (print): 0270-6474

ISSN (electronic): 1529-2401

Publisher: Society for Neuroscience

URL: http://www.jneurosci.org/content/19/23/10584.full.pdf+html

PubMed id: 10575054


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