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The Journal of Neurophysiology Vol. 82 No. 6 December 1999, pp. 3327-3338
Copyright ©1999 by the American Physiological Society
1Lehrstuhl für Neurobiologie, Fakultät für Biologie, Universität Bielefeld, D-33501 Bielefeld, Germany; and 2Centre for Visual Sciences, Research School for Biological Sciences, Australian National University, Canberra, ACT 2600, Australia
Dürr, Volker and
Martin Egelhaaf.
In Vivo Calcium Accumulation in Presynaptic and Postsynaptic
Dendrites of Visual Interneurons. J. Neurophysiol. 82: 3327-3338, 1999. In this comparative in vivo study of
dendritic calcium accumulation, we describe the time course and spatial
integration properties of two classes of visual interneurons in the
lobula plate of the blowfly. Calcium accumulation was measured during
visual motion stimulation, ensuring synaptic activation of the neurons
within their natural spatial and temporal operating range. The compared cell classes, centrifugal horizontal (CH) and horizontal system (HS)
cells, are known to receive retinotopic input of similar direction selectivity, but to differ in morphology, biophysics, presence of dendrodendritic synapses, and computational task. 1) The time course of motion-induced calcium
accumulation was highly invariant with respect to stimulus parameters
such as pattern contrast and size. In HS cells, the rise of
[Ca2+]i can be described by a single
exponential with a time constant of 5-6 s. The initial rise of
[Ca2+]i in CH cells was much faster (
1 s). The decay time constant in both cell classes was
estimated to be at least 3.5 times longer than the corresponding rise
time constant. 2) The
voltage-[Ca2+]i relationship was best
described by an expansive nonlinearity in HS cells and an approximately
linear relationship in CH cells. 3) Both cell classes
displayed a size-dependent saturation nonlinearity of the calcium
accumulation. Although in CH cells calcium saturation was
indistinguishable from saturation of the membrane potential, saturation
of the two response parameters differed in HS cells. 4)
There was spatial overlap of the calcium signal in response to
nonoverlapping visual stimuli. Both the area and the amplitude of the
overlap profile was larger in CH cells than in HS cells. Thus calcium
accumulation in CH cells is spatially blurred to a greater extent than
in HS cells. 5) The described differences between the
two cell classes may reflect the following computational tasks of these
neurons: CH cells relay retinotopic information within the lobula plate
via dendritic synapses with pronounced spatial low-pass filtering. HS
cells are output neurons of the lobula plate, in which the slow, local
calcium accumulation may be suitable for local modulatory functions.
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