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The Journal of Neurophysiology Vol. 87 No. 5 May 2002, pp. 2262-2270
Copyright ©2002 by the American Physiological Society
Department of Physiology, University of Wisconsin, Madison, Wisconsin 53706
Ferragamo, Michael J. and
Donata Oertel.
Octopus Cells of the Mammalian Ventral Cochlear Nucleus Sense the
Rate of Depolarization. J. Neurophysiol. 87: 2262-2270, 2002. Whole cell patch recordings in slices show
that the probability of firing of action potentials in octopus cells of
the ventral cochlear nucleus depends on the dynamic properties of
depolarization. Octopus cells fired only when the rate of rise of a
depolarization exceeded a threshold value that varied between 5 and 15 mV/ms among cells. The threshold rate of rise was independent of
whether depolarizations were evoked synaptically or by the
intracellular injection of current. Previous work showed that octopus
cells are contacted by many auditory nerve fibers, each providing less than 1-mV depolarization. Summation of synaptic input from multiple fibers is required for an octopus cell to reach threshold. In firing
only when synaptic depolarization exceeds a threshold rate, octopus
cells fire selectively when synaptic input is sufficiently large and
synchronized for the small, brief unitary excitatory postsynaptic
potentials (EPSPs) to sum to produce a rapidly rising depolarization.
The sensitivity to rate of depolarization is governed by a
low-threshold,
-dendrotoxin-sensitive potassium conductance (gKL). This conductance also shapes
the peaks of action potentials, contributing to the precision in their
timing. Firing in neighboring T stellate cells depends much less
strongly on the rate of rise. They lack strong
-dendrotoxin-sensitive conductances. Octopus cells appear to be
specialized to detect synchronization in the activation of groups of
auditory nerve fibers, a common pattern in responses to natural sounds,
and convey its occurrence with temporal precision.
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