Bursting Activity in Weakly Electric Fish - Computational Neuroscience: A Comprehensive Approach - page 213

8.1 Introduction

Neurons in many sensory systems tend to fire action potentials intermittently with
spikes grouped into bursts of high-frequency discharge. Functionally, bursts have
been implicated in many different phenomena, such as efficient transmission of sen-
sory information [76], regulation of information flow during slow-wave sleep [116],
selective communication between neurons [58], epileptic seizures [84], and synaptic
plasticity [94]. In recent years, evidence has accumulated that bursts indeed encode
sensory information and that they may even be more reliable indicators of important
sensory events than spikes fired in tonic mode [47, 76, 82, 86, 99, 107, 128]. To un-
derstand the biological relevance of bursts and the cellular mechanisms underlying
their generation, a wide variety of approaches are needed. In vivo recordings from
neurons in awake/behaving animals allow investigating how different firing modes
affect behavioral performance. In vitro experiments, on the other hand, offer a greater
control over the preparation and are best suited to study cellular mechanisms of burst-
ing. Finally, various levels of modeling can summarize experimental findings, test
our understanding of mechanisms, and inspire new experiments. In this chapter, we
will follow this line of investigation and review a number of recent studies of burst
firing in weakly electric fish.
The electrosensory system of South American weakly electric fish has proven to
be extremely well suited for combined neuroethological and computational studies
of information processing from systems neuroscience to the characteristics of ion
channels. In this review, we will give a brief introduction to the electrosensory sys-
tem, describe in more detail the in vivo firing properties of electrosensory pyramidal
cells in the hindbrain of these fish, and report on the potential behavioral role of
bursts. Next, we present results of in vitro studies that have elucidated some of
the cellular mechanisms underlying burst generation in pyramidal cells. This is fol-
lowed by a discussion of detailed compartmental models that successfully reproduce
in vitro bursting and reduced models offering a dynamical systems perspective on
burst mechanisms. We conclude by comparing burst firing in weakly electric fish to
other systems.

8.1.1 What is a burst?

Spike bursts have been described in a large number of systems. Voltage traces from a
selection of bursting neurons are displayed in Figures 8.1 and 8.5. As is evident from
these examples, bursts can occur on a wide range of time scales and vary in their fine
temporal structure. Because the biophysical mechanisms underlying bursts can be
so diverse, it comes as no surprise that no unique definition of bursts exists. We will
use the term here for the basic event that is part of every burst definition: a burst
is a series of action potentials fired in rapid succession, set off in frequency against
the rest of a spike train. In an interspike interval (ISI) histogram, burst spikes will
typically fall into one peak at short intervals with the rest of the intervals forming