Journal of Neuroscience, Vol 16, 380-391, Copyright © 1996 by Society for Neuroscience
Neuronal circuitry for comparison of timing in the electrosensory lateral line lobe of the African wave-type electric fish Gymnarchus niloticus
M Kawasaki and YX Guo
University of Virginia, Department of Biology, Charlottesville 22903, USA.
An African wave-type electric fish, Gymnarchus, compares timing on the
order of microseconds of sensory feedback from from its high-frequency
(approximately 400 Hz) electric organ discharges (EODs) received at
different parts of its body surfaces. This capability is essential for and
demonstrated by the jamming avoidance response (JAR). The organization of
the timing comparison mechanisms was identified in the electrosensory
lateral line lobe (ELL) in the hindbrain by field potential, extra- and
intracellular recordings, and intracellular labeling with biotinylated
agents. Timing of phase of the EOD feedback is carried by action potentials
of S-type primary afferent fibers that project to the inner cellular layer
(ICL) of the medial zone of the ELL and to the giant neurons in the ELL.
The giant neurons bilaterally project to the ICL, where neurons sensitive
to phase differences between different parts of the body occur. Although
sensitive to dynamic phase changes of several microseconds, these
differential-phase- sensitive neurons showed adaptation to steady-state
changes of phase difference over a wide range (greater than +/- 100
microseconds) and continued to respond to small modulations after the mean
difference was shifted. Gymnarchus and an independently evolved South
American electric fish, Eigenmannia, exhibit nearly identical JARs and
share a rather complex but identical set of computational algorithms for
JAR. This study showed that one of the computational steps, the timing
comparison between body surfaces, occurs in the hindbrain in Gymnarchus, in
contrast to the midbrain in Eigenmannia. Thus, similar systems with a
similar overall function may have evolved differently in different genera
by assigning a subfunction to different substructures within the brain.
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