"The discharges are not distorted. They get weaker with distance, but their temporal structure stays the same. That's one reason mormyrids evolved to be exquisitely sensitive to small timing differences in electric signals," Carlson explains.
Detecting the pulse
Weakly electric fish have several types of electroreceptors but the ones important for communication are called knollenorgans, from the German word "Knolle," or tuber, because they consist of bulbous cells buried just under the fish's skin.
The knollenorgans respond to a voltage rise, firing a time-locked spike in response to outside positive-going voltage changes.
The knollenorgans on one side of a fish's body respond to the start of a discharge and those on the opposite side respond to the end of a discharge. This lets a fish recognize a species-specific discharge by comparing the intervals between spikes coming from opposite sides of its body.
The spike time comparison occurs within the central nervous system, in a part of the brain called the extero-lateral nucleus, or EL.
When we began our work, the "standard anatomy" for the "mormyrid" brainwhat you'd find if you looked in a textbook-- says Carlson, was a two-part EL, with separate nuclei, or clumps of cells, in the anterior and posterior portions.
"We collected lots of brains in Gabon, and two collaborators, Saad Hasan, a former undergraduate at Washington University, who is now a medical student at Cornell, and Derek Miller, who is an undergraduate at Washington University, did all the histology on the brains.
"In addition to the standard anatomy, we were amazed to see another anatomy, where the EL is substantially smaller and not split into two portions.
"All the fish we looked at either had the large EL that was divided into anterior and posterior halves, or they had the small undifferentiated EL
|Contact: Diana Lutz|
Washington University in St. Louis