[[File:Knollenorgan by Viktor Franz 1921.gif|thumb|upright=1.2|A knollenorgan drawn by the German anatomist Viktor Franz, 1921. RC=receptor cell; b.m.= basal membrane; n=nerve. Line at top=skin surface.<ref name="Franz 1921"/><br/><br/>The organ is embedded in the skin of mormyrid fishes, which actively electrolocate by generating brief electrical pulses with their electric organ. The returns from the pulses, distorted by any nearby objects such as prey, are detected by the knollenorgans distributed around the fish's body.]]

A '''Knollenorgan''' is an electroreceptor in the skin of weakly electric fish of the family Mormyridae (Elephantfish) from Africa. The structure was first described by Viktor Franz (1921), a German anatomist unaware of its function. They are named after "Knolle", German for "tuberous root" which describes their structure.

== Structure and function ==

Knollenorgans contain modified epithelial cells that act as sensory transducers for electric fields. Besides these, there are supporting cells and a sensory neuron. The neuron projects to the fish's brain, specifically to the nucleus of the electrosensory lateral line lobe (nELL) of the medulla via the posterior branch of the lateral line nerve.<ref name="Bennett 1965">{{cite journal |last=Bennett |first=M. V. L. |year=1965 |title=Electroreceptors in mormyrids |journal=Cold Spring Harbor Symposia on Quantitative Biology |volume=30 |pages=245–262|doi=10.1101/SQB.1965.030.01.027 |pmid=5219479 }}</ref><ref name="Bennett 1971">{{cite journal |last=Bennett |first=M. V. L. |year=1971 |title=Electroreception |journal=Fish Physiology |volume=V |editor1=W. S. Hoar |editor2=D. J. Randall |pages=347–491 |location=New York |publisher=Academic Press|doi=10.1016/S1546-5098(08)60051-5 }}</ref><ref name="Szabo 1965">{{cite journal |last=Szabo |first=T. |year=1965 |title=Sense organs of the lateral line system in some electric fish of the Gymnotidae, Gymnarchidae, and Mormyridae |journal=Journal of Morphology |volume=117 |issue=2 |pages=229–250|doi=10.1002/jmor.1051170208 |pmid=5893615 |s2cid=33838224 }}</ref><ref name="Szabo 1974">{{cite book |last=Szabo |first=T. |chapter=Anatomy of the Specialized Lateral Line Organs of Electroreception |series=Handbook of Sensory Physiology |year=1974 |title=Electroreceptors and Other Specialized Receptors in Lower Vertrebrates |volume=III |issue=3 |editor=A. Fessard |pages=13–58 |location=Berlin |publisher=Springer-Verlag|doi=10.1007/978-3-642-65926-3_2 |isbn=978-3-642-65928-7 }}</ref>

The organs are embedded in the thickened epidermis. The receptor cells lie buried in the deeper layers of the epidermis, where they expand into a pocket in the superficial layers of the corium. The sense organ is surrounded by a basement membrane which separates corium from epidermis.<ref name="Franz 1921">{{cite journal |last=Franz |first=Viktor J. |year=1921 |title=Zur mikroscopischen Anatomie der Mormyriden |trans-title=On the Microscopic Anatomy of the Mormyrids |journal=Zoologische Jahrbücher / Abteilung für Anatomie und Ontogonie der Tiere |language=de |volume=42 |pages=91–148}}</ref> Epithelial cells form a loose plug over the sensory receptors, allowing capacity-coupled current to pass from the external environment to the sensory receptor.<ref name="Bennett 1965"/><ref name="Bennett 1971"/><ref name="Szabo 1965"/><ref name="Szabo 1974"/>

Knollenorgans lack the jelly-filled canal leading from sensory receptor cells to the external environment characteristic of the ampullae of Lorenzini found in sharks and other basal groups of fishes. Knollenorgans are sensitive to electrical stimuli at frequences between 20 hertz and 20 kilohertz, with electric fields as small as 0.1&nbsp;millivolt per centimetre. They are used to detect the weak electric organ discharges of other electric fish, usually of their own species.<ref name="Bennett 1965"/><ref name="Bennett 1971"/><ref name="Szabo 1965"/><ref name="Szabo 1974"/>

== See also ==

* Ampullae of Lorenzini – the ancestral type of electroreceptor in vertebrates

== References ==

{{reflist}}

{{Electric fish}}

Category:Fish physiology Category:Neuroethology Category:Fish nervous system Category:Weakly electric fish