Knollenorgan

Knollenorgan is the name given to an electroreceptor found in the skin of weakly electric fish of the family Mormyridae from Africa. It was first described by V. Franz (1920), a German anatomist who was unaware of its function. They are named after "Knolle", German for "tuberous root" which describes their structure in the skin.

Cell composition
The sense organs are composed of modified epithelial cells that act as sensory transducers for electric currents. Besides these, there are also supporting cells, and a sensory neuron which projects to the nucleus of the electrosensory lateral line lobe (nELL) of the medulla via the posterior branch of the lateral line nerve.

Placement
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 corium. The sense organ is surrounded by a basement membrane which separates corium from epidermis. Epithelial cells form a loose plug over the sensory receptors, allowing capacity-coupled current to pass from the external environment to the sensory receptor.

Structure and effectivity
Knollenorgans lack the jelly filled canal leading from sensory receptor cells to the external environment. Knollenorgans are sensitive 20 to 20,000 Hz electric stimuli with electric fields as small as 0.1 mV/cm. These receptor organs are used to detect the weak electric organ discharges (EODs) of other electric fish, usually of their own species. The anatomy of Knollenorgans is described by Franz (1920), Bennett (1965, 1971) and Szabo (1965, 1974).