Knollenorgan is the name given to an electroreceptor found in the skin of weakly electric fish

Electric fish

An electric fish is a fish that can generate electric fields. It is said to be electrogenic; a fish that has the ability to detect electric fields is said to be electroreceptive. Most electrogenic fish are also electroreceptive. Electric fish species can be found both in the sea and in freshwater...

 of the family Mormyridae

Mormyridae

The family Mormyridae, sometimes called "elephantfish" , are freshwater fish in the order Osteoglossiformes native to Africa. It is by far the largest family in the order with around 200 species. Members of the family are popular, if challenging, aquarium species...

 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

Epithelium

Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 that act as sensory transducers

Transduction (physiology)

In physiology, transduction is the conversion of a stimulus from one form to another.Transduction in the nervous system typically refers to stimulus alerting events wherein a mechanical/physical/etc stimulus is converted into an action potential which is transmitted along axons towards the central...

 for electric currents. Besides these, there are also supporting cells, and a sensory neuron

Sensory neuron

Sensory neurons are typically classified as the neurons responsible for converting external stimuli from the environment into internal stimuli. They are activated by sensory input , and send projections into the central nervous system that convey sensory information to the brain or spinal cord...

 which projects to the nucleus of the electrosensory lateral line

Lateral line

The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...

 lobe (nELL) of the medulla

Medulla oblongata

The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply 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

Dermis

The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...

. The sense organ is surrounded by a basement membrane which separates corium from epidermis. Epithelial cells form a loose plug over the sensory receptor

Sensory receptor

In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...

s, 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).

See also

Other electroreceptors:

• Mormyromast
• Ampullary Receptor
• Ampullae of Lorenzini
  Ampullae of Lorenzini
  The ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of jelly-filled pores. They are mostly discussed as being found in cartilaginous fishes ; however, they are also reported to be found in Chondrostei such as Reedfish and sturgeon. Lungfish have also been...
  
  
• Tuberous receptors
  Tuberous receptor
  Tuberous receptors are electroreceptors that are specialized to respond to high-frequency electrical fields , and hence are found only in fish with an active electrical sense that can generate their own electrical fields. They are mostly found on weakly electric fishes such as gymnotiforms and...
  
  in Gymnotiform fish: pulse marker, burst duration coder, probability coder, phase coder