Electric Organ Discharge

All ocean animals are surrounded by very low-frequency electric fields. Many animals have magnetic or electric senses. Electric fish interest neuroscientists today because of the extraordinarily sensitive and precise system of generating, receiving, processing, and responding behaviorally to electric signals. Seven families of fish deliver appreciable voltage outside of their bodies. A few predators kill prey electrically. The majority, weakly electric fish, use a specialized sensory guidance system for environmental navigation via echolocation. Similar to the echolocation in bats and dolphins, the weakly electric fish use electrolocation to analyse the reception and transmition of electric current for communication. This is primarily for species and sex recognition in their low visibility environments.

Electric Organ Discharge
The organ is located in the posterior of the fish and the synchronous firing of the organ in controlled by the midline medullart nucleus, called the pacemaker nucleus, located in the brain. The multinucleated cells that make up the organ are either myogenically (muscle) or neuronally (nerve) derived. The cells that make up the organ are called electrocytes. The columns of the electrocytes are innervated by supramoterneurons which carry the signal from the brain stimulating the EOD by depolarizing the elecrocytes. In hummers the electrocyte is never fully depolarized because the stimulation is constant, which results in the sinusiodal wave.

Strongly Electric Fish

Strongly electric fish (freshwater eels and catfish) kill prey and ward off predators by delivering electric shocks of several hundred volts. With many strong dischargers, the whole fish has enormous electric potential. Eels have the unique ability to discharge both weak and strong electric current. The weak current is used primarily to locate and and stun prey. The strong current is used almost exclusively as a weapon to attack prey.

 

Image by Skyler R. Chapman

 

Weakly Electric Fish
Weakly electric fish use their electric organs primarily for the detection of the rough shape, conductivity, and location of nearby objects, recognition of members of their own species, calling their mates, finding their position in a school, and enacting other behaviors critical to their survival. Weakly electric fish live in a variety of freshwater habitats in Central and South America and in Africa. The South American Gymnotiforms and African Mormyforms are so phylogenetically distant that they are thought to have evolved through convergent evolution (independently). In both orders, the fish live in shallow streams of dark murky waters so the EOD essentially replaces their vision; they sense their surroundings by emitting an EOD which creates an electric field around the fish. The fish can sense purturbations caused by objects in their electric field-called electrolocation. Weakly electric fish have one of two patterns of electric discharge, both of which are generated from modified muscle tissue usually near the tail or from tissue near the eyes. Wave fish (hummers) produce continuous sinusiodal wave signals at frequencies of 50-1000 Hz. Pulse fish (clickers) emit electrical pulses lasting approximately one millisecond which are spaced about 23ms apart. These pulses create time gaps in their field. (see hummers and clickers) Hormones are now thought to play an integral roll in the amplitude of the discharge and probably effects the anatomy of the EOD control system in many other ways. The EOD varies with sex.

EOD’s: Social Communicaion
Electric fish emit sexually dimorphic EOD’s (waveform can be altered by a steroid treatment), generated by a medullary pacemaker nucleus (PMN) composed of 2 cell types. The output neurons of the PMN synapse located on electromotor neurons on the spinal cord innervate the electric organ. The PMN only receives input from 2 sources, which are responsible for brief modulations that occur during social interactions. This capacity to emit and sense weak EODs evolved only in South American gymnotisforms and African mormyriforms. For these fish, EODs give information on the species, sex, and possibly the individual identity of another fish. Also,transient modulations in EOD frequency happen in social situations and convey information on aggressiveness, readiness to mate, etc. (See also http://www.hhmi.org/grants/lectures/97lect/behave/ (outside site) for an applet on the social and non-social behavior of Gnathonemus petersii)

Jamming Aviodance Response (JAR)
The Jamming Avoidance Response (JAR) is a very important mechanism for electric fish. When two fish with nearly the same frequency meet each other, one alters his frequency to be slightly higher and the other alters to be slightly lower. The shifts are simultaneous and reflexive. This process prevents two frequencies from interfering and jamming each other's electrical signals-allowing the fish to operate in the same area. (See JAR lab)

Electroreception
Nonconductive objects reflect the electric field-while highly conductive objects weaken the field by attracting electrons and ions from the field. The electroreceptors located in the epidermis of the head and sides of the fish percieve these changes. Voltage sensitive channels open to detect a change in the electric field-this stimulation of the cells cause the release of synaptic vescicles—release a neurotransmitter, which binds to a lateral line nerve fiber which eventually sends a signal to the brain where the information is processed. Ampularry receptors detect electric fields given off by other fish. Tuberous receptors respond to the range of the fish’s own EOD.