The electric eel, unlike most electric fish, has three separate organs which it uses for producing a charge. The need for three electric organs is to fulfill the various roles and applications of its ability to produce electricity. The Main and Hunters' organs are the high voltage producers, used for protection, fright reflexes and stunning prey. The Sachs' organ is capable only of producing low voltage pulses - its purpose is mainly electro communication and navigation. The eel, spite being slightly curved in the above picture, tends to remain straight whilst moving, using its anal fin to propel itself. This is necessary in order to maintain a uniform electric field around itself, as a more effective sensory mechanism.
The electric organs, responsible for the discharge, are composed of muscle-like cells called electrocytes. These cells resemble muscle cells in the sense that they exist at the end of axons or nerve cells, as a muscle cell would, only the electrocytes have no contracting ability. Having a distinctly disc like form, the electrocytes can be aligned as cells in a battery.
Up to 200,000 of these cells can be aligned in series within the organ....each one capable of producing a small voltage so that when discharged simultaneously, the resulting potential difference is the sum of each of these voltages. In the low voltage organs, electric discharge varies considerably from fish to fish. Some species produce a continual wave-like discharge, whereas others emit a constant pulse. The higher voltage organs emit a pulse, but they remain inactive most of the time, recharging for a large accumulated discharge in the event of danger.
Electric eels are classified as strongly electric fish as opposed to weakly electric fish, the Torpedo ray is another strongly electric fish. Electric discharge is therefore of the "pulse" like nature.
Electrocytes, like all eukaryotic cells, maintain a potential gradient across the membrane, this is done by active transport of Na+ and K+ ions through membrane pumps, protein structures which span the phospholipid bilayers forming the cell membrane. Transport of these ions is coupled to the synthesis of ATP, the standard currency of energy in all biological organisms.
Whilst not in use, the electrocytes have a net negative charge inside the cell;
This is the result of a net efflux of Na+ ions through the sodium potassium pumps. Although the intracellular concentration of K+ is high, potassium ions flow out of the cell under the influence of a concentration gradient (due to there being a higher concentration of K+ inside the cell) . The extracellular positive charge now increase as the cytosolic (inside the cell) negative charge becomes more negative......eventually the concentration gradient and the potential difference cancel each other out, a state of equilibrium has developed whereby the inside of the electrocyte is negatively charged and the outside has a net positive charge. The concentration and potential difference can be equated by the Nernst equation;
click here for a chart of cellular ion concentrations
Where R is the molar gas constant, T is the temperature (in Kelvin), z is the charge of the ion, F is the Faraday constant, Co ans Ci are the ion concentrations outside and inside the cell respectively.
When in use, ie on stimulation by a motor neuron, the innervated (side of the cell which is attached to a nerve ending) side of the cell becomes depolarised before the uninnervated side. This gives rise to a temporary potential gradient on each membrane pointing in the same direction, which is then discharged.
Leading up to the discharge, the motorneuron delivers a nerve impulse from the brain, which, when it reaches the electrocyte, causes the activation of acetylcholine. Acetylcholine can diffuse across the synaptic cleft (the gap between the end of the motorneuron and the electrocyte), and into an acetylcholine receptor on the cell membrane causing an ion channel to open. extracellular ions can then enter the cell and the depolarization occurs. The discharge of electricity out of the cell has to be synchronized with the 200,000 other electrocytes, this is done by a combination of three features in the motorneurons.
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