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 fish (sharks, rays, and chimaeras); however, they are also reported to be found in Chondrostei such as reedfish and sturgeon.Lungfish have also been reported to have them.Teleosts have re-evolved a different type of electroreceptors. They were first described by Stefano Lorenzini in 1678.

These sensory organs help fish to sense electric fields in the water. Each ampulla consists of a jelly-filled canal opening to the surface by a pore in the skin and ending blindly in a cluster of small pockets full of special jelly. The ampullae are mostly clustered into groups inside the body, each cluster having ampullae connecting with different parts of the skin, but preserving a left-right symmetry. The canal lengths vary from animal to animal, but the distribution of the pores is generally specific to each species. The ampullae pores are plainly visible as dark spots in the skin. They provide fish with an additional sense capable of detecting electric and magnetic fields as well as temperature gradients.

The ampullae detect electric fields in the water, or more precisely the potential difference between the voltage at the skin pore and the voltage at the base of the electroreceptor cells. A positive pore stimulus would decrease the rate of nerve activity coming from the electroreceptor cells, and a negative pore stimulus would increase the rate of nerve activity coming from the electroreceptor cells. Each ampulla contains a single layer of cells that contains electrically excitable receptor cells separated by supporting cells. The cells are connected by apical tight junctions so that no current leaks between the cells. The apical faces of the receptor cells have a small surface area with a high concentration of voltage dependent calcium channels and calcium activated potassium channels. Because the canal wall has a very high resistance, all of the voltage difference between the pore of the canal and the ampulla is dropped across the receptor epithelium which is about 50 microns thick. Because the basal membranes of the receptor cells have a lower resistance, most of the voltage is dropped across the apical faces which are excitable and are poised at threshold. Inward calcium current across the receptor cells depolarizes the basal faces causing presynaptic calcium release and release of excitatory transmitter onto the afferent nerve fibers. One of the first descriptions of calcium activated potassium channels was based on studies of the ampulla of Lorenzini in the skate. Large conductance calcium activated potassium channels (BK channels) have recently been demonstrated in the ampulla by cloning.

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