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<br />in doing so, some structural metal may be inadvertently
<br />lost.
<br />
<br />Results - Responses of Fish
<br />to Electric Fields
<br />
<br />Movement toward an electrode in response to an
<br />electric field is not unique to aquatic vertebrates, or even
<br />organisms with nervous systems. Even individual cells
<br />respond. Halsband (1967) noted that common carp and
<br />trout erythrocytes placed in a powerful electric field (1,000
<br />times more current density than used in electrofishing)
<br />first moved towards the anode (cataphoresis), then
<br />changed shape from oval to round, and finally
<br />disintegrated.
<br />As in water, ionic conductivity is responsible for elec-
<br />tric currents in blood and interstitial fluids of living tis-
<br />sues (Steroin et aI., 1972, 1976), but transmission of
<br />electricity to and deep within the body of a fish is com-
<br />plex. Tissues and membranes have different and some-
<br />times variable electrical qualities (e.g., conductivity,
<br />capacitance, and impedance-Steroin et aI., 1972, 1976;
<br />Sharber et a!., 1995). Skin, for example, is essentially re-
<br />sistive and dissipates much of the electrical energy as
<br />heat (perhaps some observed responses in fish are actu-
<br />ally responses to heat). Some of the electrical energy that
<br />is transmitted across skin and other tissue membranes is
<br />reportedly transferred by capacitance. Presumably, with
<br />electrolytes on both sides of a membrane (e.g., water on
<br />one side of skin and interstitial fluids and blood in capil-
<br />laries on the other side), the membrane functions some-
<br />what as a dielectric in an electrical condenser and allows
<br />a momentary current across the membrane only as ap-
<br />plied voltage is switched on, off, or suddenly increased
<br />or decreased. No current is transmitted by capacitance in
<br />PDC when the applied voltage is constant; therefore, the
<br />amount of charge transmitted by capacitance in PDC fields
<br />varies directly with frequency. But fish also exhibit very
<br />distinct, field-intensity dependent, responses under con-
<br />tinuous DC. Direct electrical stimulation of afferent nerves
<br />probably also occurs through various external sensory
<br />structures in the skin, including the lateral-line canal sys-
<br />tem. Although not mentioned in literature reviewed for
<br />this report, the gills, which are the primary sites for ionic
<br />exchange, might also have a significant role in the trans-
<br />mission of electrical current to the blood and from there
<br />via the circulatory system to nerves and other tissues
<br />throughout the body.
<br />Neurological responses to stimuli, nerve impulse
<br />transmission, and muscular actions in animals are
<br />electrochemical phenomena. In accord with the "all or
<br />none" principle of individual nerve response, each level
<br />
<br />SNYDER 21
<br />
<br />of reaction requires a stimulus of a specific minimum
<br />intensity that must arrive quickly and be maintained for a
<br />minimum time. However, if a series of stimuli below the
<br />threshold level for nerve response are received over a
<br />sufficiently short period, their effect may be cumulative
<br />and still cause. the nerve to respond according to the
<br />principle of temporal summation (Best and Taylor, 1943,
<br />as quoted by Haskell et aI., 1954; Wydoski, 1980; Emery,
<br />1984).
<br />
<br />Biarritz and Bozeman Paradigms
<br />
<br />In what has become known as the Biarritz paradigm,
<br />Blancheteau et a!. (I 96 1), Lamarque (I963, 1967a, 1990),
<br />Vibert (I963, 1 967b ), and Blancheteau (1967) developed a
<br />set of principles for nerve and muscle excitation in DC
<br />fields to explain the various responses of fishes observed
<br />in their experiments at the Biarritz Hydrobiological Sta-
<br />tion in France (Table 1). Lamarque (I 967a) summarized
<br />these principles as follows:
<br />
<br />"I. At a certain threshold, direct current initiates and
<br />maintains nerve or muscle excitation by the
<br />"autorhythm of excitation" (see Fessard 1936 and
<br />Monnier et al. 1940).
<br />2. Short nerves in an electric field are excited at a
<br />higher value of current than long nerves (Laugier,
<br />1921).
<br />3. The greater the angle between a neurone in an
<br />electric field and the direction of current flow, the
<br />greater the current necessary to excite it (Fick,
<br />cited by Charbonnel-Salle 1881).
<br />4. A neurone can only transmit its excitation to an-
<br />other neurone in the soma-axon direction.
<br />5. The stimulus being produced by catelectrotonus
<br />at the cathode, an excitation can be conveyed to
<br />the next structure only if the cathode is on the
<br />soma side with regard to the axonic endings
<br />(nonnodromic stimuli).
<br />6. Inversely, if the anode is on the soma side with
<br />regard to the axonic endings, the soma anelectro-
<br />tonus can block a normodromic stimulus from
<br />another structure, and thus create an inhibition.
<br />7. Nerve or muscle structures ofa fish in an electric
<br />field can be excited or inhibited in situ since the
<br />fish body has itself become an electric field. Ac-
<br />cording to the potential values, certain structures
<br />will be excited on account of their length (2), or
<br />their position (3); others will be inhibited (6), and
<br />yet others preserved from the action of current."
<br />
<br />Lamarque (1967a) also noted that nerve interaction
<br />with PDC is further complicated by". . . very complex
<br />physiological processes, such as chronaxies, spatial and
<br />temporal summations, synaptic delays, excitatory post-
<br />
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