11. ELECTRORECEPTION
561
spontaneous activity the central nervous system may also discriminate
changes in the responses of a single receptor more sensitively than does
the experimenter.
Although it can be accepted that the electroreceptors are sensitive
to electric fields and thereby provide perceptually useful information,
there is a possibility that these receptors also convey other kinds of
information to the central nervous system. Phasic receptors are quite insensitive to mechanical, thermal, or other modes of stimulation, but tonic
receptors may be rather sensitive to temperature and salinity changes and
also have some sensitivity to mechanical stimulation (Murray, 1967; Suga,
1967a; Szabo, 1970). The effects of salinity changes are ascribable to
changes in diffusion potentials (Murray, 1967). In the ampullae of
Lorenzini the location deep in the body would reduce sensitivity to
thermal and mechanical stimuli. The sensitivity to pressure differences
between inside and outside of the canal is not very great but might become significant during rapid swimming. Pressure changes with depth
would have little effect being communicated equally to inner and outer
faces of the receptor cells. It is an open question whether activation of
electroreceptors by nonelectric stimuli is interpreted by the animal as
electric stimuli in the same way that we see lights when our eyes are
electrically stimulated. Extensive conditioning experiments would be
required to resolve this question,
VI. EVOLUTION OF ELECTROSENSORY SYSTEMS AND
ELECTRIC ORGANS
In a chapter of The Origin of Species entitled “Difficulties of the
Theory,” Darwin raised the problem of evolution of electric organs. It
was difficult to see by what small intermediate steps the strongly electric
organs of the eel and Torpedo could have arisen, when the early stages
could have no value in defense or offense. Furthermore, the highly
specialized but weakly electric organ of the ray could have no value in
these functions. He wrote, “. . . it would be extremely bold to maintain
that no serviceable transitions are possible by which these organs might
have been gradually developed.” Only relatively recently did Lissmann
(1958) provide evidence for a transition stage by demonstration of the
electrosensory system of the weakly electric fish. The explanation became
that early stages of the weakly electric organs were selected for their
electrosensory function. Once a weakly electric organ reached a certain
size, it then began to be adaptive in defensive-offensive actions. In view
561
spontaneous activity the central nervous system may also discriminate
changes in the responses of a single receptor more sensitively than does
the experimenter.
Although it can be accepted that the electroreceptors are sensitive
to electric fields and thereby provide perceptually useful information,
there is a possibility that these receptors also convey other kinds of
information to the central nervous system. Phasic receptors are quite insensitive to mechanical, thermal, or other modes of stimulation, but tonic
receptors may be rather sensitive to temperature and salinity changes and
also have some sensitivity to mechanical stimulation (Murray, 1967; Suga,
1967a; Szabo, 1970). The effects of salinity changes are ascribable to
changes in diffusion potentials (Murray, 1967). In the ampullae of
Lorenzini the location deep in the body would reduce sensitivity to
thermal and mechanical stimuli. The sensitivity to pressure differences
between inside and outside of the canal is not very great but might become significant during rapid swimming. Pressure changes with depth
would have little effect being communicated equally to inner and outer
faces of the receptor cells. It is an open question whether activation of
electroreceptors by nonelectric stimuli is interpreted by the animal as
electric stimuli in the same way that we see lights when our eyes are
electrically stimulated. Extensive conditioning experiments would be
required to resolve this question,
VI. EVOLUTION OF ELECTROSENSORY SYSTEMS AND
ELECTRIC ORGANS
In a chapter of The Origin of Species entitled “Difficulties of the
Theory,” Darwin raised the problem of evolution of electric organs. It
was difficult to see by what small intermediate steps the strongly electric
organs of the eel and Torpedo could have arisen, when the early stages
could have no value in defense or offense. Furthermore, the highly
specialized but weakly electric organ of the ray could have no value in
these functions. He wrote, “. . . it would be extremely bold to maintain
that no serviceable transitions are possible by which these organs might
have been gradually developed.” Only relatively recently did Lissmann
(1958) provide evidence for a transition stage by demonstration of the
electrosensory system of the weakly electric fish. The explanation became
that early stages of the weakly electric organs were selected for their
electrosensory function. Once a weakly electric organ reached a certain
size, it then began to be adaptive in defensive-offensive actions. In view