11. ELECTRORECEPTION
543
lations. Although oscillations have been recorded from some mormyrid
large receptors immersed in water (Harder, 1968), it is to be expected
that these responses are infrequent under normal conditions and represent
malfunctioning of the system. An important factor in the oscillations of
submerged receptors is the conductivity of the water. The rate of spontaneous discharge of receptors submerged in low conductivity water is
substantially reduced when a small quantity of salts is added (M. V. L.
Bennett, unpublished data). It would be of interest in this respect to
know the conductivity of the natural waters in which mormyrids are
found.
The input-output relationship of synaptic membrane of phasic receptor cells differs from that of tonic receptor cells in that there is little
resting release of transmitter. It is difficult to be confident of the shape
of the potential-secretion relationship because of active processes in the
receptor cells. It is likely but uncertain that the secretory membrane
of phasic receptors does have an input-output relationship which has
a much greater slope than that at known interneuronal and neuromuscular synapses.
At some phasic receptors the number of impulses in response to a
stimulus rises sharply after a certain threshold is reached and then
saturates ( Hagiwara and Morita, 1963; Szabo and Hagiwara, 1967). Since
these receptors are activated by the electric organ and presumably operate in electrolocation, it makes biological sense for them to have a high
threshold that lies in the range of stimuli provided by the organ and to
be very sensitive to changes above this threshold, that is, to have a low
incremental threshold once absolute threshold has been reached. These
receptors would then be best able to signal small influences of objects in
distorting the field of the electric organ.
It is difficult to be confident of the amplitudes of the membrane responses of the receptor cells. The maximum values of the externally recorded responses are 10-20 mV when the skin is dry, but significant
loading may still be present. Nonetheless, on the reasonable assumption
that permeability changes underlie the responses, a small voltage must
produce a fairly large change in permeability, and an impulse of 10-20
mV amplitude might be sufficient to move a sensitive receptor cell
through the entire cycle of permeability changes associated with the
response. Since loading blocks receptor oscillations, it probably reduces
the amplitude of the transmembrane potentials, and at large receptors of
mormyrids there is marked shortening of the externally recorded response
(Fig. 3 3 ) .
The large receptors of mormyrids and ampullae of Lorenzini are
apparently comparable in terms of the energy required to stimulate, about
543
lations. Although oscillations have been recorded from some mormyrid
large receptors immersed in water (Harder, 1968), it is to be expected
that these responses are infrequent under normal conditions and represent
malfunctioning of the system. An important factor in the oscillations of
submerged receptors is the conductivity of the water. The rate of spontaneous discharge of receptors submerged in low conductivity water is
substantially reduced when a small quantity of salts is added (M. V. L.
Bennett, unpublished data). It would be of interest in this respect to
know the conductivity of the natural waters in which mormyrids are
found.
The input-output relationship of synaptic membrane of phasic receptor cells differs from that of tonic receptor cells in that there is little
resting release of transmitter. It is difficult to be confident of the shape
of the potential-secretion relationship because of active processes in the
receptor cells. It is likely but uncertain that the secretory membrane
of phasic receptors does have an input-output relationship which has
a much greater slope than that at known interneuronal and neuromuscular synapses.
At some phasic receptors the number of impulses in response to a
stimulus rises sharply after a certain threshold is reached and then
saturates ( Hagiwara and Morita, 1963; Szabo and Hagiwara, 1967). Since
these receptors are activated by the electric organ and presumably operate in electrolocation, it makes biological sense for them to have a high
threshold that lies in the range of stimuli provided by the organ and to
be very sensitive to changes above this threshold, that is, to have a low
incremental threshold once absolute threshold has been reached. These
receptors would then be best able to signal small influences of objects in
distorting the field of the electric organ.
It is difficult to be confident of the amplitudes of the membrane responses of the receptor cells. The maximum values of the externally recorded responses are 10-20 mV when the skin is dry, but significant
loading may still be present. Nonetheless, on the reasonable assumption
that permeability changes underlie the responses, a small voltage must
produce a fairly large change in permeability, and an impulse of 10-20
mV amplitude might be sufficient to move a sensitive receptor cell
through the entire cycle of permeability changes associated with the
response. Since loading blocks receptor oscillations, it probably reduces
the amplitude of the transmembrane potentials, and at large receptors of
mormyrids there is marked shortening of the externally recorded response
(Fig. 3 3 ) .
The large receptors of mormyrids and ampullae of Lorenzini are
apparently comparable in terms of the energy required to stimulate, about