450
M. V. L. BENNETT
standing is frequency of firing. The electric organs of sternarchids discharge constantly at frequencies from 700 to 1700/ sec, Eigenmannia
operates at 2sO-600/ sec, and Gymnarchus at about 250/sec. Sternop ygus
discharges at 50-100/sec, but it is like Eigenmannia on a 50% duty cycle
in which the interval between pulses is about equal to the pulse duration. The higher values are unequaled for maintained frequency by
muscle or other nerve. In fish other than the sternarchids the nerveelectrocyte synapse is chemically transmitting ( and cholinergic ) so that
chemically mediated transmission can also operate at high frequencies.
However, in gymnotids and Gymnarchus the synapses between the
controlling neurons, which fire at the full organ frequency, may well
all be electrically transmitting ( see Section I11 ).
The high frequency of maintained activity suggests that there
could be a considerable power output per unit weight of organ. As yet
no satisfactory data are available, but many of the cells contain large
numbers of mitochondria suggesting a high metabolic rate (Schwartz
et al., 1971). The peak pulse power of the strongly electric organs is
very large, but the pulses can be emitted at a high rate only for short
times. The energy output presumably represents ions running down
preexisting concentration gradients that are restored relatively slowly.
Electrocytes of strongly electric organs do not have high densities of
mitochondria ( Mathewson et al., 1961; Wachtel, 1964; Sheridan, 1965;
Bloom and Barnett, 1966), and it would be predicted that the power
output per gram of cell that could be maintained would be lower than
for high frequency cells.
In marine electrocytes the site of production of electric energy is
somewhat paradoxically the innervated face which is inexcitable in the
strongly electric fish. It is across this face that the potential is found
during organ discharge. In the eel the contribution of the two faces
is about equal, as it is in many cells of which both faces generate spikes.
For electrocytes in which one face acts as a capacitance the energy is
produced entirely across the innervated face.
F. Adaptation and Convergent EvoIution in EIectric Organs
The evolutionary origin of electric organs will be discussed in the
following chapter, because it appears intimately linked with the development of electrosensory systems. Ignoring the intermediate stages
of evolution, one notes a number of convergences in electric organ
systems that extend beyond simply the production of electricity (Table
I V ) . Some of these convergences have obvious adaptive value. For
example, it was recognized early that electric organs are relatively
M. V. L. BENNETT
standing is frequency of firing. The electric organs of sternarchids discharge constantly at frequencies from 700 to 1700/ sec, Eigenmannia
operates at 2sO-600/ sec, and Gymnarchus at about 250/sec. Sternop ygus
discharges at 50-100/sec, but it is like Eigenmannia on a 50% duty cycle
in which the interval between pulses is about equal to the pulse duration. The higher values are unequaled for maintained frequency by
muscle or other nerve. In fish other than the sternarchids the nerveelectrocyte synapse is chemically transmitting ( and cholinergic ) so that
chemically mediated transmission can also operate at high frequencies.
However, in gymnotids and Gymnarchus the synapses between the
controlling neurons, which fire at the full organ frequency, may well
all be electrically transmitting ( see Section I11 ).
The high frequency of maintained activity suggests that there
could be a considerable power output per unit weight of organ. As yet
no satisfactory data are available, but many of the cells contain large
numbers of mitochondria suggesting a high metabolic rate (Schwartz
et al., 1971). The peak pulse power of the strongly electric organs is
very large, but the pulses can be emitted at a high rate only for short
times. The energy output presumably represents ions running down
preexisting concentration gradients that are restored relatively slowly.
Electrocytes of strongly electric organs do not have high densities of
mitochondria ( Mathewson et al., 1961; Wachtel, 1964; Sheridan, 1965;
Bloom and Barnett, 1966), and it would be predicted that the power
output per gram of cell that could be maintained would be lower than
for high frequency cells.
In marine electrocytes the site of production of electric energy is
somewhat paradoxically the innervated face which is inexcitable in the
strongly electric fish. It is across this face that the potential is found
during organ discharge. In the eel the contribution of the two faces
is about equal, as it is in many cells of which both faces generate spikes.
For electrocytes in which one face acts as a capacitance the energy is
produced entirely across the innervated face.
F. Adaptation and Convergent EvoIution in EIectric Organs
The evolutionary origin of electric organs will be discussed in the
following chapter, because it appears intimately linked with the development of electrosensory systems. Ignoring the intermediate stages
of evolution, one notes a number of convergences in electric organ
systems that extend beyond simply the production of electricity (Table
I V ) . Some of these convergences have obvious adaptive value. For
example, it was recognized early that electric organs are relatively