436
M. V. L. BENNETT
stimulus ( C ) is the same as the threshold for firing at the termination
of an anodal stimulus ( D ) . Furthermore, if brief pulses are used to
evoke responses of the innervated face during maintained current pulses,
the amplitudes of the responses are the same as without maintained
current, although they occur when the uninnervated membrane is either
markedly depolarized or hyperpolarized.
One may ask if the membranes of the electrocytes have unique
properties that give rise to the unusual function of the uninnervated
face, From the equivalent circuit (Fig. 59) it is clear that intracellularly
applied current and differential recording across the innervated face
allow evaluation of the resistance of this face ri. The resistances rel or
rez can be evaluated by differentially recording across the cell while
passing current through an electrode external to one or the other face.
Where monopolar recording shows the external potentials outside the
two faces to be equal, then rel = rez. The time constant of decay of
potentials following either intra- or extracellular stimulation is given
by ( rel + ren + r i ) C. Thus from the measured resistances and time
constant one can obtain the capacity.
Although the data available are restricted, preliminary values are
given in Fig. 59. These assume a uniform planar membrane on each
surface. The resistance of the innervated face is quite reasonable. The
capacity of the uninnervated face is if anything too small on the assumption that its capacity is the usual 1 pF/cm2 and takes into account the
great increase in membrane area resulting from the numerous tubules
and processes of this face. The measurements indicate that the resistance
of the uninnervated face is at least 50 times that of the innervated face.
The great increase in area observed on a fine structural level means
that the uninnervated membrane has a resistance of perhaps 250-500
times that of the innervated face. This is a high value, perhaps 200,000
a ern2, but it is not unprecedentedly great (Bennett and Trinkaus, 1970).
If the capacity of the innervated face were the usual 1 pF/cm2 its time
constant would be 0.4 msec. This value is much shorter than the time
constant of the uninnervated face and is in reasonable agreement with
the experimental observations. The significance of an organ discharge
without net current flow is discussed in Section 11, F.
4. MORMYRIDS
The mormyrids are found in freshwaters of tropical Africa. They are
a large group of some 11 genera and large numbers of species. All that
have been studied are weakly electric. A key to the genera is presented
in Table I11 and Fig. 60. Identification as to species is generally un-
M. V. L. BENNETT
stimulus ( C ) is the same as the threshold for firing at the termination
of an anodal stimulus ( D ) . Furthermore, if brief pulses are used to
evoke responses of the innervated face during maintained current pulses,
the amplitudes of the responses are the same as without maintained
current, although they occur when the uninnervated membrane is either
markedly depolarized or hyperpolarized.
One may ask if the membranes of the electrocytes have unique
properties that give rise to the unusual function of the uninnervated
face, From the equivalent circuit (Fig. 59) it is clear that intracellularly
applied current and differential recording across the innervated face
allow evaluation of the resistance of this face ri. The resistances rel or
rez can be evaluated by differentially recording across the cell while
passing current through an electrode external to one or the other face.
Where monopolar recording shows the external potentials outside the
two faces to be equal, then rel = rez. The time constant of decay of
potentials following either intra- or extracellular stimulation is given
by ( rel + ren + r i ) C. Thus from the measured resistances and time
constant one can obtain the capacity.
Although the data available are restricted, preliminary values are
given in Fig. 59. These assume a uniform planar membrane on each
surface. The resistance of the innervated face is quite reasonable. The
capacity of the uninnervated face is if anything too small on the assumption that its capacity is the usual 1 pF/cm2 and takes into account the
great increase in membrane area resulting from the numerous tubules
and processes of this face. The measurements indicate that the resistance
of the uninnervated face is at least 50 times that of the innervated face.
The great increase in area observed on a fine structural level means
that the uninnervated membrane has a resistance of perhaps 250-500
times that of the innervated face. This is a high value, perhaps 200,000
a ern2, but it is not unprecedentedly great (Bennett and Trinkaus, 1970).
If the capacity of the innervated face were the usual 1 pF/cm2 its time
constant would be 0.4 msec. This value is much shorter than the time
constant of the uninnervated face and is in reasonable agreement with
the experimental observations. The significance of an organ discharge
without net current flow is discussed in Section 11, F.
4. MORMYRIDS
The mormyrids are found in freshwaters of tropical Africa. They are
a large group of some 11 genera and large numbers of species. All that
have been studied are weakly electric. A key to the genera is presented
in Table I11 and Fig. 60. Identification as to species is generally un-