10. ELECTRIC ORGANS
361
tial generating sites may be the only responsive elements in a membrane
as well as occurring intermixed in membrane of the preceding kinds.
(4) Low resistance inexcitable membrane. This membrane has a
low resistance owing to a large potassium conductance, and it generates
a resting potential; its properties do not change when the potential across
it is changed. In some electrocytes there may be a significant C1
conductance.
( 5 ) High capacity inexcitable membrane. This membrane has few
sites for ionic movement, and its resistance is high. As a result, during
activity virtually all of the current through it is capacitative. Its capacity
per unit area of plasma membrane is probably similar to that of other
membranes, that is, 1 pF/cm2. Where membrane of this kind has been
studied by electron microscopy, the surface is highly convoluted on an
ultrastructural level, and therefore the capacity per unit area referred to
macroscopic area is larger than the values for ordinary cells. [A similar
situation obtains in muscle because of the transverse tubular system (cf.
Gage and Eisenberg, 1969) .] In electrocytes of the gymnotid Sternopygus
one membrane has an effective capacity so large that it probably
results from shift in ionic distributions on either side of the membrane
(see Section 11, C, 1, d).
In different electrocytes the two faces are made up of different combinations of the foregoing kinds of membrane. The equivalent circuits at
rest and during activity are diagrammed in Fig. 4; they will be discussed
in detail in sections concerning the different kinds of fish in which they
occur.
There is good indirect evidence that sites mediating the different
components of electrically excitable responses are independent of each
other on a microscopic scale and that they are also separate from PSP
generating sites (which often occur in the same general region of the
cell). To be sure the spatial resolution of electrophysiological techniques
may not allow measurement of the separation of different kinds of membrane responsiveness. Nonetheless, physicochemical considerations and
separability of different kinds of electric activity under various electrical
and pharmacological treatments strongly support the idea of separate
shown by dashes. ( E ) Electrocytes of the electric catfish. The stalk face is of
higher threshold and generates a smaller spike (indicated by a smaller battery
symbol) and the external potential is entirely negative on the nonstalk side distant
from the stalk. ( F ) Electrocytes of Gymnarchzrs. The uninnervated face acts as a
series capacity and the external response has no net current flow. The summation
of a second response (2’,3’) on the first is shown by dashes. Electrocytes of
sternarchids and Eigenmanniu may operate similarly. From Bennett ( 1970).
361
tial generating sites may be the only responsive elements in a membrane
as well as occurring intermixed in membrane of the preceding kinds.
(4) Low resistance inexcitable membrane. This membrane has a
low resistance owing to a large potassium conductance, and it generates
a resting potential; its properties do not change when the potential across
it is changed. In some electrocytes there may be a significant C1
conductance.
( 5 ) High capacity inexcitable membrane. This membrane has few
sites for ionic movement, and its resistance is high. As a result, during
activity virtually all of the current through it is capacitative. Its capacity
per unit area of plasma membrane is probably similar to that of other
membranes, that is, 1 pF/cm2. Where membrane of this kind has been
studied by electron microscopy, the surface is highly convoluted on an
ultrastructural level, and therefore the capacity per unit area referred to
macroscopic area is larger than the values for ordinary cells. [A similar
situation obtains in muscle because of the transverse tubular system (cf.
Gage and Eisenberg, 1969) .] In electrocytes of the gymnotid Sternopygus
one membrane has an effective capacity so large that it probably
results from shift in ionic distributions on either side of the membrane
(see Section 11, C, 1, d).
In different electrocytes the two faces are made up of different combinations of the foregoing kinds of membrane. The equivalent circuits at
rest and during activity are diagrammed in Fig. 4; they will be discussed
in detail in sections concerning the different kinds of fish in which they
occur.
There is good indirect evidence that sites mediating the different
components of electrically excitable responses are independent of each
other on a microscopic scale and that they are also separate from PSP
generating sites (which often occur in the same general region of the
cell). To be sure the spatial resolution of electrophysiological techniques
may not allow measurement of the separation of different kinds of membrane responsiveness. Nonetheless, physicochemical considerations and
separability of different kinds of electric activity under various electrical
and pharmacological treatments strongly support the idea of separate
shown by dashes. ( E ) Electrocytes of the electric catfish. The stalk face is of
higher threshold and generates a smaller spike (indicated by a smaller battery
symbol) and the external potential is entirely negative on the nonstalk side distant
from the stalk. ( F ) Electrocytes of Gymnarchzrs. The uninnervated face acts as a
series capacity and the external response has no net current flow. The summation
of a second response (2’,3’) on the first is shown by dashes. Electrocytes of
sternarchids and Eigenmanniu may operate similarly. From Bennett ( 1970).