M. V. L. BENNETT
458
over muscle is that they do not move when stimulated. However, the
large size and low resistance is often a disadvantage because the membrane potential may not remain uniform over the cell when it is stinwlated by a single intracellular microelectrode; that is, the cells are not
“space-clamped ( cf. Bennett, 1970). Thus, qualitative results can be
obtained using microelectrodes, but quantitative characterization of
membranes in terms of Hodgkin-Huxley parameters as has been carried
out for other tissues may not be possible (Hodgkin and Huxley, 1952;
Frankenhaeuser and Huxley, 1964).
Because electrocytes have a uniform orientation, it is sometimes
feasible to use external electrodes on columns of cells or even on the
entire electric organ. This kind of preparation allows satisfactory impedance measurements which can be referred to the single cell (e.g., Fig.
41; see also Albe-Fessard, 1950b). Crude but useful current clamp
measurements may also be possible where the properties of the
individual cells are sufficiently alike that the current density through
them is essentially uniform (e.g., Bennett et al., 1961). The uniform
orientation also has allowed measurements of thermal and optical
changes associated with activity; most important, these changes can be
studied under different degrees of electrical loading of the tissue (Cohen
et al., 1969; other references in Bennett, 1970).
Single eel electrocytes can be isolated. The single cell can then be
placed between two baths and transcellular current restricted to a limited
area by pressing the innervated face of the cell against a plastic sheet
with a m a l l hole through it. Current application by external electrodes is
more or less uniform because the uninnervated face is inexcitable and of
low resistance. This preparation has been used in voltage clamp and
impedance measurements, although it is not clear that space clamping is
possible even under these conditions (Nakamura et al., 1965; Morlock et
al., 1969; cf. Bennett, 1970). As shown in respect to the squid axon, space
clamping may fail if access resistance through the surrounding solution
is too large compared to membrane resistance (K. S. Cole, 1968). It is
difficult or impossible to clamp the squid axons showing the largest inward currents, and eel electrocytes pass considerably larger currents.
Another problem is whether the small stalks on the innervated faces protrude far enough to be nonisopotential with neighboring membrane (as
in several weakly electric gymnotids, Figs. 28 and 31). In the eel
preparation the effects of membrane outside the edges of the window
also must be evaluated. These factors require careful analysis as did
voltage clamp of the squid giant axon, and it will be necessary to use
exploring microelectrodes to verify space clamping. In spite of the
possible or real shortcomings, gymnotid electrocytes allow voltage clamp-
458
over muscle is that they do not move when stimulated. However, the
large size and low resistance is often a disadvantage because the membrane potential may not remain uniform over the cell when it is stinwlated by a single intracellular microelectrode; that is, the cells are not
“space-clamped ( cf. Bennett, 1970). Thus, qualitative results can be
obtained using microelectrodes, but quantitative characterization of
membranes in terms of Hodgkin-Huxley parameters as has been carried
out for other tissues may not be possible (Hodgkin and Huxley, 1952;
Frankenhaeuser and Huxley, 1964).
Because electrocytes have a uniform orientation, it is sometimes
feasible to use external electrodes on columns of cells or even on the
entire electric organ. This kind of preparation allows satisfactory impedance measurements which can be referred to the single cell (e.g., Fig.
41; see also Albe-Fessard, 1950b). Crude but useful current clamp
measurements may also be possible where the properties of the
individual cells are sufficiently alike that the current density through
them is essentially uniform (e.g., Bennett et al., 1961). The uniform
orientation also has allowed measurements of thermal and optical
changes associated with activity; most important, these changes can be
studied under different degrees of electrical loading of the tissue (Cohen
et al., 1969; other references in Bennett, 1970).
Single eel electrocytes can be isolated. The single cell can then be
placed between two baths and transcellular current restricted to a limited
area by pressing the innervated face of the cell against a plastic sheet
with a m a l l hole through it. Current application by external electrodes is
more or less uniform because the uninnervated face is inexcitable and of
low resistance. This preparation has been used in voltage clamp and
impedance measurements, although it is not clear that space clamping is
possible even under these conditions (Nakamura et al., 1965; Morlock et
al., 1969; cf. Bennett, 1970). As shown in respect to the squid axon, space
clamping may fail if access resistance through the surrounding solution
is too large compared to membrane resistance (K. S. Cole, 1968). It is
difficult or impossible to clamp the squid axons showing the largest inward currents, and eel electrocytes pass considerably larger currents.
Another problem is whether the small stalks on the innervated faces protrude far enough to be nonisopotential with neighboring membrane (as
in several weakly electric gymnotids, Figs. 28 and 31). In the eel
preparation the effects of membrane outside the edges of the window
also must be evaluated. These factors require careful analysis as did
voltage clamp of the squid giant axon, and it will be necessary to use
exploring microelectrodes to verify space clamping. In spite of the
possible or real shortcomings, gymnotid electrocytes allow voltage clamp-