10. ELECTRIC ORGANS
359
(Adrian et al., 1970; cf. Bennett, 1970). It can also be considered a kind
of inactivation ( Bennett and Grundfest, 1966).
The resting membrane potential is apparently largely determined by
potassium selective sites and the concentration gradient of potassium,
although the cells may also be significantly permeable to chloride. In
some electrocytes large hyperpolarizations cause the conductance of the
resting membrane to decrease which can lead to what have been termed
hyperpolarizing responses ( Bennett and Grundfest, 1966). Since the
conductance decreases by a large fraction of the resting value, these
responses must involve a change in the potassium selective sites of the
resting membrane. There is no known physiological significance of these
responses.
In myogenic electrocytes another kind of a response is mediated by
sites sensitive to the neurotransmitter acetylcholine released by the presynaptic nerve fibers. The transmitter increases the permeability at these
sites, presumably to sodium and potassium ions, and the resulting current
flow generates a postsynaptic potential (PSP). The change in conductance is virtually independent of the membrane potential, a property
which has been termed electrical inexcitability ( Grundfest, 1957; Bennett, 1964). An important consequence of there being an electrically inexcitable conductance increase is useful in establishing the chemical
mediation of a postsynaptic response (Bennett, 1966). If the membrane
is made sufficiently inside positive prior to the release of transmitter, the
PSP current that would have depolarized the cell reverses and flows to
make the inside of the cell less positive; the response is inverted. The
potential at which the current reverses (loosely speaking the equilibrium
potential by analogy with Nernst potentials) is generally slightly negative to the zero potential; this is the primary reason for believing that
permeability is increased to both sodium and potassium (see N.
Takeuchi, 1963).
Sites of the kinds described are combined in a number of ways in
membranes comprising the faces of different electrocytes. These combinations are as follows:
(1) Spike generating membrane. This membrane contains sodium
sites, and may or may not have potassium activation. Probably anomalous
rectification is present in spike generating membranes of all myogenic
organs. PSP generating sites may or may not be present.
( 2 ) Membrane exhibiting delayed rectification without spike activity.
This membrane appears to be spike generating membrane that has lost
the sodium mechanism. PSP generating sites may or may not be present.
( 3 ) Postsynaptic potential generating membrane. Postsynaptic poten-
359
(Adrian et al., 1970; cf. Bennett, 1970). It can also be considered a kind
of inactivation ( Bennett and Grundfest, 1966).
The resting membrane potential is apparently largely determined by
potassium selective sites and the concentration gradient of potassium,
although the cells may also be significantly permeable to chloride. In
some electrocytes large hyperpolarizations cause the conductance of the
resting membrane to decrease which can lead to what have been termed
hyperpolarizing responses ( Bennett and Grundfest, 1966). Since the
conductance decreases by a large fraction of the resting value, these
responses must involve a change in the potassium selective sites of the
resting membrane. There is no known physiological significance of these
responses.
In myogenic electrocytes another kind of a response is mediated by
sites sensitive to the neurotransmitter acetylcholine released by the presynaptic nerve fibers. The transmitter increases the permeability at these
sites, presumably to sodium and potassium ions, and the resulting current
flow generates a postsynaptic potential (PSP). The change in conductance is virtually independent of the membrane potential, a property
which has been termed electrical inexcitability ( Grundfest, 1957; Bennett, 1964). An important consequence of there being an electrically inexcitable conductance increase is useful in establishing the chemical
mediation of a postsynaptic response (Bennett, 1966). If the membrane
is made sufficiently inside positive prior to the release of transmitter, the
PSP current that would have depolarized the cell reverses and flows to
make the inside of the cell less positive; the response is inverted. The
potential at which the current reverses (loosely speaking the equilibrium
potential by analogy with Nernst potentials) is generally slightly negative to the zero potential; this is the primary reason for believing that
permeability is increased to both sodium and potassium (see N.
Takeuchi, 1963).
Sites of the kinds described are combined in a number of ways in
membranes comprising the faces of different electrocytes. These combinations are as follows:
(1) Spike generating membrane. This membrane contains sodium
sites, and may or may not have potassium activation. Probably anomalous
rectification is present in spike generating membranes of all myogenic
organs. PSP generating sites may or may not be present.
( 2 ) Membrane exhibiting delayed rectification without spike activity.
This membrane appears to be spike generating membrane that has lost
the sodium mechanism. PSP generating sites may or may not be present.
( 3 ) Postsynaptic potential generating membrane. Postsynaptic poten-