186
ERNEST SCHOFFENIELS
bathing fluid. It is now well established that a living membrane is
affected by the ionic atmosphere: Ca-free or K-rich media decrease the
nerve specific transverse resistance (181-182).
A change in membrane
resistance may reflect an alteration in ionic fluxes or a structural reorganization of the membrane. The problem is thus to define at the molecular
level the relations existing between a membrane and its surroundings.
This has been considered in some detail in a previous volume of this
series (Volume II, Chapter 10) as well as in another book (95).
The general conclusion to be drawn is that the structure of the membrane is different according to the ionic composition of the bathing fluid
and is directly related to the concentration ratios of the various cations.
The reader is thus referred to the above-mentioned publications for a
more complete discussion of this matter (see also reference 182a).
3. Action Potential
The effect of Ca on the action potential is rather well known (183).
An elevation in Ca concentration augments the threshold, increases the
specific resistance, and accelerates the accommodation, whereas a decrease in Ca concentration results in opposite effects and eventually
brings the nerve fiber to a state of spontaneous activity (180, 181, 184,
185).
If one studies the relationships between outside concentration and
electrical activity in the squid giant axon (186), it can be shown that,
at low Ca concentration, there is an increase in Κ permeability and a
subsequent depolarization. Sodium influx increases. Thus the passive
permeability of the membrane to Na and Κ is directly related to Ca
concentration. In Ca-free medium, the increase in Na conductance leading to an action potential is accelerated under the cathode, the reverse
being true at the anode. In the same conditions the increase in Κ conductance at the cathode is also accelerated. These results show clearly
the particular importance devolved upon Ca ions in the generation of
an action potential. This is also well demonstrated by the fact that in
Ca-free medium the Κ conductance increases and the electrical activity
is abolished. Magnesium behaves, as far as the giant axon of the squid
is concerned, like Ca but is nevertheless less effective. This could be
explained if one considers that the affinity of Ca for the cellular structure
involved is greater than that for Mg.
The above results may be summarized by saying that an increase in
Ca concentration is equivalent to a hyperpolarization, a decrease in Ca
concentration is equivalent to a depolarization.
That is why it has been suggested that the first step in the genera-
ERNEST SCHOFFENIELS
bathing fluid. It is now well established that a living membrane is
affected by the ionic atmosphere: Ca-free or K-rich media decrease the
nerve specific transverse resistance (181-182).
A change in membrane
resistance may reflect an alteration in ionic fluxes or a structural reorganization of the membrane. The problem is thus to define at the molecular
level the relations existing between a membrane and its surroundings.
This has been considered in some detail in a previous volume of this
series (Volume II, Chapter 10) as well as in another book (95).
The general conclusion to be drawn is that the structure of the membrane is different according to the ionic composition of the bathing fluid
and is directly related to the concentration ratios of the various cations.
The reader is thus referred to the above-mentioned publications for a
more complete discussion of this matter (see also reference 182a).
3. Action Potential
The effect of Ca on the action potential is rather well known (183).
An elevation in Ca concentration augments the threshold, increases the
specific resistance, and accelerates the accommodation, whereas a decrease in Ca concentration results in opposite effects and eventually
brings the nerve fiber to a state of spontaneous activity (180, 181, 184,
185).
If one studies the relationships between outside concentration and
electrical activity in the squid giant axon (186), it can be shown that,
at low Ca concentration, there is an increase in Κ permeability and a
subsequent depolarization. Sodium influx increases. Thus the passive
permeability of the membrane to Na and Κ is directly related to Ca
concentration. In Ca-free medium, the increase in Na conductance leading to an action potential is accelerated under the cathode, the reverse
being true at the anode. In the same conditions the increase in Κ conductance at the cathode is also accelerated. These results show clearly
the particular importance devolved upon Ca ions in the generation of
an action potential. This is also well demonstrated by the fact that in
Ca-free medium the Κ conductance increases and the electrical activity
is abolished. Magnesium behaves, as far as the giant axon of the squid
is concerned, like Ca but is nevertheless less effective. This could be
explained if one considers that the affinity of Ca for the cellular structure
involved is greater than that for Mg.
The above results may be summarized by saying that an increase in
Ca concentration is equivalent to a hyperpolarization, a decrease in Ca
concentration is equivalent to a depolarization.
That is why it has been suggested that the first step in the genera-