356
M. V. L. BENNETT
ductance that may or may not vary during activity. Alternatively, each
membrane can be considered to consist of a single branch with a single
internal potential and conductance, both of which can change. The
membrane capacity, which may or may not be significant in electrocytes,
is in parallel with the other branch or branches of the membrane
equivalent. Generally, but not always, the resistance of the cytoplasm
is negligible, and in the equivalent circuit of the entire cell the two membranes are directly connected together but oriented in the opposite
direction. The circuit of the electrocyte can then be represented as a
three terminal network, two terminals just external to each face and one
inside the cell (Fig. 2D). The external terminals can be considered to
be connected by a resistive path through the tissue and medium surrounding the cell. Another resistance leading to the indifferent electrode
is required if monopolar recording is employed. This resistance is connected into the resistance of the external path thereby dividing it into
two components. The placement of the connection depends on the symmetry with respect to the indifferent electrode of the material making up
the external resistance. There are thus three resistances in the external
path which are labeled T,,, re2, and res in Fig. 2D.
Consider what is recorded when one membrane of a cell generates a
spike and the other membrane does not change its properties (Fig. 2E ) .
Appropriate differential recordings show the potentials across each face
and across the external medium. The recording across the active face
gives the internal potential of the membrane less whatever internal
voltage drop there is due to current flow through the opposed, inactive
face and external path. Differential recording across the inactive face
gives the passive voltage drop across this face that results from current
flow. The potential across the external path is the voltage drop across the
external resistances and also the difference between the potentials across
the opposed membranes. Monopolar recording outside the two faces
gives potentials of opposite sign; for a conventional depolarizing response
the recording is negative going outside the active face and positive going
outside the inactive face. The potentials are smaller than when differential recording is used because they represent voltage drops across part
of the resistance around the external circuit ( r e l and rr2 in Fig. 2D). A
monopolar recording by an intracellular electrode cannot distinguish
between the two faces. The spike is smaller than that recorded across
the active face by the voltage drop across T , ~ in Fig. 2D. The spike is
larger than the passive drop across the inactive face by the voltage drop
across rez.
Obviously the same kind of inferences can be made from monopolar
and differential recording since monopolar records can be subtracted to
M. V. L. BENNETT
ductance that may or may not vary during activity. Alternatively, each
membrane can be considered to consist of a single branch with a single
internal potential and conductance, both of which can change. The
membrane capacity, which may or may not be significant in electrocytes,
is in parallel with the other branch or branches of the membrane
equivalent. Generally, but not always, the resistance of the cytoplasm
is negligible, and in the equivalent circuit of the entire cell the two membranes are directly connected together but oriented in the opposite
direction. The circuit of the electrocyte can then be represented as a
three terminal network, two terminals just external to each face and one
inside the cell (Fig. 2D). The external terminals can be considered to
be connected by a resistive path through the tissue and medium surrounding the cell. Another resistance leading to the indifferent electrode
is required if monopolar recording is employed. This resistance is connected into the resistance of the external path thereby dividing it into
two components. The placement of the connection depends on the symmetry with respect to the indifferent electrode of the material making up
the external resistance. There are thus three resistances in the external
path which are labeled T,,, re2, and res in Fig. 2D.
Consider what is recorded when one membrane of a cell generates a
spike and the other membrane does not change its properties (Fig. 2E ) .
Appropriate differential recordings show the potentials across each face
and across the external medium. The recording across the active face
gives the internal potential of the membrane less whatever internal
voltage drop there is due to current flow through the opposed, inactive
face and external path. Differential recording across the inactive face
gives the passive voltage drop across this face that results from current
flow. The potential across the external path is the voltage drop across the
external resistances and also the difference between the potentials across
the opposed membranes. Monopolar recording outside the two faces
gives potentials of opposite sign; for a conventional depolarizing response
the recording is negative going outside the active face and positive going
outside the inactive face. The potentials are smaller than when differential recording is used because they represent voltage drops across part
of the resistance around the external circuit ( r e l and rr2 in Fig. 2D). A
monopolar recording by an intracellular electrode cannot distinguish
between the two faces. The spike is smaller than that recorded across
the active face by the voltage drop across T , ~ in Fig. 2D. The spike is
larger than the passive drop across the inactive face by the voltage drop
across rez.
Obviously the same kind of inferences can be made from monopolar
and differential recording since monopolar records can be subtracted to