10. ELECTRIC ORGANS
463
cells and are not merely a result of proximity. Other less direct methods
of demonstrating electrotonic transmission may be useful; for example,
a PSP of very short latency can be assumed to be electrically mediated
(cf. Bennett, 1966).
The morphological basis of electrotonic coupling has been investigated in a number of systems (Pappas and Bennett, 1966; Bennett et al.,
1967a,b,c; Kriebel et al., 1969; Pappas et al., 1971; these papers include
other references ) . In every case close membrane appositions occur (Figs.
71 and 72) which are rare or absent in neighboring regions and which do
not occur at synapses for which there is evidence that transmission is
chemically mediated. These close appositions are believed to be the site
of current passage between cells. They are probably what have recently
been termed gap junctions (Revel and Karnovsky, 1967; Brightman and
Reese, 1969). This name arises from the appearance in perpendicular
sections of a 20-30 A gap between membranes following suitable fixation
procedures. The gap is penetrable by marker substances applied in the
extracellular space. The gap is not uniform but is made up of a more or
less hexagonal lattice of channels. There is evidence that in the spaces
outlined by the lattice, channels separated from extracellular space cross
the junctional complex and interconnect the cell cytoplasms. The intercytoplasmic channels are perhaps 10A in diameter and provide sites for
movement of ions and other small molecules between cytoplasms of the
coupled cells (Payton et al., 1969; Pappas et al., 1971). This class of
junctions is distinct from tight junctions, which are appositions where
extracellular space appears completely occluded in perpendicular sections, and where there is no hexagonal structure in tangential sections
( Brightman and Reese, 1969). (Both types were formerly called tight
junctions. The current nomenclature in this area is confusing and will
probably be revised when there is more general argreement as to morphological and physiological properties of the junctions and criteria for
their identification. ) These latter junctions generally occur in epithelia
where they form complete rings around cells (zmulae occludentes;
singular, zonula occludens) that prevent transepithelial leakage through
intercellular clefts ( Farquhar and Palade, 1963; Brightman and Reese,
1969). There is no evidence that zonulae occludentes form low resistance
channels between cell cytoplasms because no cells are known to be both
electronically coupled and joined exclusively by these junctions.
Electrotonic coupling between cells of the same kind can be mediated
by electrotonic synapses between somata or dendrites (Fig. 71). Alternatively, cells can be coupled by way of presynaptic fibers that form
electrotonic synapses on the cells; current then spreads from one postsynaptic cell to another through the presynaptic fibers (Fig. 72). (Since
463
cells and are not merely a result of proximity. Other less direct methods
of demonstrating electrotonic transmission may be useful; for example,
a PSP of very short latency can be assumed to be electrically mediated
(cf. Bennett, 1966).
The morphological basis of electrotonic coupling has been investigated in a number of systems (Pappas and Bennett, 1966; Bennett et al.,
1967a,b,c; Kriebel et al., 1969; Pappas et al., 1971; these papers include
other references ) . In every case close membrane appositions occur (Figs.
71 and 72) which are rare or absent in neighboring regions and which do
not occur at synapses for which there is evidence that transmission is
chemically mediated. These close appositions are believed to be the site
of current passage between cells. They are probably what have recently
been termed gap junctions (Revel and Karnovsky, 1967; Brightman and
Reese, 1969). This name arises from the appearance in perpendicular
sections of a 20-30 A gap between membranes following suitable fixation
procedures. The gap is penetrable by marker substances applied in the
extracellular space. The gap is not uniform but is made up of a more or
less hexagonal lattice of channels. There is evidence that in the spaces
outlined by the lattice, channels separated from extracellular space cross
the junctional complex and interconnect the cell cytoplasms. The intercytoplasmic channels are perhaps 10A in diameter and provide sites for
movement of ions and other small molecules between cytoplasms of the
coupled cells (Payton et al., 1969; Pappas et al., 1971). This class of
junctions is distinct from tight junctions, which are appositions where
extracellular space appears completely occluded in perpendicular sections, and where there is no hexagonal structure in tangential sections
( Brightman and Reese, 1969). (Both types were formerly called tight
junctions. The current nomenclature in this area is confusing and will
probably be revised when there is more general argreement as to morphological and physiological properties of the junctions and criteria for
their identification. ) These latter junctions generally occur in epithelia
where they form complete rings around cells (zmulae occludentes;
singular, zonula occludens) that prevent transepithelial leakage through
intercellular clefts ( Farquhar and Palade, 1963; Brightman and Reese,
1969). There is no evidence that zonulae occludentes form low resistance
channels between cell cytoplasms because no cells are known to be both
electronically coupled and joined exclusively by these junctions.
Electrotonic coupling between cells of the same kind can be mediated
by electrotonic synapses between somata or dendrites (Fig. 71). Alternatively, cells can be coupled by way of presynaptic fibers that form
electrotonic synapses on the cells; current then spreads from one postsynaptic cell to another through the presynaptic fibers (Fig. 72). (Since