Ephaptic Transmission and Conduction Velocity of an Action Potential in Chara Internodal Cells Placed in Parallel and in Contact with One Another

When two separated Chara internodal cells were kept in contactover a length of 14 mm or more in moist air, an action potentialof one cell could be transmitted to the other cell in about40% of cases (ephaptic transmission). The action potential ofthe former cell was sometimes eliminated transiently when anew action potential was elicited in the latter cell. The newaction potential reactivated the former cell. The conductionvelocity of the action potential was reduced from 0.30?0.11cm/s (mean?SD) to 0.15?0.05 cm/s by ephaptic transmission ofthe action potential. Substitution of an artificial pond waterfor moist air or contact over a shorter length (4 mm insteadof 14 mm or more) between the two cells reduced the couplingratio, the ratio of the change in membrane potential of onecell to that of the other cell, and no transmission was observed.After ephaptic transmission, the action potentials of the twocells were conducted at almost the same velocity along the cells.The velocity was increased to 0.68?0.06 cm/s by simultaneousconduction of the action potentials in the two adjacent cells.When the simultaneously conducted action potentials reachedthe node of one cell, at the point at which another cell madecontact, they were able very frequently to trigger a new actionpotential in the adjoining cell. (Received September 28, 1989; Accepted April 5, 1990)