Studies on Stimulation in Nitella.

Abstract

Waves of negativity occurring spontaneously in Nitella, sometimes in regular rhythm, have been previously described. 1 Similar negative variations, passing down the cell, may be induced in a variety of ways, by touching, bending, by application of solutions or by small direct 2 potentials. 3 They resemble the action currents of muscle and nerve in form and magnitude but last longer and are propagated more slowly.

Nitella appears to follow in general the laws of stimulation of muscle and nerve but there are some interesting exceptions. For example, it is far from obeying the all or none law completely since the P.D. (normally positive 4 when the cell is in contact with dilute solutions) may disappear partially or completely at the stimulated spot, and conduction with a decrement occurs. 5 The normal P.D. returns in from 5 to 30 seconds. The electrical (polarization) resistance falls off simultaneously with the P.D. but it takes much longer to return to normal. The period during which recovery takes place constitutes the refractory period after which the cell may again be stimulated.

The stimulus is transmitted along the cell 6 at a rate of from 1 to 2 cm. per second and may pass across the “node” to a neighboring cell (or even through two aqueous contacts for a distance of half an inch or more to another cell, e. g., to a cell from a different plant). The fact that it is unable as a rule to pass a spot whose positive P.D. has been (reversibly) lowered to a sufficient extent (e. g., by an opposed potential or by the application of 0.05 M KCl) lends support to the idea that the wave of stimulation consists of a local current flowing outward through the protoplasm and through the cell wall to a neighboring spot which is less positive (and then back through the protoplasm and sap).