MECHANISMS OF BIOELECTRIC ACTIVITY IN ELECTRIC TISSUE : I. THE RESPONSE TO INDIRECT AND DIRECT STIMULATION OF ELECTROPLAQUES OF ELECTROPHORUS ELECTRICUS

1. A preparation is described consisting of one or several layers of innervated cells of the electric organ of Electrophorus electricus. 2. Each plaque is multiply innervated and only at its caudal face. The nerve fibers may derive from two or more different nerve trunks. 3. During activity the innervated face becomes negative relative to the non-innervated. 4. The first electrical response of the cell to an increasing neural volley is graded and has the character of a prepotential. At a critical size of the prepotential the cell discharges with an all-or-nothing spike. 5. Both responses have durations of about 2 msec. 6. A neural volley which does not cause the spike discharge facilitates the discharge of the cell by a second subsequent volley in the same nerve (temporal facilitation). 7. The period of facilitation lasts ca. 900 msec. During the first 100 msec., the facilitation is large enough to cause a spike. In the later portion only the prepotential is facilitated. No electrical concomitant has been detected. 8. Neural volleys reaching the plaque from different trunks interact at the cell to produce a period of facilitation lasting only about 2 msec. This interaction is interpreted as spatial summation. 9. In a population of cells, simultaneous stimulation of 2 nerves causes a smaller discharge than the sum of the two isolated responses (occlusion). 10. Cells denervated for 7 weeks or more can be excited directly, but only by a current flow outward through the caudal face. 11. Weak direct stimulation causes a prepotential in the denervated plaque. On increasing the stimulus the prepotential increases to a critical size when a spike develops. The duration of both responses is about 2 msec. 12. The absolutely refractory period of the denervated cell is about 1.5 msec. and relative refractoriness lasts about 15 msec. 13. Direct stimulation causes slight facilitation lasting as long as 200 msec. 14. Repetitive stimulation of the nerve at low frequencies (2 to 3 per second) causes rapid "fatigue" of transmission. The denervated plaque, however, responds for several minutes to repetitive direct stimulation at high frequencies (25 per second).