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If the hydrostatic pressure in the enteron of Hydra is made more than 2-4 mm of water greater than the outside pressure, the animal becomes distended, indicating that the normal enteron pressure is less than this. Positive enteron pressure attenuates the spontaneous, negative-going electrical spikes across the body wall, which are called contraction pulses (CP's) because of their relation to column contraction. Pressure has little effect on the transepithelial resting potential, The low frequency electrical impedance of the column is nonlinear. The impedance tends to increase as the transepithelial potential is made more negative. The nonlinearity has both initial and delayed components. The De impedance of the column near the resting potential averages 100 kohms (approximately 5 kohms-cm2). The phase between transepithelial potential and imposed sinusoidal current approaches --90 ° with increasing current frequency. Bode plots of the column impedance and the phase lag indicate that the column has three or more time constants. CP's show several unusual features: (a) their amplitude and frequency are essentially independent of the transepithelial potential when the latter is altered by imposed current; (b) there is practically no change in column impedance during CP firing; (c) when the transepithelial potential is clamped at zero, CP's continue to appear spontaneously as current spikes. These features are consistent with the hypothesis that the CP-generating membrane forms but a small fraction of the total transverse impedance of the column.

There is a maintained electrical potential of 15 to 40 millivolts across the two epithelial layers forming the body wall of Hydra, the inside of the animal being positive. Negativegoing (depolarizing) spikes are recorded spontaneously and sometimes in response to depolarizing current pulses. These spikes usually overshoot the zero potential level. The large size of the spikes and the orientation of the potential difference across the body wall indicate that this electrical activity is epithelial rather than nervous in origin.

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