ELECTRICAL ACTIVITY AND BEHAVIOR IN THE SOLITARY HYDROID CORYMORPHA PALMA. II. CONDUCTING SYSTEMS

¹ Department of Biological Sciences, University of California, Santa Barbara, California 93106

1. The triggered pulses (TPs) most commonly produced by Corymorpha in response to electrical stimulation appear similar to spontaneously occurring stalk pulses (SPs) in amplitude, waveform, and duration. Average conduction velocities in the triggered and spontaneous pulse systems are 15.7 cm/sec and 15.9 cm/sec, respectively. The TPS is non-polarized. Some animals show a relatively consistent threshold of activation; in others the threshold varies over time.

2. The behavioral response to electrical activation of the TPS varies from an almost imperceptible contraction to a vigorous inward flexion of the proximal tentacles associated with strong stalk contraction. In general, there is a clear relation between the length of a burst of electrical activity and the strength of response.

3. A small percentage (< 20%) of the Corymorpha examined showed a distal opening response, in which all distal tentacles simultaneously flared aborally, to electrical stimulation. In a few animals this response occurred in the absence of other behavior and was associated with a single small electrical pulse; in others it was associated with contraction and a burst of TPs. Only two of more than forty animals gave a consistent distal opening response. Available data do not provide an adequate basis for establishing the DOS as a system distinct from the TPS.

4. Evidence for more than one stalk conducting system is provided by the presence of two sizes of pulses which are conducted at different velocities.

5. Experiments involving the selective destruction of endoderm or ectoderm with a glass needle were used to establish that triggered stalk pulses are conducted in the ectoderm.

6. Three electrodes lined up transversely across the stalk of a Corymorpha all record rather similar pulses during a spontaneous stalk burst.

7. Electrical recording from various sorts of bridge preparations provides no evidence for preferential longitudinal conduction tracts.

8. The sympathomimetric drugs tyramine, ephedrine, and phenylpropanolamine tend to cause an immediate increase in electrical activity and grouping of hydranth pulses into short high-frequency bursts. Associated with this alteration in electrical activity is an increase in the frequency of concert behavior. Another sympathomimetic drug, D-amphetamine, causes no clear change in electrical activity.

9. Tetrodotoxin (10^-5 g/ml) has no effect on either spontaneous or triggered pulses for periods of up to 312 minutes.

10. Ca⁺⁺-free sea water causes whole Corymorpha to become motionless without apparent relaxation of the muscles. Accompanying this effect is a decrease in pulse amplitude and frequency, although pulses continue long after the animal is motionless. On return to sea water the distal tentacles are in motion within 30 seconds, followed by the proximal tentacles and proboscis within a few minutes. Electrical pulses quickly regain their full amplitude and pulse frequency begins to rise.

11. MgCl₂ has various effects depending on the concentration and length of exposure. The first effect is to eliminate obvious muscular activity while leaving electrical activity apparently unchanged. A few minutes later electrical activity is also reversibly eliminated. Exposure of the central portion of an isolated stalk to isotonic MgCl₂ while the two ends remained in sea water results in the reversible elimination of large pulses in the portion of the stalk exposed to MgCl₂ without interfering with conduction through the area or with the generation of large pulses at either end of the stalk. This result is interpreted as indicating that the epitheliomuscular cells are locally triggered to generate large electrical pulses by activity in the nerve net.