โ Scribed by Robert K. Josephson
No coin nor oath required. For personal study only.
Spontaneous electrical potentials can be recorded from a microelectrode in the hydranth or distal stalk of the hydroid Tubularia crocea. These potentials appear as repeating single pulses and short bursts of pulses. The duration of the pulses is usually about 40 msec.
Evidence indicates the electrical potentials are due to activity in a nerve net. The patterns of activity imply that the system generating it has spontaneity, feed-back and formation of patterned bursts. These properties emphasize the complexity which can be found in even a "simple" nervous system like that of coelenterates.
The most typical pattern of activity is single pulses, repeating often fairly rhythmically at a frequency of from 1/1-5 sec-1/30 sec, interrupted at regular (10 sec to several min) or irregular intervals by a burst of 3-12 pulses at a frequency of about 5/sec. There is usually a post-burst-pause. Activity can appear as only single pulses, pulses and bursts, or only bursts in different preparations or in the same preparation at different times.
Single pulses are rarely and bursts never restricted to one portion of the distal polyp. The conduction velocity of potentials in the upper stalk is about 42 cm/sec. Patterns of activity can be recorded from both isolated hydranths and decapitated stalks.
Of the several types of spontaneous polyp movements, only concerted elevation of the proximal tentacles is clearly correlated with electrical activity. Such concerted elevation always occurred during a burst, following a burst, or following single pulses immediately after a burst. Movements of individual proximal tentacles are sometimes associated with bursts, but usually have no electrical correlates. Bursts often occur with no associated tentacle movements. No correlation was found between electrical activity and movements of distal tentacles, or between single pulses other than those immediately following bursts and visible polyp behavior. Excess Mg 2+ first abolishes spontaneous polyp movements, then causes a slow reduction in the amplitude of the recorded potentials, but does not seem to affect the frequency of the potentials.
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