ABSTRACT
The tension reflex in a number of isolated intact segments of the earthworm exhibits a definite threshold to weight. The value of the threshold in the series of preparations tested, varies between 0·1 and 1·0 g., measured as direct tension applied to the preparation. There is also a threshold for the duration of application of the weight. In the figure shown this lies above 0·5 sec.
After an applied stimulus has been removed the peristaltic response of a preparation may continue for a finite length of time which seldom exceeds 3 min. The period for which the movement persists after the stimulus has been removed depends on the magnitude of the stimulus applied but not on the height of the contractions occurring. This fact is taken to support the view that the continuation of the response after the stimulus has been removed is a true after-discharge, due to a persistence of excitement in some region of the reflex arc.
The after-discharge consists of a co-ordinated movement of both muscle sets which may be perfectly rhythmic. The after-discharge may be inhibited by suitable stimuli. The frequency of the after-discharge may be increased by the application of a stimulus of a different type from that which elicited the original response.
When the preparation is submitted to a continuous tension, the response dies away within a period which does not normally exceed 25 min. The rate of beat in these circumstances shows three phases : a phase of acceleration, a plateau phase and a phase of decline. After the disappearance of a reflex under continuous stimulation, the response can be elicited once more by the addition of further tension.
The frequency of beat in any one preparation shows a well-defined upper limit which, over the series of experiments carried out, was never observed to exceed 1 beat in 2·0 sec. The maximum frequency of beat is obtained with tensions intermediate between those of threshold value and those near the upper limit of endurance of the preparation. Lower and higher tensions than that producing maximum frequency produce submaximal rates of beat. The range of variation between rates of beat produced by a series of different tensions is less than the range of tensions used.
After a tension reflex has been evoked, an increase in the weight applied may accelerate a submaximal frequency of beat, but cannot force the rhythm above its maximum. Large increments in weight lower the rate of beat. The rate of beat of a tension reflex may be accelerated by the application of a tactile stimulus and the frequency of a response to tactile stimulation may be accelerated by the application of tension. By whatever stimulus the acceleration of beat of a reflex is carried out, it is not accompanied by any break in the rhythm.
