Electrophysiological Investigations of the Heart of Squilla Mantis

1. With an external hook electrode placed upon the ganglionic nerve trunk of the isolated heart of Squilla mantis a burst of a small number (3-12) of nerve impulses was recorded at each heart beat. 2. The number of impulses per burst showed a direct correlation with interval between bursts. 3. The only consistent feature of impulse pattern within the bursts was a lengthening of the intervals between impulses towards the ends of the bursts. 4. Electrodes at two points on the ganglionic nerve trunk each recorded the same number of impulses at a burst. The delay between the two recording points was the same for all impulses, and usually all the impulses were, in a given heart, recorded travelling in the same direction, though this could be either forwards or backwards along the chain. 5. It is suggested that each cell in the chain of 16 fires in succession the same number of times during a burst and that the impulses travel along the same ‘firing channel’ within the ganglionic nerve trunk. 6. Cells near the two ends of the chain showed the greatest spontaneity when isolated by transverse cuts or ligatures. Coupled with the records obtained from two points, this suggests that the bursts are initiated in the front or back regions of the chain, but not invariably by the same one of the 16 cells.

Electrophysiological Investigations of the Heart of Squilla Mantis

1. There was a seasonal variation in the strength of the effect which extracts of the pericardial organs of Squilla mantis exert on the animal's heart. The extract was most effective in the spring and summer. 2. An investigation was made of the way in which the pericardial organ extract acts at a cellular level to increase the amplitude and frequency of the heart beat. 3. The extract did not affect the rate of impulse firing within the ganglionic nerve trunk burst. It increased the ratio of the number of impulses per burst: burst interval. 4. It initiated burst firing in complete g.n.t.'s which had become quiescent, and in those which had previously given only one impulse in response to an applied stimulus. 5. The junction potentials recorded intracellularly from Squilla heart muscle increased in height on adding pericardial extract. Some of this increase resulted from facilitation of the junction potentials at the increased heart rate, but 10-20% of the increase was rate-independent. 6. These results suggest that the extract acts at a minimum of two primary sites within the heart.