Some aspects of the neurophysiology of Archachatina (Calachatina) marginata (Swainson)

Abstract

The work reported in this paper deals with electro-physiological aspects of the nervous system and is restricted to the pleuro-visceral ganglia, the pallial and visceral nerves. A method for making the preparation is given and the recording equipment is briefly described. In the pallial nerves, compound action potentials frequently reached 2 to 4 mV, showing a constancy of form which may reflect a pattern of neural relationships in the ganglia. Latency in the pleuro-visceral ganglia, measured during tactile stimulation, varied from 25 to 30 ms to slightly more than 1 s. Measured during electrical stimulation, the probable shortest latency is 15 to 17 ms for the large fibre responses and 28 to 36 ms for the slow fibres. A fast efferent spike, descending in the stimulated (left pallial) nerve, showed time relations similar to those of the slower efferents in the middle fine right nerve. This suggests the existence of an integration centre in the abdominal ganglion. Analyses of large, compound potentials in well-stretched nerves indicated that: (i) Reflex potentials, although more fully separated, retained their regularity. (ii) Wide separation of recording electrodes prevented adequate separation of individual potentials and led to large, irregular wave forms. (iii) Estimates of maximum nerve lengths based on the shell breadth (Nisbet 1961 b ) were close to the functional limit. (iv) Marked decline of potential heights occurred at 25 to 30 mm from the ganglia. (v) Latency of nerve response to maximum stimulation varied by ± 3 ms. Single, large fibre potentials were found to undergo a sudden change in character at ca . 30 mm from the ganglia. Initially fast spikes (100 to 125 cm/s) lost their high velocities, potential heights declined, rise times and durations increased in length. In the visceral nerve these changes appear to be due to synapse formation but in the pallial nerves they are more probably due to the sudden formation of numbers of fine collaterals from the large fibres prior to their entry into the body wall. An increase in conduction velocity is often observed 10 to 15 mm from the ganglia. This may be due to persistent contraction and folding of the nerve fibres in the proximal portion of the nerve, giving conduction paths which are longer than the measured distances. Unlike Mya (Horridge 1958), in Archachatina responses of the ganglia to single, maximal shocks applied to a pallial nerve are small and the reflex bursts in response to tactile stimulation show rapid adaptation. Responses to repetitive stimulation at high voltage levels ( ca . 10 V) suggest that the giant cells exert a pacemaker function upon adjacent neurones and that secondary motor co-ordination centres may be present in the pleural ganglia.