An Experimental Study of the Tactile System of Lymnaea Stagnalis (L.)

In addition to histological investigations and extirpation experiments on the CNS of L. stagnalis, carried out in our laboratory, a neurophysiological study of this CNS was desirable. As the extirpations involved cutted central connections, it seemed logical to study the pathways in the CNS. Central tactile pathways were investigated with the aid of stimulus response techniques. Mechanical stimulation by means of a hair or a needle was used. Responses were recorded in central stumps of peripheral nerves, with the aid of glass capillary sucking electrodes. Without stimulation potentials are recorded, which are caused by activities in motor nerve fibres. This spontaneous activity was studied qualitatively on film and quantitatively by means of a level selector. The influence of temperature and pH was investigated. The results obtained indicate that when long periods (24 hours) are considered, a decrease in activity can be observed (steeper as the temperature is higher). When shorter periods (2-3 hours) are analysed slow oscillations can sometimes be found, and finally fast oscillations (bursts) can be observed in periods of minutes. The bursts are practically always found in preparations kept at constant temperature. Temperature changes stimulate general activity but suppress the occurrence of bursts, during periods of at least half an hour. A rapid lowering of the pH has a stimulating effect upon the general activity, during approximately half an hour. When recordings (with micro-electrodes) ofganglion cells are compared with recordings of complete nerves, similarities in firing pattern can be observed. Responses upon tactile stimulation had to be discriminated from this spontaneous activity. In part of the experiments, this has been done with the aid of an averaged response technique. It could be demonstrated that in all central stumps of peripheral nerves investigated, a response upon tactile stimulation of the lip can be evoked. When the left lip was stimulated and simultaneous recordings were made of the left fronto-labial nerve and of one nerve emerging from another ganglion, the fronto-labial nerve was always shown to have the lowest threshold. Experiments in which various connections between central ganglia were cut, resulted only in the abolishment of stimulus-response relations when no nervous connection between the lip nerve and the recorded nerve was left intact. However, stimulus intensities had to be higher to evoke a response in proportion to the number of connections which was interrupted. The cerebral intercommissure does not fit in this scheme. The tactile signals that are transmitted through this connection adapt faster than those in other ones. Averaged responses from central stumps of peripheral nerves upon stimulation of the lip are different when stimuli of high and of low intensity are used. It appeared that high intensity stimulation did not cause identical individual responses. As it was felt that a complicated sensory input could be involved, a special study was made of the lip-nerve preparation. The responses in the lip nerve upon tactile stimulation of the lip can be divided into two types: fast and slow potentials. The two types have different characteristics with respect to stimulus intensity and stimulus interval. The slow potentials are transmitted through many small nerve fibres, and the fast potentials through less numerous, larger nerve fibres. The skin areas, where the two types can be evoked, do not overlap completely, which means that possibly two types of sense cells are involved. Hence the two types probably belong to separate systems. The reactions of the snail upon tactile stimulation can also be divided into two types: a, virtually no reaction during creeping or positive thigmotaxis during righting reactions, and b, escape reactions during increased stimulation. On the basis of the adaption and the sensitivity to increased stimulus intensities of the slow potentials, the escape behaviour may be associated with this type. The fast potentials may be associated with more proper tactile functions as testing of the substrate, etc. As both reaction types involve integrated behaviour, it can be expected that both systems use central connections.