A Neurophysiological Study of the Peripheral Tactile System of the Pond Snail Lymnaea Stagnalis (L.)

1. Mechano-sensitive neurones of the body wall of the pond snail Lymnaea stagnalis were studied with electrophysiological techniques. 2. The tactile innervation areas of all skin nerves were determined. Each innervation area has one or more super-sensitive regions, the innervation centres. Most of these centres are situated in the anterior part of the animal. This explains the important tactile behavioural functions of these body parts. 3. As found in other animals the territories of the nerves sprouting from the same ganglion overlap considerably, whereas those of nerves of different ganglia overlap to a much lesser extent. The latter differs from the situation in vertebrates, but is found in many invertebrates. 4. The pedal ganglia appeared to be important integrating and relay centres of tactile information. 5. With the aid of synaptic blocking agents primary and higher order sensory (h.o.s.) neurones could be distinguished. 6. Primary touch sensitive neurones (pTSN) and primary stretch sensitive neurones (pSSN) are present in the body wall. 7. Two types of pTSN's were distinguished (I and II). As type II rapidly ceases to fire upon repeated stimulation, it probably has an alarm function. 8. The properties of pTSN type I were studied in detail and a structural model of this cell could be made. The cell body is situated in the CNS. It sends one afferent to the periphery where it branches in the same way as the peripheral nerve in which it runs. In the skin the afferent ends with many dendritic trees, the receptors. The spike generating regions are situated at the base of the trees. This cell has remarkable similarities to vertebrates touch sensitive neurones. 9. In the foot pTSN's type I with large receptive fields were found. It is supposed that they function in intensity discrimination and movement detection. Furthermore, these pTSN's possibly function in an alarm system. 10. In the anterior part of the animal the receptive fields are small. With these pTSN's the animal probably localizes tactile stimuli. 11. Indications were obtained that still another pTSN is involved in peripheral reflexes only and does not have axons in peripheral nerves. 12. Extremely phasic pSSN's were found in the body wall and in the external organs, and less phasic pSSN's in the muscles of the buccal mass. The latter are supposed to function in modulating muscular activities. 13. A number of the tactile h.o.s. neurones were also studied. H.o.s. neurones sensitive to touch, pressure and stroke, or stretch stimuli were found. Models of the first two types were made. The cell body of these neurones is located in the CNS, and integration of sensory information takes place in peripheral ganglia. 14. The properties of the h.o.s. neurones agree well with the general properties of the h.o.s. neurones of other invertebrates. It is suggested that these neurones function in avoidance behaviour and in modulation of central excitabilities. 15. Some h.o.s. neurones have a motor function in addition and are involved in peripheral reflexes.