Aspects of the neural organization of Littorina littorea (L.)

An investigation has been made of the nervous mechanisms mediating the withdrawal response of Littorina littorea in order to establish aspects of the functional organization of its nervous system. Observations of the withdrawal response, elicited by tactile stimulation of the tentacles and leading to partial withdrawal of the animal into its shell, revealed that the response consisted of a fast contractile phase followed by a prolonged relaxation and extension phase, without a period of prolonged contraction. The effects of tactile stimulation on isolated tentacles were determined and the roles of the tentacle and cerebral ganglia in mediating tentacle responses were established. Four classes of response, corresponding to four types of mechanosensory neurones, were recorded from the tentacle nerves. Two morphologically separate pathways were traced from the right tentacle between the paired cerebral, pleural and pedal ganglia to a nerve shown to innervate the columellar muscle and to originate from the left pleural ganglion. It was shown that the two pathways were excited by different classes of tentacle response. A comparison of 'normal' contractions of the columella muscle with those induced by either of the two pathways showed that both pathways initiated apparently normal muscle contractions. These consisted of a rapid contractile phase followed by a prolonged relaxation phase of similar character to that of the withdrawal response of the intact animal. An explanation of the double system of pathways has been suggested after consideration of the properties of the sensory neurones found in the tentacles. One pathway is thought to function both as a high threshold slowly adapting system and also as a low threshold system, while the other functions as a high threshold rapidly adapting system. The nerves, ganglia, tentacles and columellar muscle were examined histologically and some limited studies have been made of spontaneous central activity and conduction velocity.