Neurophysiological correlates of tactile stimulus-induced whole-body eversion, a novel type of behavior in the snail Helix pomatia L

Abstract

Repeated weak tactile stimuli to the exposed skin of the foot of withdrawn snails ( Helix pomatia L.) result in a rapid whole-body eversion response not previously described. We studied the neurophysiological correlates of this novel type of behavior in semi-intact preparations consisting of the foot, pneumostome and mantle collar attached to the CNS. The pneumostome opening component of the eversion response is retained in semi-intact preparations and can be triggered by a series of weak tactile stimuli to the foot. The same stimuli also strongly excite a giant neuron (LPd7) in the pedal ganglion. This cell has axon branches in the anal and left pallial nerve which innervate the pneumostome and mantle collar. Intracellular stimulation of LPd7 causes the pneumostome to open. Bursts of spikes in LPd7 also lead to contractions of a specific group of muscles in the mantle collar innervated by the left pallial nerve. Both responses are retained when the CNS is bathed in high Mg 2+/0 Ca 2+ saline but the mantle response has a much shorter latency than the opening of the pneumostome. We conclude that LPd7 is an efferent neuron which causes opening of the pneumostome via a peripheral pathway in the mantle collar and may also be a motoneuron of muscles in the mantle collar. Besides its pneumostome-opener and putative mantle motoneuron function, the LPd7 cell also triggers movements of the foot which are also part of the eversion sequence in intact snails. The LPd7 cell receives excitatory input from neurons which trigger withdrawal reactions in active snails. The excitation is due to peripheral interactions between the two cell types and leads to opening of the closed pneumostome following brief aversive stimuli. We suggest that LPd7 is part of a novel putative neuronal network underlying the described eversive reaction to tactile stimuli.