Respiratory Physiology & Neurobiology 154 (2006) 61–72 Modulation of aerial respiratory behaviour in a pond snail  Ken Lukowiak ∗ , Kara Martens, Mike Orr, Kashif Parvez, David Rosenegger, Susan Sangha Hotchkiss Brain Institute, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada, T2N 4N1 Accepted 10 February 2006 Abstract Aerial respiratory in Lymnaea is driven by a three-neuron CPG whose sufficiency and necessity has been directly demonstrated. While this CPG is ‘hard-wired’ it displays a tremendous amount of plasticity. That is, it is possible by employing specific training procedures to alter how it functions in a specific hypoxic environment. Thus, it is possible to study directly the causal mechanisms of long-term memory formation, forgetting, and modulation of the memory at a single cell level. Thus, it is possible to use a relatively simple three-neuron CPG to study not only important questions concerning regulation of important homeostatic mechanisms but to also use it to study how learning and non-declarative memory are mediated at a cellular level. © 2006 Elsevier B.V. All rights reserved. Keywords: Respiratory CPG; Learning; Memory; Lymnaea 1. Introductory remarks As physiologists in one-way or another our research is concerned with ‘the where’, the ‘why’ and ‘the how’ of homeostasis and quite possibly the neural control of respiration is the quintessential homeostatic system. Our laboratory has focused on the neuronal mecha- nisms underlying control of two molluscan (Aplysia and Lymnaea) respiratory systems. However, our pri- mary reason for using these model systems was not to  This paper is part of a special issue entitled: “Frontiers in Com- parative Physiology II: Respiratory Rhythm, Pattern and Responses to Environmental Change”, guest edited by W.K. Milsom, F.L. Pow- ell and G.S. Mitchell. ∗ Corresponding author. Tel.: +1 403 220 6872; fax: +1 403 283 2700. E-mail address: lukowiak@ucalgary.ca (K. Lukowiak). study respiratory behaviour per se; but rather to use these systems to study the neuronal mechanisms of learning, memory, and forgetting. We will concentrate this review on key aspects of aerial respiration in Lym- naea and show how this behaviour can be modified by experience, the environment, and ageing. Homœstasis is defined as in the following manner: “to designate stability of the organism” and according to the OED was first used in the English language by Walter Canon in 1926. Unsurprisingly then the word may imply to some the necessity of constrained plas- ticity in the neural system that controls a specific home- ostatic behaviour; however, we feel that the ability to maintain homeostasis can only come about because there is plasticity within the neural control system. Let us explain what we mean by this. The ‘hard-wired’ neural system that controls aerial respiration in Lym- 1569-9048/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.resp.2006.02.009