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… A simple idea began to grown in my mind: that the entire nervous system could be regarded as a system of superimposed sensory-motor loops. Starting from simple pathways involved in locomotion, a kind of neuromechanical loop evolved in vertebrates which inevitably gave organisms greater flexibility and autonomy in the environment. In parallel, they developing internal states which inevitably led towards an awareness of the world, of the body and of the inner self. I was co-ordinating the undergraduate topic ‘Sensory Motor Systems’ at the time. I was teaching about the neuroscience of locomotion in the lamprey and higher vertebrates, including humans. The explanatory power of a concept of superimposed internal neural loops began to grow.
In 2007-2008, I was invited by Christos Pantelis, a Melbourne psychiatrist, to give a talk to the Melbourne Neuropsychiatry Centre on “How can we bridge the gap between neurons and mental states?“ That event triggered a long, productive correspondence on the subject. Then a series of historical quirks, triggered by four main events, dictated my return to experiments in the laboratory.
In 2008, I was invited to talk at a symposium on Consciousness organised by Seong-Seng Tan at the ANS annual meeting in Hobart. My title was “Beyond dualism; the physics of brain interaction with a 4- dimensional world“. It was based on the concepts I had developed over the previous two decades starting from the “Fountains of Shizuoka” in 1993. The idea was to graphically represent transiently stable dynamic states, with all their spatial and temporal coordinates, thus generating four-dimensional objects. Because morphology and function were sections of the same 4-D object, the anatomy and physiology of living organisms could be investigated together. This should also apply to any events in the nervous system. I argued that it could resolve how to portray ‘mental-neural’ processes and overcome many philosophical mind-brain issues.
Participating in the Hobart Symposium was Peter Slezak, a Sydney philosopher. During the conference breaks, we discussed the problem of the neural correlates of consciousness. We agree that they were still some way from being identified. Peter added “Surely at least you have found the neural correlates of peristalsis!”. To my own surprise, I had to answer that no one had achieved that either! This was one of the most important influences which made me return to work on peristalsis. Since then, I have often commented that, while we may not know how the brain shapes ideas, but we don’t even understand how the gut shapes faeces!
The second event involved my colleague Nick Spencer. An Australian, he had been a student of Mollie Holman at Monash. He had spent a decade in Reno Nevada but had moved back in 2007 to Australia to join Physiology at Flinders. He set up an independent research laboratory with some of my old equipment. During this time he asked my opinion about a new commercial gadget for constructing DMaps. This had been developed in the USA by colleagues copying our original methods. I was unimpressed by the system which seemed rather limited to small species. I had been interested in re- investigating the movements of the rabbit intestine which I knew had well-developed myogenic mechanisms and neurogenic peristalsis.
This period, in 2009, coincided with a visit by of Grant Hennig, my former PhD student, who had originally developed the computer method of constructing spatio-temporal maps from videos of intestinal movements for DMaps. He had updated his methods and they were compatible with the new generation of Macintosh computers. I could once again apply the DMaps method this time to record intestinal motor patterns in the rabbit intestine.
The fourth crucial event which encouraged my return to experimental work was the visit of Phil Dinning. He had been invited by Nick Spencer and Simon Brookes to bring a revolutionary new manometry probe, which Phil had developed with John Arkwright. It was able to record intraluminal pressures at higher resolution than previous probes and could tie together studies in intact humans with studies of isolated gut in the laboratory.
Return to the experimental labs
I returned to experimental science as a guest in the laboratories of my younger colleagues, Nick Spencer, Simon Brookes, and Phil Dinning. Now my younger mentors. In this fruitful period, mainly based in Nick’s laboratory, I worked on new methods to record and portray intestinal motor patterns, combining spatio-temporal maps of diameter with pressure recorded with Phil’s high resolution manometry. We first studied the rabbit colon. This herbivore has some anatomical similarities with both the guinea pig, but also, with its taeniated colon, with humans.
We then applied this new methodology to the guinea pig, studying the motor patterns involved in formation and propulsion of faecal pellets. In collaboration with clinical investigators, we addressed the possibility of using these new methods in humans. More recently, I have joined Nick Spencer in his long-term interest in mouse colonic motility, applying spatio-temporal mapping to this small preparation.
Let’s consider these steps in some detail.
Developing the new spatio-temporal mapping methods
A few years earlier, we had studied the effects of mechanical stretching to flat preparations of intestine while recording changes in force and length (Brookes et al 1999; Brookes et al 2001). With Simon, we reasoned that tissue compliance, portrayed as a length/tension curve is an appropriate way to portray the physical state of the muscle. It can incorporate changes of both length and of force, better than simple concepts of isometric and isotonic contraction and relaxation.
John Arkwright, at the CSIRO in Sydney, had built the novel manometry probe used by Phil Dinning in human subjects. It was based on a novel use of fibre-optic technology. John also moved to Flinders, in the School of Engineering, boosting the capabilities of our team. With Phil Dinning, we recorded videos of segments of rabbit small intestine and, in parallel, high resolution intraluminal pressure using the new manometry probe (Dinning et al 2011). The technology associated with this sophisticated device was the subject of several technical publications. (Arkwright et al 2011; Arkwright et al 2012; Arkwright et al 2013; Arkwright et al 2015).
We then began to investigate the mechanical complexities of colonic motility in isolated preparations of rabbit colon (Dinning et al 2012). With the availability of a manometry probe and the video analysis, which we had already developed, we acquired the ability to indirectly record forces which determine movements of the gut. We did this by recording local intraluminal pressures (kinetics) and, in parallel, the changes in the shape of the gut (kinematics). Applying some simple physical concepts, based on compliance of the intestinal wall, enabled us to identify where in the gut active contractions and relaxations were occurring during propulsive movements (Costa et al 2013; Wiklendt et al 2013).
This methodological approach enabled us, for the first time, to describe the mechanical states of the intestinal muscle during neurally-mediated motor patterns (neurogenic) and to distinguish them from myogenic motor patterns generated by the net of intestinal pacemaker cells (the interstitial cells of Cajal). We then applied this analysis to several different animal species confirming its utility (Costa et al 2013b).
We were ready to integrate our research with the classic findings of Bayliss and Starling in 1899 with their ‘Law of the Intestine’. We could explain how propulsion involved polarised enteric pathways and provided the experimental evidence that propulsion is a self-sustaining process which involves continuous sequential activation of a “neuromechanical loop”. The loop consists of mechanically sensitive enteric circuits which, when activated by a bolus, activate a polarised mechanical responses of the circular muscle. This leads to contraction orally and relaxation anally – the Law of the Intestine. The bolus being pushed forward then activates a neuromechanical loop at its new location, re-initiating the propulsive process (Dinning et al 2014).
We confirmed that neuromechanical loops are also responsible for the propulsion of pellets in the guinea pig distal colon. We also revealed ex vivo that the faecal pellets are formed at the junction between the proximal and distal colon (Costa et al 2015).
I was determined to investigate the nature of the other motor patterns in the guinea pig colon that were not directly involved in propulsion of faecal material already formed into pellets. We identified different motor patterns in the two functional sections of the colon, proximal and distal (Costa et al 2017; Costa et al 2019). We then revealed how these new neurally mediated motor patterns in the guinea pig distal colon interact with the neuromechanical loop during pellet propulsion (Costa et al 2019b). We completed this work by identifying the motor patterns responsible for the actual formation of the pellets at the flexure between the proximal and distal colon (Costa et al 2021).
For me, these publications closed a long historical cycle. It started experimentally in 1976 (Costa and Furness 1976) and was conceptually reinvigorated in 2008 (at the Hobart ANS Symposium on consciousness). In a simple system it brought together principles of intestinal mechanics and knowledge of enteric circuits to explain motor activity. Who would have thought that the humblest body function of shaping and propelling faecal pellets could give such satisfaction – and not just to the guinea pig.
I was fortunate to join Nick Spencer’s project on the mouse colon. This had been part of his research focus over the decades since his PhD with Mollie Holman. During his time in Reno, Nick had developed some advanced recording methods which helped him set up one of the most effective Australian laboratories of basic neuro-gastroenterology. The extended team included the laboratories of Simon Brookes and of Phil Dinning at Flinders, and spans questions about the important molecules, cells tissues and organs from experimental animals to human patients.
Nick continued his intensive studies on the mouse colon. I was fortunate to be part of his team which revealed a new neural pattern. This involves coordinated firing of motor neurons at ~2Hz and appears to modulate several colonic motor patterns (Sorrensen et al 2016); Hibberd et al 2017; Hibberd et al 2018; Spencer et al 2018; Spencer et al 2020; Wiklendt et al 2020; Spencer et al 2021). My contribution was to demonstrate that the mouse colon distinct motor patterns in the proximal and distal colon (Costa et al 2020; Costa et al 2021b). My contribution was to demonstrate that the mouse colon had, similar to the guinea pig, distinct motor patterns in the proximal and distal colon (Costa et al 2020; Costa et al 2021b).
We also demonstrated that the spontaneous motor complexes in the mouse are correlated along the entire small and large intestine, suggesting that the enteric nervous system is functionally connected along the entire gastrointestinal tract (Costa et al 2019c).
Extending studies of intact isolated segment of intestine to human specimens led to the confirmation of significant similarities between experimental animals and humans (Carbone et al 2013; Kuizenga et al 2015).
One important development arose from the our combination of kinematic and kinetics to identify mechanical states of the muscle. From this the possibility of using intraluminal impedance as a proxy for diameter of the gut was tested (see above). Impedance measurements make it possible to investigate the mechanical state of the muscles in live humans in vivo, where video recording is not feasible. This idea was validated from measurements of movement of gas in the lumen, which is often the case in vivo (Mohd et al 2017). This lays the groundwork for studies in live human patients of gas movement which is suspected as a cause of some abdominal pain.
The conceptual frame of mechanical states of the muscle was applied to human subjects in collaborations with clinical investigators at Flinders. We described for the first time a complete list of motor patterns of the human colonic motility recorded by the high-resolution manometry probe (Dinning et al 2014b).
We then successfully extended our methods to a variety of clinical conditions of the colon and the oesophagus, opening a new era in the clinical studies of digestive tract functions (Loganathan et al 2013; Omari et al 2014; Omari et al 2015; Omari et al 2016; Dinning et al 2015; Leibbrandt et al 2016; Wessel et al 2016; Cock et al 2020; Mohd et al 2020; Raju et al 2013).