4.6 The Idea of a Neuromechanical Loop

The idea of neuromechanical loops arose during decades of investigations of the Enteric Nervous System which has been my own research field of a lifetime. From these studies some general principles emerged applicable to other parts of the nervous system.

The Enteric Nervous System consists of neural circuits embedded within the gut wall and are probably one of the best-known mammalian neural circuits. It includes intrinsic primary afferent neurons, interneurons and motor neurons. They subserve neural pathways investigated primarily as ‘enteric reflexes’ elicited by distension of the gut wall, resulting in the ascending excitatory reflex, which contracts the intestinal muscle behind (ie on the oral side of) a bolus of intestinal contents, and the descending inhibitory reflex which relaxes the muscle ahead of (ie on the anal side) the bolus.

These movements of the gut, involved in the controlled propulsion of nutrients along the digestive tube, is known as peristalsis. Until recently, peristalsis also was regarded as a simple reflex (the peristaltic reflex). However, we could better explain peristalsis as a sequential activation of enteric pathways forming neuromechanical loops. We proposed that peristalsis is a form of intestinal locomotion rather than a simple reflex, thereby allowing this behaviour to adapt to variations in the intraluminal contents of the gut. The structural polarity of the enteric neural circuits, revealed by neuroanatomical studies, ensures the correct direction of propulsion of contents along the gut. This convergence of structural and functional studies led to a unifying framework crossing historical distinctions between anatomical and physiological observations.

*Functional analysis of propulsive peristaltic activity in the gut.*

*Diagram of the enteric neural pathways underlying the ascending excitatory reflex (red) and the descending inhibitory reflex (blue). These pathways interact to generate and regulate peristalsis* *in response to inputs from local sensory neurons (grey).*

The concept of a neuromechanical loop to explain peristalsis. Changes in the activity of the neural circuits (“neuro”) change the contractile behaviour of the gut muscle (“mechanical”) which in turn alter the activity of the neural circuits via their local sensory inputs. Oral end to the left, anal end to the right.
As in the preceding diagram, **red** indicates activity of the ascending excitatory pathway leading to contraction of the gut muscles, and **blue** indicates the activity of the descending inhibitory pathway leading to relaxation of the gut muscles. Thus the gut contents are propelled in the oral to anal direction.

To read more about how I came to develop these concepts and their experimental evidence, see my Memoir here and here.

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