Discuss the roles of feedback and feed­forward in the control of . Feedback and feed-forward responses are very important in the control of digestion. They aid in the progression of ingesta through the ; they help with the rates and types of secretions from different areas of the gut; to a lesser extent, they also help control the rates of digestion and absorption in each part of the gut. Feed-forward systems prepare the remainder of the GI tract for what is to come, whereas feedback mechanisms enable the guts to respond to the volume, type and movement of contents. Different can act over a long distance or merely create a response locally.

Motility Local reflexes = , mixing and secretion, mediated by sensory fibres entirely within the ENS.

Peristalsis = gut motility patterns that propel food in the anal direction, usually only involves a short segment of the at any one time.

Peristaltic = stretching of gut muscle wall elicits contraction of longitudinal and circular muscle behind a bolus, mediated by Ach, but relaxation of the muscle in front of the bolus, mediated by NO.

Local stretch -> enterochromaffin cells in mucosa release (5-HT) -> sensory neurones stimulated -> -> neurones on oral side release Ach, on anal side release NO ± ATP -> contraction on oral side, relaxation on anal side.

Vagovagal reflexes = both afferent and efferent arms of a reflex arc are carried by the vagus, occurs with fibres transmitting signals to the spinal cord and brainstem. Involved in certain reflexes e.g. receptive relaxation – vagovagal reflex responsive to oesophageal or stomach stretch causes the fundus to relax using VIP to accommodate extra food with little pressure increase, also pain reflexes. Sensory fibres have cell bodies in dorsal root ganglia.

Long-range reflexes = used to coordinate activity of different parts of the gut. Sensory fibres have cell bodies in the ENS, usually involves a synapse in the prevertebral ganglia. May use gastrointestinal hormones. Can be feedback – reducing motility in proximal sections if the contents are moving too fast – or feed-forward – enhancing the motility of the distal parts in order to make room for what is to come. Includes:

- Ileal brake = unabsorbed nutrients in the ileum, particularly fat, reduce the motility and secretion of more proximal parts of GI tract, may involve PYY and GLP- 1(incretin). Feedback.

- Gastrocolic reflex = food entering stomach promotes the motility of the colon, resulting in increased urge to defaecate. Feed-forward.

- Duodenocolic reflex = similar to above, encourages mass movements in colon – waves of prolonged contraction alongside relaxation of the haustra.

- Gastroileal reflex = peristaltic activity in the ileum is increased in response to a full stomach, so more chyme is forced through the iliocaecal sphincter