SAQs Flashcards
Describe how the vago-vagal reflex works and its effects
The vagovagal reflex controls the contraction of gastrointestinal muscle in response to distension of the tract by food. The reflex allows for the accommodation of large amounts of food in the gastrointestinal tract.
The vagal afferent fibres are activated during the gastric phase of digestion, when the corpus and fundus are distended secondary to the entry of a food bolus. The stimulation of the mechanical receptors in the gastric mucosa stimulates the vagus afferents. The completion of the reflex circuit by vagal efferents leads to the stimulation of postganglionic muscarinic nerves. These nerves release acetylcholine to secrete 2 end effects. One is stimulation of release of H+ from parietal cells in the body of the stomach. The other is stimulation of histamine release from ECL cells in the lamina propria of the body of the stomach. Simultaneous vagal stimulation of the peptidergic neurons leads to the secretion of gastrin-releasing-peptide. The Delta cells are inhibited to reduce the inhibition of gastrin release
Describe the G-protein cycle following stimulation of G-protein-coupled receptors
Before stimulation, G-protein is bound to the receptor and GDP is bound to the alpha subunit of the G-protein. When a drug binds to and stimulates the receptor, there’s a change in the receptor’s conformation, and GTP binds to the alpha subunit of the G-protein. The alpha subunit dissociates from the receptor, breaks away into the local environment and activates processes that produce a biological response. GTP is then dephosphorylated to GDP by intrinsic G-alpha subunit GTPase activity. The G-protein alpha, beta and gamma subunits then re-associate and bind with the unbound receptor
