The action potentials of the distal stomach are myogenic (i.e., an inherent property of the muscle) and occur in the absence of any neurotransmitters or other chemical messengers. The myogenic characteristics of the action potential are modulated by motor neurons in the gastric ENS. Neurotransmitters primarily affect the amplitude of the plateau phase of the action potential and, thereby, control the strength of the contractile event triggered by the plateau phase. Neurotransmitters, such as ACh from excitatory motor neurons, increase the amplitude of the plateau
Onset of terminal antral contraction
Complete terminal antral contraction
kGastric retropulsion. Jet-like retropulsion through the orifice of the antral contraction triturates solid particles in the stomach. The force for retropulsion is increased pressure in the terminal antrum as the trailing antral contraction approaches the closed pylorus.
phase and of the contraction initiated by the plateau. Inhibitory neurotransmitters, such as NE and VIP, decrease the amplitude of the plateau and the strength of the associated contraction.
The magnitude of the effects of neurotransmitters increases with increasing concentration of the transmitter substance at the gastric musculature. Higher frequencies of action potential discharged by motor neurons release greater amounts of neurotransmitter. In this way, motor neurons determine, through the actions of their neuro-transmitters on the plateau phase, whether the trailing contraction of the propulsive complex of the distal stomach occurs. With sufficient release of transmitter, the plateau exceeds the threshold for contraction. Beyond threshold, the strength of contraction is determined by the amount of neurotransmitter released and present at receptors on the muscles.
The action potentials in the terminal antrum and pylorus differ somewhat in configuration from those in the more proximal regions. The principal difference is the occurrence of spike potentials on the plateau phase (see Fig. 26.25), which trigger short-duration phasic contractions superimposed on the phasic contraction associated with the plateau. These may contribute to the sphincteric function of the pylorus in preventing a reflux of duodenal contents back into the stomach.
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