Phases of digestion

[2] The cephalic phase of digestion is the stage in which the stomach responds to the mere sight, smell, taste, or thought of food.

These sensory and mental inputs converge on the hypothalamus to induce the responses needed for preparing the gastrointestinal tract for food processing, which relays signals to the medulla oblongata.

[1] This enhanced secretory activity brought on by the thought or sight of food is a conditioned reflex.

1) In the body of the stomach, the vagal postganglionic muscarinic nerves release acetylcholine (ACh) which stimulates parietal cell H+ secretion.

2) In the lamina propria of the body of the stomach the ACh released from the vagal endings triggers histamine secretion from ECL cells.

Vagovagal reflex: Distention, or stretching, activates an afferent pathway which in turn stimulates efferent response from the dorsal nucleus of the vagus nerve.

[2] Small peptides also buffer stomach acid so the pH does not fall excessively low.

ACh is secreted by parasympathetic nerve fibers of both the short and long reflex ,.ml; pathways.

[2] The intestinal phase is a stage in which the duodenum responds to arriving chyme and moderates gastric activity through hormones and nervous reflexes.

Peptones stimulate an unknown endocrine cell to release an additional humoral signal, "enterooxytonin".

Osmolarity due to products of digestion stimulate acid secretion The acid and semi-digested fats in the duodenum trigger the enterogastric reflex – the duodenum sends inhibitory signals to the stomach by way of the enteric nervous system, and sends signals to the medulla that (1) inhibit the vagal nuclei, thus reducing vagal stimulation of the stomach, and (2) stimulate sympathetic neurons, which send inhibitory signals to the stomach.

The effect of this is that gastrin secretion declines and the pyloric sphincter contracts tightly to limit the admission of more chyme into the duodenum.

Originally called gastric-inhibitory peptide, it is no longer thought to have a significant effect on the stomach, but to be more concerned with stimulating insulin secretion in preparation for processing the nutrients about to be absorbed by the small intestine.