GIT Part 3 salivary secretion Mastication

The process of digestion (mastication & Salivary secretion)

1. Mastication (Chewing) Chewing is controlled by the somatic nerves to the skeletal muscles of the mouth and jaw. In addition to the voluntary control of these muscles, rhythmical chewing motions are reflexively activated by the pressure of food against the gums, hard palate at the roof of the mouth, and tongue. Activation of these mechanoreceptors leads to reflexive inhibition of the muscles holding the jaw closed. The resulting relaxation of the jaw reduces the pressure on the various mechanoreceptors, leading to a new cycle of contraction and relaxation.

2. Mastication (Chewing) Makes it easier for food to be swallowed. Facilitates mixing of food with secretions. Also prolongs the subjective taste of food.

3. Saliva The first secretion encountered when food is ingested. P roduced by three pairs of salivary glands that drain into the oral cavity. Has a number of organic constituents that facilitate it to performs its functions. For example: Mucins help lubricate the food bolus; amylase mediate the initiation of starch digestion, immunoglobin A and lysoyzmes protect the oral cavity from bacteria.

4. The Salivary G lands C onsist of acini that produce the primary secretion that is essentially identical in its composition to plasma. I s modified as it flows from the acini out into ducts that eventually coalesce and deliver the saliva into the mouth. Na + and Cl – are extracted while K + and bicarbonate are added. T he ducts are relatively impermeable to water so the loss of NaCl renders the saliva hypotonic, particularly at low secretion rates.

5. The Salivary Glands As the rate of secretion increases, there is less time for NaCl to be extracted and the tonicity of the saliva rises, but it always stays somewhat hypotonic with respect to plasma. Also, a lkaline; important to neutralize any gastric secretions that reflux into the oesophagus . Overall, the three pairs of salivary glands that drain into the mouth supply 1000 to 1500 mL of saliva per day and the volume of saliva secreted per gram of tissue is the largest secretion of any of the body’s exocrine glands.

6. The Salivary Glands A re extremely active when maximally stimulated, secreting their own weight in saliva every minute. To accomplish this, they are richly endowed with surrounding blood vessels that dilate when salivary secretion is initiated. Thus, increased secretion of saliva is caused by a large increase in blood flow to the salivary glands, mediated by both neural activity and paracrine/autocrine agents released by the active cells in the salivary gland.

7. The regulation of salivary secretion A lmost entirely controlled by neural influences. Both sympathetic and parasympathetic neurons, with the latter being more predominant (greater response). Sympathetic input slightly modifies the composition of saliva (particularly by increasing proteinaceous content), but has little influence on volume. There is barely any hormonal regulation of salivary secretion.

8. Regulation of Salivary Secretion Secretion is initiated even before the meal is taken into the mouth as a result of central triggers that are prompted by thinking about, seeing, or smelling food. Further s ecretion is triggered by reflexes that are stimulated by the physical act of chewing. This reflex response is initiated by chemoreceptors (acidic fruit juices are a particularly strong stimulus) and pressure receptors in the walls of the mouth and on the tongue. Salivary secretion is also prompted by nausea, but inhibited by fear or during sleep.

9. Regulation of salivary secretion by the parasympathetic nervous system.

10. Regulation of Salivary Secretion In summary, in the absence of ingested material, a low rate of salivary secretion keeps the mouth moist. In the presence of food, salivary secretion increases markedly.