51. Food intake, salivary secretion, process of swallowing Flashcards
Food intake :
- Dogs
- Cats
- Horses
- Ruminants
- Pigs
-Dogs:
quickly and in great amounts.
-Cats:
eat more regularly but a smaller amount.
-Horses:
with the help of their lips practically encircle rough fodder, after which it is bit off with their corner-teeth.
-Ruminants:
pull the rough fodder into their mouth with their tongue twisted around it, and then pressing it between their lower incisor-teeth and the upper edentate edge, tear it off. By opening their mouth wide they take in the cereal fodder, so to say, biting into it.
-Pig:
nuzzle up to the ground with their nose-ring, then with the help of their extended lower lip they place the food inside the mouth.
Process of chewing
- Reflex mechanism.
- Mechanoreceptors of the mucosa in the oral cavity are stimulated by foodstuff, the chewing muscles are reflexively inhibited, and the mouth opens.
- The opening of the mouth stretches, and their reflex tone facilitation closes.
- The reflex motion happens on the side of the stimulated receptors and the reflex centre is in the brain stem.
- Chewing is initiated by the cortex, and it controls reflex motions.
- Afferent stimuli arising from the mechanoreceptors, regulate the contraction and relaxation of the tongue- and cheek muscles.
- This partly ensures to keep the bite between the chewing surfaces, and partly to ensure that the mucosa of the tongue and the cheek are not hurt. In horses, pigs and ruminants the mandible’s sideways grinding movement is significant in the mechanical fragmentation of plant fodder.
Process of swallowing:
Phases
Phase 1: Passing the bolus from the oral cavity to the pharinx
Phase 2: Passing the bolus from the pharynx to the oesophagus
Phase 3: Passing the bolus in the oesophagus
Process of swallowing – phase I
Step 1 :
From the fodder, fragmented and mixed with saliva during chewing, the tongue forms boluses, which are placed on the medial line.
The process begins voluntarily, but after its initiation it is continued as a sequence reflexes and can’t be stopped voluntarily.
Step 2:
Soft palate rises and closes the dorsal opening of the nasal-pharyngeal cavity. Breathing is inhibited, the larynx rises and the glottis closes.
Step 3:
The pressure induced by the motions of the tongue presses the bolus across the base of the tongue into the extended pharynx. The bolus presses the epiglottis backwards, and this way the entrance of the pharynx closes.
Process of swallowing – phase II
Step 4:
At the pharyngooesophagial border the fibres of the m. cricopharyngeus form the pharyngooesophagial sphincter.
When the bolus reaches the pharynx, the longitudinal and the transverse muscles contract and pull the wall of the pharynx on the bolus.
Step 5:
–The pressure in the pharyngeal cavity suddenly
and powerfully increases, and at the same time the pharyngooesophagial sphincter relaxes, after which the bolus is pushed to the lower-pressured cavity of the oesophagus. The sphincter is opened only for a short time, then suddenly it closes tightly, and by this it inhibits the bolus from re-entering the pharynx. At the same time the contraction of muscles in the oesophagus is initiated, the pressure increases and the peristaltic wave helps the bolus to pass further.
Process of swallowing – phase III
When the bolus reaches the upper part of the oesophagus, a peristaltic wave begins, which pushes the bolus in the direction of the stomach and continuously progresses from the pharynx until the stomach.
- The medium of the oesophagus and the stomach is separated by a longer-shorter section - cardia-, in which the pressure exceeds the one in the gastric cavity, as in the oesophagus. This section is also called the physiological gastrooesophageal sphincter, because, though it does not have a separable and thickened circular muscle layer, its myogenic tone is reflexively larger.
- When the bolus gets to this section by the effect of the peristaltic motion, the tone of the cardia decreases, but it exceeds the pressure in the stomach. Right after the passing of the peristaltic wave the pressure of the cardia doubles, and this inhibits the sliding back of the bolus against the temporary increased gastric pressure (regurgitation).
Swallowing receptors
Swallowing receptors are in the anterior and posterior pillars of the fauces, the tonsils, the soft palate and the base of the tongue, and the posterior pharyngeal wall.
Salivary secretion:
- RU: Saliva is important from the point of view of the function of the forestomachs (the HCO3- content of the saliva neutralizes the essential fatty acids formed in the rumen, so the pH of the rumen is not changed).
- The quantity and quality of the saliva are adjusted to the food that gets into the mouth. In the case of dry food a large amount of saliva is secreted.
- In the case of food having low or high pH, the quantity and its mucin content of the saliva is increased. The saliva of the carnivores is also rich in mucin.
-In monogastric animals food intake stimulates the secretion of seromucinous saliva which makes chewing and swallowing easier. Salivary secretion in ruminants is
continuous.
Regulation of salivary secretion
The regulation of the quantity and composition of saliva is mainly a neural reflex effect. Hormonal effects influence the saliva only to a small degree (e.g.: aldosterone)
Reflex of salivary secretion
- The reflex of salivary secretion by peripheral mechanical or chemical stimuli.
- Afferentation occurs by receiving chemical stimuli through the taste buds of the tongue, as well as through the receptors of the olfactory epithelium.
- Mechanoreceptors of the epithelium transmit stimuli towards the centre regarding the structure of the food (smooth, rough, dry, wet), and influences the quantity and character of the saliva (serous - mucinous).
Conditional reflexes can be built (vision, hearing, etc.) upon these reflexes.
-The reflex of salivary secretion can be observed after the activation of the salivary center in the myelencephalon which is influenced by effects of the central nervous system.
sympathetic praeganglionic fibers of the salivary glands :
arise from the upper thoracic segments of the spinal cord, and then go to the cranial cervical ganglion, from where postganglionic fibres reach the glands through different plexuses.
Parasympathetic fibers of the salivary glands:
-Parasympathetic fibers run in the collateral of glossopharyngeal nerve (parotid), or in the facial nerve (mandibular, sublingual).
The innervation of salivary glands:
-The innervation of salivary glands is carried out via parasympathetic (cholinergic) or sympathetic (adrenergic) efferentation.
Regulation of salivary secretion
- For a proper stimulus of the parasympathetic nerve a high quantity of saliva is produced, poor in mucin and slightly viscous; this process can be blocked by atropine.
- Acetylcholine released from the parasympathetic nerve fibers has its effect via the IP3 activating system of the acini cells.
The Na+ absorption of the tubules is significantly modified by the aldosterone concentration of blood.
- On a parasympathetic effect an intense atropine- resistant vasodilatation occurs due to vasodilators, like VIP and bradykinin.
- The regulation of salivary secretion in the acini is controlled mainly by parasympathetic and sympathetic nervous systems.
Parasympathetic saliva
Acetylcholine, which is released from parasympathetic postganglionic fibers, binds to muscarine type acetylcholine receptors via the IP3 activating system. This mechanism increases the intracellular Ca2+ level of the cells which leads to an increased Cl- ion secretion toward the lumen. Parallel with this, Na+ secretion is increased, too.
-The higher enzyme secretion is due to the activation of the DAG and protein kinase C mechanisms.
