Hello :) This post will discuss the acts of prehension, mastication and deglutition. I'll describe the mechanics of prehension, mastication, and swallowing. We'll also take a look at the main salivary glands in animals as well as the main constituents of saliva and the process of salivary excretion. We'll finish off by looking at how the structure of the oesophageal wall may differ between species.
Prehension and Mastication
Prehension is the act of getting food into the mouth. Domestic animals do this by using their lips, teeth, tongue and by head and jaw movements. Some animals may also use their front legs and digits to grip their food. Most animals drink by drawing water into the oral cavity with suction. Dogs and cats drink by lapping water with the tongue. Birds, on the other hand, fill the oral cavity with water and then raise the tip of their beak, draining water into the pharynx. Prehension is a voluntary reflex and the trigeminal, facial and hypoglossal nerves provide motor innervation to the lips, tongue and muscles of the jaw. Sensory information is also important and this is derived from olfactory, optic and trigeminal (sensation from oral mucosa, lips and teeth) input.
Mastication involves the mechanical breakdown of food and allows mixing with saliva. This is also under voluntary control. The tongue and buccal muscles position the food within the mouth while the teeth cut or grind the food. Several nerves are involved in this process:
- The trigeminal nerve provides sensory input to the teeth and motor input to the jaw muscles.
- The facial nerve provides motor and sensory input to the tongue and pharynx.
- The glossopharyngeal nerve provides sensory input to the caudal third of the tongue.
- The hypoglossal nerve provides motor innervation to the tongue.
Most of the salivary secretion in animals comes from the paired parotid, mandibular and sublingual salivary glands. Dogs also have zygomatic glands.
Saliva is formed in a two-step process:
- Acini secret fluid containing amylase, lysozyme, mucin and electrolytes in similar concentrations to those found in the blood plasma.
- During the passage through the ducts of the salivary gland, ionic composition of the saliva becomes modified. This is because Na+ and Cl- are reabsorbed while K+ and HCO3- are secreted.
If saliva is flowing at a high rate, there is less time for its modification in the salivary ducts. This leads to an isotonic saliva. A low saliva flow rate leads to less Na+ and Cl- and more K+ in the saliva. Here the saliva is hypotonic.
Saliva is mainly water (98%) but also contains mucin, which forms mucus when mixed with water and acts as a lubricant. Amylase is also present and initiates the breakdown of starch. Bicarbonate is there too and this neutralises acids produced by the bacteria in the mouth and prevents acid damage to the enamel on the teeth. Saliva is very important as a buffer in ruminants. Urea also diffuses from the blood into the saliva. In ruminants this is important as it acts as a nitrogen source for ruminal microorganisms.
The major differences between species seen in terms of saliva composition is mainly the amount of bicarbonate secreted and the flow rate. Ruminant animals excrete much more bicarbonate in their saliva and produce much more saliva per day when compared to non-ruminant animals.
Regulation of Salivary Secretion
Salivary secretion is regulated by the autonomic nervous system. The volume and composition of saliva depends on the balance between the parasympathetic and sympathetic divisions of the ANS. Parasympathetic stimulation results in a high volume of saliva that has a watery consistency. Sympathetic stimulation results in constriction of the blood vessels supplying the salivary glands. This causes a low volume of saliva which has a viscous consistency to be produced.
The act of swallowing occurs in several phases. The initial phase of swallowing is voluntary. However the remaining phases are under involuntary control.
- Oral Phase: following prehension and chewing, a bolus is formed from the food. The bolus is then moved caudally towards the pharynx. The upward movement and pressure of the base of the tongue pushes the bolus against the palate, moving it caudally. This involves the trigeminal, facial and glossopharyngeal nerves.
- Pharyngeal Phase: In this phase, the bolus is moved towards the oesophagus. The pathway back into the oral cavity is closed by the muscles of the mouth and tongue. The passage of the bolus into the nasopharynx is also prevented by the reflex elevation of the soft palate and apposition of the palatopharyngeal folds. The opening of the trachea is protected by the closure of the glottis and the tipping of the epiglottis. This diverts food away from the glottis. In addition, the swallowing centre of the medulla inhibits the respiratory centre which ceases respiration. All this involves the glossopharyngeal and vagus nerves which are initiated from the swallowing centre.
- Cricopharyngeal phase: In the absence of swallowing, the cricopharyngeus and thyropharyngeus muscles contract (this is caused by vagus nerve activity). This ensures that the upper oesophageal sphincter remains closed, preventing air from entering the oseophagus during respiration. During swallowing, the vagus nerve is inhibited and this causes the crico- and thyropharyngeal muscles to relax. This opens the upper oesophageal sphincter allowing the bolus to pass into the oesophagus. A mechanism in the central nervous system senses the size of the bolus and calculates the amount of time the sphincter is to remain open. After the bolus has moved into the oesophagus, the sphincter closes and this prevents the aspiration of the oesophageal contents.
- Oesophageal Phase: As the bolus enters the oesophagus, the primary peristaltic contraction wave is initiated by the swallowing centre. If the primary wave ends before the bolus reaches the stomach, a secondary wave is rapidly generated by local oesophageal distention. The pressure generated by these actions varies. It will be lower for liquids and higher for solids.
- Gastro-Oesophageal Phase: The muscles of the gastro-oesophageal region act as a sphincter and maintain a high pressure region between the oesophagus and the stomach. This prevents the reflux of gastric contents. This sphincter open to allow for the eructation of gas. This is mediated by the vagus nerve.
The Structure of the Oesophageal Wall
- The oesophageal wall has four layers:inner mucosa
- outer connective tissue
In dogs and ruminants, the muscularis is entirely striated muscle. However, in the horse, cat and pig, the muscularis is striated at its origin but changes to smooth muscle as the oseophagus passes through the thorax.
That's all for this post, see you next time :)