Digestive enzymes act only on the exposed surfaces of food particles, consequently, how fast digestion proceeds depends on the total surface area available to the digestive enzymes.
Chewing is important because it breaks up large food particles and mixes the food with secretions of the salivary glands. This wetting and grinding of food to very fine homogenous matter aids swallowing and prevents excoriation of the esophagus and other segments of the gastrointestinal tract.
Without chewing and proper grinding of food in the mouth, gastric emptying would be extremely slow and the transit time for food through the gut would increase significantly.
Perhaps we swallow certain foods without chewing, but chewing is particularly important for certain fruits because most fruits have indigestible cellulose membranes that envelope their nutrient-rich portions. Chewing helps break up the cellulose barrier to liberate the nutrient-rich portions for digestion and assimilation.
Teeth help to chew food; with incisors designed for cutting food particles and molars designed for grinding of food. Jaw muscles provide the force needed for cutting and grinding food. Much grinding of food occurs in the molars. Jaw muscles are responsible for chewing, and they receive nerve fibers from motor branch of trigeminal nerve (cranial nerve V).
Swallowing also called deglutition is the process whereby food and other oral contents pass from the mouth (oral cavity) to the stomach through the pharynx and esophagus.
Swallowing, a complicated mechanism comprises three phases:
- A voluntary phase that initiates the swallowing process
- Pharyngeal phase that is involuntary and involves the passage of food bolus from pharynx into the esophagus
- An esophageal phase, another involuntary phase that transports food bolus through the esophagus to stomach.
Voluntary phase moves food and other oral content to the pharynx
After mastication, the tongue rolls the ingested food into a bolus or a mass. Tongue rolls the bolus into the pharynx in the voluntary phase of swallowing. The tongue collects oral contents on itself before moving upwards and backward against the palate. This action squeezes the food and rolls it posteriorly propelling it backward into the pharynx. Thus, the voluntary phase of swallowing delivers food from the mouth into the pharynx.
From this point, swallowing becomes entirely a reflex response, which proceeds on its own and we are unable to stop it.
Pharyngeal phase of swallowing moves food masses and other materials from pharynx to esophagus
Neuronal areas of the reticular substance of the medulla and lower pons automatically initiate the successive stages of swallowing in an orderly sequence. Neuronal areas in the medulla and pons responsible for controlling swallowing collectively called deglutition or swallowing center initiate and control the involuntary phases of swallowing. Thus, the pharyngeal phase of swallowing is purely a reflex act initiated by the distension of the pharyngeal ring and stimulation of sensory receptors of this area.
As part of the reflex act, the swallowing center inhibits the respiratory center of medulla oblongata halting respiration during this phase of swallowing to allow swallowing to proceed.
As the mass of food reaches the pharynx, the distension of the pharyngeal ring where you have the pharyngeal opening and the surrounding tonsillar pillars stimulates certain epithelial sensory receptors that control swallowing. Afferent impulses picked by the receptors of this area travel through afferent fibers of the trigeminal nerve (cranial nerve V), glossopharyngeal nerve (cranial nerve IX), and vagus nerve (cranial nerve X) to medulla oblongata in brainstem.
The swallowing center formed principally by two brainstem nuclei – the nucleus of tractus solitaries and nucleus ambigus – receives the afferent /sensory impulses and integrate the impulses. The nucleus of tractus solitarius receives virtually all sensory impulses from the mouth. After integrating the afferent impulses from the pharynx, the swallowing center sends efferent/motor outputs to muscles of the pharynx, tongue, and upper portion of esophagus through efferent fibers of trigeminal nerve (cranial nerve V), glossopharyngeal nerve (cranial nerve IX), vagus nerve (cranial nerve X), hypoglossal nerve (cranial nerve XII).
The efferent/motor signals reaching the pharynx starts/initiates a wave of involuntary contraction in the pharyngeal muscles; the contraction eventually pushes the bolus into the esophagus.
The swallowing reflex is a complex one, during the pharyngeal phase of swallowing, there is a reflex inhibition of the respiratory center to ensure that food is unable to pass into the windpipe (trachea). Note that pharynx is a common passage for both food and air. Thus, as food passes through the pharynx, the glottis closes to ensure that food particles do not pass into the trachea.
In summary, the following events occur as part of the reflex response:
- The trachea closes
- The upper esophageal sphincter also called pharyngoesophageal sphincter relaxes to allow food and other ingested materials move freely from pharynx to the esophagus.When one is not swallowing, the upper esophageal sphincter closes tightly to prevent air from going into the esophagus during breathing. The epiglottis flings backward covering the opening of the larynx preventing food from entering the nose and trachea.
- As the upper esophageal sphincter relaxes the entire upper esophageal musculature contracts, this contraction begins from the upper portion of the pharynx before spreading downward over the middle and lower portions of the pharynx. This pharyngeal contraction propels the food bolus into the esophagus. The pattern of the pharyngeal contraction is peristaltic and is purely involuntary. The entire pharyngeal phase of swallowing occurs in less than 2 seconds.
Esophageal phase of swallowing
Primarily, esophagus is a conduit for the passage of food from pharynx to the stomach. In the esophageal phase of swallowing, esophagus propels/conducts food from pharynx to the stomach. Peristalsis is most prominent in the esophagus where it propels food bolus from pharynx to stomach. Esophagus shows two types of peristaltic waves:
- Primary peristaltic wave
- Secondary peristaltic wave
The primary peristaltic wave
The peristaltic wave that starts in the pharynx, during pharyngeal phase of swallowing, often continues down into the esophagus. Normally, this peristaltic wave, which begins from pharynx pass all the way through the esophagus to the stomach in less than 10 seconds. However, fluids and liquids and semi-solid foods ingested when one is standing upright usually fall by gravity to reach the lower esophagus ahead of the peristaltic wave.
Secondary peristaltic wave
Most often, the primary peristaltic wave fails to move the entire esophageal content into the stomach, when this happens, the esophageal distension triggers another peristaltic wave called secondary peristaltic wave. Secondary peristaltic wave continues until all esophageal luminal content empties into the stomach.
The myenteric nerve plexus initiates the secondary peristaltic waves.
Therefore, in the esophageal phase of swallowing, a peristaltic ring of contraction of esophageal muscles forms behind a food bolus propelling the bolus downward along the esophagus all the way to the stomach at an estimated speed of 4cm/second.