Chewing and swallowing. Gastric motor activity. Chewing (Mastication).
Chewing-
Mastication (chewing) is an intermittent rhythmic act in which the tongue-, facial-, and jaw-muscles act in coordination to position the food between the teeth, cut it up, and then prepare it for swallowing.
Chewing serves three primary functions:
- Mixing Food with Saliva: This lubricates food, making it easier to swallow.
- Reducing Food Particle Size: While particle size doesn't affect digestion, smaller pieces are easier to swallow.
- Mixing Carbohydrates with Salivary Amylase: This begins the process of carbohydrate digestion.
Chewing involves both voluntary and involuntary components:
- Involuntary Chewing: Reflexes initiated by food in the mouth. Sensory information from mechanoreceptors is relayed to the brainstem, which orchestrates a rhythmic chewing pattern.
- Voluntary Chewing: Can override the involuntary reflex at any time.
Chewing cycle:
The pathway of the mandible during chewing is called a chewing cycle.
It is characterized by opening, closing, retrusive, protrusive and lateral jaw movements.
Phases:
1) Opening phase: Intake of bolus in the mouth – mouth opens by reflex inhibition of elevators. It is divided into two types: Slow opening and Rapid opening.
2) Closing Phase: Gradual transition from isotonic contraction to isometric contraction of the elevators.
3) Intercuspal phase: Tooth to tooth contact occurs.
Swallowing (Deglutition):
Swallowing is a complex process involving three phases:
- Oral Phase: Voluntary. The tongue pushes the food bolus (ball of chewed food) towards the pharynx, triggering the involuntary swallowing reflex.
- Pharyngeal Phase: Involuntary and controlled by the swallowing center in the medulla. This phase ensures safe passage of the bolus into the esophagus:
- The soft palate elevates to prevent food from entering the nasopharynx.
- The epiglottis covers the larynx, and the larynx moves upward, preventing food from entering the trachea.
- The upper esophageal sphincter relaxes, allowing the bolus to enter the esophagus.
- A peristaltic wave in the pharynx propels the food through the open sphincter.
- Breathing is temporarily inhibited during this phase.
- Esophageal Phase: Partly controlled by the swallowing reflex and partly by the enteric nervous system. This phase transports the bolus to the stomach:
- The upper esophageal sphincter closes to prevent reflux into the pharynx.
- A primary peristaltic wave, coordinated by the swallowing reflex, propels the food down the esophagus.
- If necessary, secondary peristaltic waves, initiated by the enteric nervous system, clear any remaining food.
- As the bolus approaches the stomach, the lower esophageal sphincter relaxes (mediated by VIP released from vagus nerve fibers), allowing it to enter.