Swallowing Mechanism: Three phases

Summary

This lecture explains the complex process of swallowing, involving both voluntary and involuntary actions. It's divided into three phases: oral, pharyngeal, and esophageal. The oral phase is voluntary, where food is broken down and formed into a bolus. The pharyngeal phase is involuntary, involving six sub-phases to prevent food from entering the respiratory tract. Lastly, the esophageal phase uses peristalsis to push the bolus into the stomach, with the upper and lower esophageal sphincters regulating passage and preventing reflux.

Takeaways

• 🧠 Swallowing is a complex reflex governed by both the brainstem and muscles of the pharynx and esophagus.
• 🫁 The pharynx acts as a shared conduit for the respiratory and digestive systems, making swallowing a coordinated process to avoid food entering the respiratory tract.
• 🔄 Swallowing is divided into three main phases: oral (voluntary), pharyngeal, and esophageal (both involuntary).
• 👅 The oral or buccal phase involves voluntary actions like chewing, lubricating food with saliva, and pushing the food bolus to the back of the pharynx using the tongue.
• 👂 Afferent nerves, like the trigeminal and glossopharyngeal, detect the bolus in the pharynx and trigger the involuntary pharyngeal phase.
• 🚫 During the pharyngeal phase, several motor actions occur, including the closing of the soft palate to prevent food from entering the nasal cavity.
• 🔒 The vocal folds close, and the larynx rises to ensure the airway is blocked, preventing food from entering the trachea.
• 📉 The esophageal phase involves peristaltic waves, which push the food down into the esophagus. The upper esophageal sphincter relaxes to allow food passage and then tightens again.
• 🔁 If the bolus doesn’t clear the esophagus in one peristaltic wave, secondary peristalsis is triggered to ensure complete passage into the stomach.
• 🌬️ Swallowing momentarily inhibits the respiratory center, temporarily halting breathing during the 6-10 seconds it takes for food to reach the stomach.

Q & A

What are the three main phases of swallowing?

The three main phases of swallowing are the oral (buccal) phase, the pharyngeal phase, and the esophageal phase.

Which phase of swallowing is voluntary, and which phases are involuntary?

The oral (buccal) phase is voluntary, while the pharyngeal and esophageal phases are involuntary.

What is the role of the hypoglossal nerve during swallowing?

The hypoglossal nerve (cranial nerve XII) innervates the tongue and is responsible for controlling tongue movements during the oral phase, helping push the bolus of food towards the back of the pharynx.

How is the pharyngeal phase initiated?

The pharyngeal phase is initiated when the bolus of food touches the afferent receptors in the back of the pharynx, particularly around the tonsillar pillars, which send sensory signals to the brainstem.

What prevents food from entering the nasal cavity during swallowing?

The soft palate elevates during swallowing, closing off the nasal cavity to prevent food from entering the nasal passage.

What is the role of the epiglottis during swallowing?

The epiglottis covers the laryngeal inlet to prevent food from entering the trachea, ensuring that the bolus of food is directed towards the esophagus.

What happens to the vocal cords during the pharyngeal phase of swallowing?

During the pharyngeal phase, the vocal cords close to protect the airway and prevent food or liquid from entering the lungs.

How does the upper esophageal sphincter function during swallowing?

The upper esophageal sphincter normally remains closed with a resting pressure of around 50 mmHg. However, during swallowing, it relaxes and opens (reducing the pressure to zero) to allow the bolus to enter the esophagus.

What is primary peristalsis in the esophageal phase?

Primary peristalsis refers to the initial wave of muscle contractions that move the bolus down the esophagus following the pharyngeal phase.

What triggers secondary peristalsis in the esophagus?

Secondary peristalsis is triggered when the bolus remains in the esophagus, causing a stretch that is detected by sensory vagal afferents, which send signals to the brainstem to initiate a second wave of contractions to push the remaining food into the stomach.