Swallowing

Overview
  • Swallowing is the act of moving a bolus of ingested material from the oral cavity, through the pharynx and esophagus, and into the stomach whilst avoiding the possibility of aspiration. The swallowing motion can be thought of as occurring in a sequence of stages which utilize different sets of musculature as food is guided down the alimentary tract.
Stages
  • Voluntary Stage
    • The voluntary stage is characterized by the voluntary lifting of the tongue which pushes a bolus of food backward into the upper pharynx. A large number of sensory receptors in the upper pharynx can sense the presence of food and then initiate the subsequent involuntary steps which complete the act of swallowing.
  • Pharyngeal Stage
    • The Pharyngeal Stage of swallowing is characterized by upward movement of the soft palate covering the Posterior Nares and thus preventing reflux of food into the nose. Additionally, the vocal cords tightly appose and the epiglottis covers the opening of the larynx, thus preventing aspiration. During this stage the upper esophageal sphincter also relaxes, allowing movement of food into the esophagus and a peristaltic wave, initiated in the pharynx, moves the food bolus into the patent opening of the esophagus. During the entire pharyngeal stage breathing is inhibited as an additional protection against aspiration.
  • Esophageal Stage
    • The peristaltic wave begun by the pharynx continues as the 'Primary Peristaltic Wave' of the esophagus which can conduct the bolus to the stomach in about 5-10 seconds. If the bolus remains in the esophagus then a 'Secondary Peristaltic Wave' will be initiated just above the position of the remnant bolus. Prior to the peristaltic wave reaching the bottom of the esophagus, the lower esophageal sphincter relaxes, allowing the bolus to pass into the stomach. In addition, the stomach also relaxes creating space for new material to enter, in a process termed "Receptive Relaxation".
Musculature
  • All of the swallowing musculature in of the oral cavity, tongue, pharynx, and upper third of the esophagus is skeletal muscle and is thus completely controlled by motor neurons. In contrast, all the swallowing musculature in the lower two-thirds of the esophagus is smooth muscle and is thus controlled by the myenteric plexus with modulation by the autonomic nervous system.
Regulation
  • Voluntary Stage:
    • The voluntary stage is completely controlled voluntarily.
  • Pharyngeal and Esophageal Stages:
    • These stages are largely coordinated by the "Swallowing Center" in the medulla of the brainstem as part of the "Swallowing Reflex". Although peristaltic swallowing motions in the lower two-thirds of the esophagus composed of smooth muscle can be completely coordinated locally through the myenteric plexus, in practice the brainstem exerts heavy control on the lower esophageal peristalsis via the autonomic nervous system.
Esophageal Pressures
  • The esophagus exists essentially in the same compartment as the intrapleural space which normally possesses a negative pressure (See: Breathing Cycle). This is problematic because in the absence of physical barriers air might be sucked into the esophagus from the pharynx or gastric contents might be sucked in from the stomach. The solution is provided by the upper and lower esophageal sphincter which tonically seal the openings of the esophagus and only open transiently to allow passage of a food bolus.