Gagging is defined as the swallowing-vomiting reflex activity of the elevation of the soft palate followed by a reverse peristalsis of the upper digestive tract. Swallowing is the taking in of a substance through the mouth and the pharynx and into the esophagus. It is a combination of a voluntary act and a series of reflex actions. Once begun, the process operates automatically. The swallowing reflex is a rigidly ordered sequence of events that propels food from the mouth to the stomach while concurrently inhibiting breathing and preventing food from entering the trachea.
The nerves involved with swallowing are the sensory and motor branches of the trigeminal nerve (CN V), the hypoglossal nerve (CN XII)，the facial nerve (CN VII), and the glossopharyngeal nerve (CN IX). The afferent limb of the swallowing reflex begins with tactile receptors, most notably those near the opening of the pharynx.Sensory impulses from these receptors are transmitted to certain areas in the medulla. The central integrating areas for swallowing lie in the medulla and lower pons, called the swallowing center.Motor impulses travel from the swallowing center to the musculature of the pharynx and upper esophagus via various cranial nerves.
The process of swallowing, called deglutition,can be divided into three phases in the dog and cat (and also in human beings) : (1) the oral (or voluntary) phase, (2) the pharyngeal phase, and (3) the esophageal phase (Figure 36-1). The oral phaseis initiated by separation of a bolus of food from the mass in the mouth with the tip of the tongue. The bolus is moved in a dorsocaudal direction in the mouth by pressing first with the tip of the tongue and then with the more caudal tongue segment against the hard palate. The bolus is forced into the pharynx, where it stimulates the tactile receptors that initiate the swallowing reflex. In the cat the oral phase is much longer, marked by bolus accumulation in the valleculae (喉頭蓋谷＝喉頭蓋の裏側).
The pharyngeal phaseof swallowing occurs in less than 1 second. During this phase, breathing is reflexly inhibited. The soft palate is pulled dorsally and the palatopharyngeal folds move medially, preventing reflux of food into the nasopharynx. The pharyngeal passage narrows, directing the bolus caudally. Concurrently, the vocal cords move medially and the epiglottis covers the opening of the larynx, preventing food from entering the trachea. The cranial esophageal sphincter relaxes to accept the bolus. The dorsal constrictor muscles of the pharynx contract strongly to force the bolus deep into the pharynx. Peristalsis of these muscles initiates the movement of the bolus caudally through the cranial esophageal sphincter into the esophagus.
The esophageal phase of swallowing is partly controlled by the swallowing center. The cranial esophageal (pharyngoesophageal) sphincter, or cricopharyngeus muscle, reflexly constricts after the bolus passes caudal to the sphincter, preventing regurgitation of the bolus. A peristaltic wave, which begins just caudal to the cranial esophageal sphincter, traverses the entire esophagus in approximately 10 seconds. The initial peristaltic wave is termed primary peristalsisand is controlled by the swallowing center. If primary peristalsis is insufficient to clear the bolus from the esophagus, distension of the esophagus stimulates secondary peristalsis,which is mediated in part by the swallowing reflex and by local stretch receptors.
During primary and secondary peristalsis, the caudal esophageal (gastroesophageal) sphincter relaxes to receive the bolus into the stomach. In human beings and dogs, the cranial one third of the tunica muscularis of the esophagus is composed predominantly of striated muscle, the caudal one third is composed predominantly of smooth muscle, and the middle one third is a mixture of these two muscle types. In the cat, the esophagus changes abruptly from striated to smooth muscle at the heart. Somatic nerve fibers from the vagus nerve (CN X) form motor endplates on striated muscle fibers. Visceral motor nerves are preganglionic parasympathetic fibers that innervate the smooth muscle cells.
Gagging often is associated with retching. Retchingis an involuntary and ineffectual attempt at vomiting,and the causes of retching are similar to those that cause vomiting. Another physical sign that resembles gagging is expectoration, which involves clearing the airway of mucus and discharges without nausea. For cats, expectoration can be a normal means of clearing the airway of hair after grooming.
- Gengler W. Gagging In: Ettinger SJ,Feldman EC, eds.Textbook of Veterinary Internal Medicine. 6th ed. St. Louis, MO: Elsevier Saunders, 2005;127-128.