The swallowing act consists of a preparatory phase and three transport phases. The first phase corresponds to the oral transport phase of the food pulp, during which the swallowing reflex is triggered. Swallowing reflex disorders of the oral transport phase are often directly related to neurogenic diseases or muscle and connective tissue diseases.
What is the oral transport phase?
The swallowing act consists of a preparatory phase and three transport phases. The first phase corresponds to the oral transport phase of the food pulp, during which the swallowing reflex is triggered. Every day, humans swallow between 1000 and 3000 times. During swallowing, the food pulp is transported away through the pharynx and esophagus into the stomach. At the same time, the act of swallowing cleans the esophagus and removes, for example, residues of gastric acid that may damage the sensitive mucous membrane of the esophagus. The swallowing act consists of different phases. The preparation of the swallowing process takes place under voluntary control, such as chewing. Irritation of the base of the tongue activates the swallowing reflex. A reflex arc leads to the act of swallowing, which is opened by the oral transport phase. All subsequent processes are beyond voluntary control. A total of 26 pairs of muscles are involved in the act of swallowing. In addition to the anatomical structures of the oral cavity and its boundaries, the pharyngeal, laryngeal, esophageal and gastric structures play a role in swallowing. For the oral transport phase of swallowing, the oral cavity and its adjacent structures play the main role. All swallowing movements and the interaction of the muscle pairs involved are coordinated by the so-called swallowing center of the brain. This center is located in the brainstem and involves higher suprabulbar as well as cortical brain areas.
Function and task
In the narrow definition, each swallowing act consists of three phases, which are also called transport phases. The three transport phases are preceded by food intake. The first transport phase corresponds to the oral transport phase through the oral structures. This is followed by a pharyngeal transport phase and an esophageal transport phase. The oral transport phase of swallowing largely escapes voluntary control. Only a minor part of the movements involved is voluntary and can be consciously controlled. After completion of the oral preparation phase, the lips close. In this way, saliva can no longer escape from the mouth. In addition, closing the lips prevents air from entering the mouth so that no air is swallowed. The cheek muscles then contract. At the beginning of the actual swallowing process, the tongue presses against the hard palate. In this way, the hard palate serves as an abutment within the swallowing process. The pulp of chewed food now migrates towards the pharynx. This migration takes place by means of backward undulating movements assisted by the styloglossus muscle and the hyoglossus muscle. These two muscles pull the tongue backwards from the hard palate in a wave-like motion. This movement pushes the food pulp across the narrowness of the pharynx and into the throat. The food pulp eventually touches the base of the tongue or the back wall of the pharynx. Sensitive sensory cells from the mechanoreceptor group are located in these structures. The sensory cells register the touch stimulus and transmit the stimulus to the central nervous system via afferent nerve pathways. In the central nervous system, the excitation is switched to motor nerves and travels along these nerves to the muscles that realize the actual swallowing process. From the moment the food pulp touches the base of the tongue or the back of the throat, the swallowing process can no longer be controlled voluntarily within the oral phase. The muscle movements triggered later are reflexive and thus elude voluntary control.
Diseases and complaints
Swallowing disorders are grouped under the term dysphagia. Neurogenic connections and diseases are the most common cause, especially for disorders of the oral transport phase in the sense of a restricted or absent swallowing reflex. As a result of a stroke, a craniocerebral injury, meningitis or a degenerative brain disease such as Parkinson’s disease, the swallowing reflex can be disturbed in the oral transport phase.Such dysphagia is just as frequent in the context of the autoimmune disease multiple sclerosis. The diseases and phenomena mentioned above predominantly lead to dysphagia when they injure tissue of the swallowing center. Tissue injuries in the brain lead to permanent damage in most cases. Brain tissue is highly specialized and often cannot fully recover from damage. In addition, injuries in the context of the aforementioned diseases and events leave scars. In the area of these scars, the nerve cells of the brain are no longer fully functional. However, a neurogenic cause does not always have to underlie a disturbance of the oral transport phase. Muscle diseases such as muscular atrophy or connective tissue diseases such as scleroderma also cause swallowing problems. The same applies to tumors in the pharyngeal and spinal cord or brain areas. Oral transport can also be complicated by congenital malformations, such as a cleft lip and palate. Equally well, surgeries or severe injuries in the oral area can show negative effects on the oral transport phase. In the elderly, disturbances of the oral transport phase are often to be interpreted as an age-physiological phenomenon without disease value. In many cases, persons of a certain age no longer swallow efficiently. This is often referred to as presbyphagia. The older people become, the more the reaction time of their muscles and nerves is delayed. Reduced muscle strength due to natural muscle loss in old age, age-related tooth loss, dry mucous membranes due to age physiology and ossification of the jaw also interfere with the act of swallowing. In addition, coordination disorders can occur, especially in old age, which make swallowing and the oral transport phase more difficult. Specific swallowing training can often improve dysphagia.
