What Is the Organized Muscular Sequence Involved in Swallowing?
Whether it is the initiation of swallowing or whether it is the programmed sequence in the pharyngeal and esophageal phases of digestion, the act of food bolus movement from the oral cavity to the digestive tract is under the command of the swallowing center of the brain, present within your brain stem.
As we know, human swallowing or the act of chewing food and ingesting it via your oral cavity, where the food gets transformed into a bolus that is eventually meant for further digestion down your gastrointestinal tract - is characterized by different phases; this is quite a complex muscular activity both from the oral and the gastric regions. Just even at the beginning of a swallow, there are several muscles involved, such as the mylohyoid, the geniohyoid, the posterior part of the tongue, the superior constrictor muscle, the palatoglossus, the palatopharyngeus muscle, the styloglossus, and the stylohyoid muscles. Electromyography (a diagnostic procedure used to measure the electrical activity of muscles at rest and during contraction) experiments reveal that all these muscles are involved even in a single act of swallowing food.
So you can understand that the act of swallowing is a multidimensional one, wherein several muscles are simultaneously involved in passing the food from the oral cavity into the gut and into the gastrointestinal system for further digestion. Amongst these phases of swallowing, the oral and preparatory digestion phases are under voluntary control; that is, the phase of food when you are chewing orally, and it gets converted by your teeth and oral enzymes into a bolus, followed by the bolus passing into the gastrointestinal tract - the next two phases, in contrast, are completely involuntary. The next two phases in swallowing, that is, the pharyngeal and the esophageal phase, are involuntary, and they take place in an organized fashion or are stimulated rather by a series of events to ensure the safe passage of food, as in a motor sequence of swallowing.
What Is the Physiologic Role of the Brainstem in Controlling Swallowing?
The Physiologic role of the brainstem in controlling swallowing is as follows:
Recognizing the Neurophysiology:
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Dysphagia (difficulty or discomfort in swallowing) can occur when any phase of swallowing is affected: oral, preparatory, pharyngeal, or esophageal.
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It may be a temporary condition caused by gastric or oral disorders or by an underlying systemic or immunocompromising condition.
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Different healthcare providers, such as physicians, otolaryngologists (ENT specialists), dentists, or maxillofacial surgeons, need to recognize the clinical features and underlying causes of dysphagia.
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Proper diagnosis and management are crucial for effective treatment.
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Dysphagia is a neurophysiologic process regulated by the brainstem.
Cranial Nerves Regulation of Physiology:
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The swallowing center is located in the human brain stem, which essentially comprises three major components.
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The sensory input from select cranial nerves triggers and provides feedback to the swallowing center of the brain.
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Only after these inputs are received by the brain stem the motor output would originate in an organized fashion that is dictated by the brain stem - involving a sequence of swallowing that originates from the motor nuclei, distributed over to the muscles controlled by different cranial nerves.
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The essential cranial nerves relay sensory input to the brainstem and transmit motor output to the muscles they control, playing a crucial role in the digestion process. These cranial nerves are mainly CN V, VII, IX, and X, which trigger the sensory output of the brain stem. The motor output received hence over the muscles controlled by these cranial nerves, an action innately performed by the human brain stem, is at the root of the swallowing act or physiology.
What Is the Anatomy of the Swallowing Centre?
So, what comprises the swallowing center in the brain? This is basically the nucleus tractus solitaries (NTS) alongside the adjacent reticular formation that is situated dorsally, with its central parts of the reticular formation surrounding the nucleus ambiguous for the programming of the motor sequence. It is important for dentists, surgeons, neurologists, physicians, and otolaryngologists to note that the motor outputs derived from the brain stem's swallowing center described above are facilitated through several important areas in the brain. These are the motor neurons facilitating the act of swallowing, scattered amongst the nucleus ambiguus, the facial, trigeminal, and hypoglossal motor nuclei. Even the motor neurons present within the cervical spinal cord region as well relay these responses to your oral cavity gut and the esophageal tract.
Further medical researchers have understood That there is a separate interneuronal network according to neurophysiologic experiments and readings that showcase the fact that these neurons within this network system become activated when there are any of the four phases of swallowing, which we discussed earlier. This interneuronal network is mainly present within the brain stem, though it is not any single precise location according to researchers, and rather is a combined set of dorsal and ventral groups of nerve cells/neurons that are associated with the act of swallowing.
Why Is Vomit Reflex Not Controlled by the Swallowing Centre of Brain Stem?
Other than clinical dysphagia or difficulty in swallowing food, even the act of nausea and vomiting are known to be only biologically protective human mechanisms when your body is exposed to possible ingested toxins. This protective reflex, whether triggered in medical conditions like gastric reflux, pyloric stenosis (a condition where the opening between the stomach and small intestine narrows, often in infants), cardiac infarction (heart attack), or increased intracranial pressure or because of side effects of therapeutic regimens post radiation therapy or medications - is however, not controlled by the brain stem's medullary center with the emetic group of neurons that stimulate the vomit response.
Conclusion
Research studies hence clearly demonstrate the impact of the brain stem in the swallowing act; the swallowing center is mainly responsible for the organized sequence of the different phases of food chewing and digestion that follow. When the appropriate motor response is affected in clinical conditions or diseases, then the cause or the diagnosis made by the oral or general physician should hence be accurate to ensure a proper management strategy for the affected patients.
It is important that physicians, otolaryngologists, dentists, and maxillofacial surgeons dealing with the important parts of the head and neck who should be aware of the orogastric system or tract should hence be aware that there is a deeper neurophysiologic process that determines the act of swallowing. In complicated clinical cases, hence, there would be a need for not only interdisciplinary collaboration but also a consultation with the neurologist because the physiology of swallowing is such that it is controlled innately by the brain stem structures.
