15 Neurophysiology of Swallowing
Central pattern generators (CPG) are used throughout physiology. They are functional units of neural circuits capable of generating patterned or rhythmic output either spontaneously (e.g., respiration), or by volitional command or specific stimulus (e.g., walking, chewing, coughing, or swallowing) without dependence on peripheral feedback. In general, pattern generators contain a specific input, an interpreter, and a patterned output. This is achieved through coordinated interaction between afferent fibers and their sensory nuclei, motor nuclei, efferent fibers, peripheral nerves and muscles. Although CPGs may receive cortical input, it is not required.
Of particular interest to swallow function is shared circuitry across pattern generators. There is overlap between the swallow CPG and the respiratory CPG. This allows the swallow CPG temporary influence over the respiratory CPG, thus permitting the occurrence of swallow-related respiratory cessation (deglutitive apnea).
There is also overlap in the swallow CPG and the cough CPG (Troche, Brandimore, Okum, Davenport, & Hegland, 2014). The combined effort of these two CPGs provides airway protection. The swallow CPG closes off the airway during the swallow; the cough CPG expels any material that violates the airway entrance.
Chewing Pattern Generator
The CPG for chewing is located in the brainstem (Box 1.58). It can be activated by input from the cortex or from specific peripheral inputs to the brainstem near the motor nucleus of CN V and the surrounding reticular formation. The result is a rhythmic pattern of jaw opening and closing. This pattern can be modulated to incorporate adequate bite force or size of the chewing gesture (modulating the timing of the open and close phase, or the amplitude of jaw muscle contraction). Peripheral inputs aid in this modulation, and also provide input to excite jaw reflexes as needed. For example, if while chewing the texture of the bolus is suddenly altered (like finding a small pebble in your pasta), reflexes would engage to open the jaw. Although cortical inputs help modulate chewing, the rhythmic pattern of chewing will continue even if the cortex is separated from the brainstem.
Box 1.58
Schematic of chewing pattern generator
Image
Swallow Pattern Generator
To trigger the pharyngeal swallow, the CPG for swallowing receives a stimuli from oropharyngeal afferents that feed to the nucleus tractus solitarius, (NTS) and the surrounding reticular formation in the brainstem (Box 1.59). The neurons for the swallow CPG are located in the medulla of the brainstem. These neurons can be divided into two groups–the dorsal and ventral group. The dorsal group, which incorporates the NTS and the surrounding reticular formation, serves as a command center for swallowing. The input is interpreted to determine if it is adequate to trigger a swallow. In order to trigger a swallow, the stimuli must excite multiple oropharyngeal receptor fields, t