Control of the central swallowing program by inputs from the peripheral receptors. A review.
Swallowing is a complex motor sequence, usually divided into a buccopharyngeal stage (coordinated contractions of several muscles of the mouth, pharynx and larynx) and an esophageal stage, called primary peristalsis. This motor sequence depends on the activity of medullary interneurons belonging to the swallowing center which program through excitatory and inhibitory connections the sequential excitation of motoneurons and vagal preganglionic neurons responsible for the whole motor sequence. The activity of the medullary swallowing neurons can occur without feedback phenomena: it is truly a central activity indicating that swallowing depends on a central network which may function without afferent support. However, the swallowing neurons receive a strong afferent input suggesting the involvement of sensory feedbacks during swallowing. The swallowing neurons present a short latency activation on electrical stimulation of the peripheral afferent fibers supplying the region of the tract which is under their control. In addition, the neurons are activated by localized distensions of the swallowing tract, this distension having to be done more and more distally when the neuronal discharge occurs later and later during swallowing. Furthermore the swallowing discharge of the central neurons is increased either when a bolus is swallowed or during a slight distension of the corresponding region of the tract. Thus, under physiological conditions, swallowing neurons receive sensory information from pharyngeal and esophageal receptors and the central program may be modified by peripheral afferents that adjust the motor sequence to the size of the swallowed bolus. The inputs from the peripheral receptors can also exert inhibitory effects depending on the central connections between the swallowing neurons.(ABSTRACT TRUNCATED AT 250 WORDS)