Title

The motor control of swallowing

Date of Completion

January 1997

Keywords

Biology, Neuroscience|Biology, Animal Physiology

Degree

Ph.D.

Abstract

Swallowing is one of the most fundamental yet least understood physiologic functions, involving the coordinated actions of twenty-eight muscles and five cranial nerves. In addition to structural complexity, swallowing is characterized by functional complexity: swallowing must be coordinated with other physiologic functions, namely respiration and mastication.^ Despite these structural and functional complexities, swallowing is believed to be controlled primarily through a pattern-generated brainstem reflex, characterized by immutable relationships among the different structures involved with swallowing. However, a number of studies have reported extensive intra- and intersubject variability during normal swallowing. The variability reported in these studies, as well as evidence of adaptation to central (cerebral cortex or brainstem) or peripheral (upper aerodigestive tract) injury or disease suggests that the motor control of swallowing may require a more complex organizational scheme than can be facilitated through a brainstem reflex.^ The purpose of this series of investigations was to determine if the motor control of swallowing could be characterized by one of three schemes: (1) a sequence or pattern of simple reflexes, (2) one or more known reflexive or pre-programmed modes that operate through a programmed, sequential process, (3) a complex interactive organization of subsystems, encompassing both transport and airway protection subsystems.^ A total of eleven human subjects were studied in these investigations: five normal adult subjects and six subjects who had partial glossectomies for treatment of a malignant tumor. The normal subjects were studied under normal or perturbed swallowing conditions using a combined quantitative videofluoroscopic technique and electromyography. The glossectomy subjects were studied using quantitative videofluoroscopy alone.^ The results of these investigations showed significant intra- and intersubject variablility for all measured parameters of normal swallowing, contradicting the primary role of a simple reflex. Normal subjects demonstrated individual, novel, adaptive maneuvers to perturbed swallowing, instead of a programmed, sequential response. Glossectomy subjects likewise showed unique, novel adaptive maneuvers to a loss of tongue tissue, with evidence of structural-functional reorganization.^ These findings suggest that swallowing may be characterized as a complex motor skill with control of the system effected through a heterarchical organization of subsystems. ^