Neurobiology

DEVELOPMENTAL DISABILITIES RESEARCH REVIEWS 14: 77 – 86 (2008)

THE NEUROBIOLOGY OF SWALLOWING AND DYSPHAGIA
Arthur J. Miller*
Division of Orthodontics, Department of Orofacial Sciences, School of Dentistry, University of California at San Francisco, San Francisco, California

The neurobiological study of swallowing and its dysfunction, defined as dysphagia, has evolved over two centuriesbeginning with electrical stimulation applied directly to the central nervous system, and then followed by systematic investigations that have used lesioning, transmagnetic stimulation, magnetoencephalography, and functional magnetic resonance imaging. The field has evolved from mapping the central neural pathway and peripheral nerves, to defining the importance of specific regions of the lowerbrain stem in terms of interneurons that provide sequential control for multiple muscles in the most complex reflex elicited by the nervous system, the pharyngeal phase of swallowing. The field is now emerging into defining how the higher cortical regions interact with this brain stem control and is providing a broader perspective of how the intact nervous system functions to control the three phasesof swallowing (i.e., oral, pharyngeal, and esophageal). Much of the present interest focuses on how to retrain a damaged nervous system using a variety of stimulus techniques, which follow funda' 2008 Wiley-Liss, Inc. mentals in rehabilitation of the nervous system.
Dev Disabil Res Rev 2008;14:77–86.

Key Words: neuroscience; swallowing; brain stem; cortex; rehabilitation

BASIC DEFINITIONOF SWALLOWING wallowing describes a complex function in which food and liquid are transported from the oral region to the stomach in what appears as a well-coordinated function. This review focuses on the adult as the developing central nervous system (CNS) in the newborn and infant functions differently and relies more on brain stem control and sensory feedback for sustained motor output. Thecontrol of the oral, pharyngeal, and esophageal phases represent a coordination among the brain stem and cortical central pathways, and the enteric nervous systems of the esophagus that control the smooth muscle segments. Four cranial motor nuclei have motoneurons involved in the complete swallow and include the trigeminal motor nucleus (Vth), the facial motor nucleus (VIIth), the nucleus ambiguus(NA), and the hypoglossal nucleus (XII), and include motoneurons of the upper cervical regions (Table 1). Sensory input proceeds into specific regions of the trigeminal sensory nuclei (Vth) and the nucleus tractus solitarius (NTS) of the brain stem. Swallowing often occurs with mastication, and the complex movements of the jaw, hyoid bone, and larynx [Palmer et al., 1992, 1997, 2007; Hiiemae andPalmer, 1999, 2003]. The oral phase of swallowing uses the tongue to propel the food posteriorly [Ardran et al., 1953; Abd-El-Malek, 1955;

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Shawker et al., 1984; Kennedy and Kent, 1988; Dodds et al., 1990; Kahrilas, 1993; Kahrilas et al., 1993, 1996; Tasko et al., 2002; Ono et al., 2004; Hori et al., 2005, 2006; Wilson and Green, 2006]. In the oral phase, the tongue demonstrates a wave-likemotion along the central groove that provides a propulsive force. Once the boluses are transported to the region of the pharynx, the pharyngeal phase of swallowing begins with the jaw-closing muscles stabilizing the mandible in a closed position so that the suprahyoid muscles can pull the hyoid bone forward and rostrally, and contract with more activity than seen during chewing. The posterior tonguepushes downward and backward into the pharynx. The pharyngeal wall then moves forward to meet the tongue and a descending wave of contraction moves the bolus. The submental muscles raise the hyoid bone followed by the infrahyoid and pharyngeal muscles raising the larynx. The anterior and superior movements of the hyoid and larynx assist in protecting the airway as the vocal cords adduct. The...