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ORAL AND PHARYNGEAL REFLEXES IN THE MAMMALIAN NERVOUS SYSTEM: THEIR DIVERSE RANGE IN COMPLEXITY AND THE PIVOTAL ROLE OF THE TONGUE
A.J. Miller
Department of Growth and Development, School of Dentistry, and Department of Physiology, School of Medicine, University of California at San Francisco, San Francisco, CA 94143-0438; amiller{at}itsa.uscf.edu
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Figure 1. The lingual-hypoglossal reflex. This reflex has been directly analyzed by electrical stimulation of the nerve and extracellular recording from hypoglossal nerve branches. In a second approach, intracellular recordings from hypoglossal motoneurons determining excitatory (EPSP) and inhibitory (IPSP) synaptic potentials are obtained. Mechanical stimulation of the tongue will also evoke these synaptic potentials. An example of the reflex effects is given in one study in the anesthetized cat (Takata, 1981), in which a polysynaptic reflex had different proportions of excitatory and inhibitory effects, depending upon whether the motoneurons innervated the tongue-protruding or -retruding muscles. (The drawing of the dissection of the tongue muscles is adapted with permission from Fregosi and Fuller [1997].)
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Figure 2. The masseter-hypoglossal reflex. Electrical stimulation of the masseter nerve induces polysynaptic reflexes in hypoglossal motoneurons. The primary effect is inhibitory on most hypoglossal motoneurons, with some excitatory inputs on motoneurons innervating the genioglossus and intrinsic muscles of the tongue.
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Figure 3. The glossopharyngeal-hypoglossal reflex. Electrical stimulation of the glossopharyngeal nerve innervating the posterior tongue and pharynx induces polysynaptic reflexes in hypoglossal motoneurons. A similar effect occurs with stimulation of the superior laryngeal nerve that innervates the hypopharynx and larynx. The primary effect is excitation of motoneurons innervating the genioglossus and tongue intrinsic muscles.
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Figure 4. Effect of altering the mixture of O2 and CO2 on the inspired gas levels of an anesthetized rat and on the EMG activity recorded from specific tongue muscles and the diaphragm. Tongue force (upper first trace) is measured with a transducer, and retraction is denoted by a downward direction. EMG activity is shown (lower trace) as both the original signal (HG EMG, GG EMG) and the integrated level (HG iEMG, GG iEMG). The rat breathing room air (NORMOXIA) demonstrates rhythmic EMG activity from the hyoglossus retruding muscle (HG), the genioglossus protruding muscle (GG), and the primary respiratory muscle, the diaphragm (DIA iEMG). Modifying the inspired gas mixture to 10% O2 in 90% N2 (HYPOXIA) induces hypoxia and increases rhythmic EMG activity in both tongue muscles. Tracheal occlusion is induced (vertical dotted line) to prevent further respiration. Breathing only O2 (HYPEROXIA) to induce hyperoxia has minimal effect on the tongue muscles even after tracheal occlusion. Breathing O2 enriched with CO2 (HYPEROXIC HYPERCAPNIA) increases the EMG activity in both tongue muscles and induces more tongue retraction. Tracheal occlusion enhances this hypercapnic response. (Adapted with permission from Fregosi and Fuller [1997])
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Figure 5. The EMG activity of muscles involved in one pharyngeal swallow as recorded from the anesthetized animal (e.g., cat, dog, monkey). (A) The level of EMG activity can decrease below its background level, suggesting inhibition of that muscle, and then recruit with much activity followed by further inhibition. The increased EMG activity precedes the contraction of the muscle. The diaphragm ceases all activity during a pharyngeal swallow (adapted with permission from Doty and Bosma JF [1956]). (B) Location of some of the muscles involved in the earliest onset of the pharyngeal swallow (adapted with permission from Dickson and Dickson [1982]).
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Figure 6. Example of discharges recorded from fibers of the two primary sensory nerves that innervate the pharynx and larynx. (A) The distribution of sensory afferent fibers for the glossopharyngeal nerve (IX) and the superior laryngeal nerve (SLN), which is a branch of the vagal nerve (X). (B) The sensory fibers for both the glossopharyngeal and superior laryngeal nerves synapse in the nucleus tractus solitarius (NTS). (C) Different discharge patterns of sensory fibers from the superior laryngeal nerve recorded from laboratory animals during the inducement of particular reflexes (e.g., apnea, cough) (adapted with permission from Storey [1968b]). (D) Different patterns of electrical stimulation applied to the superior laryngeal nerve of laboratory animals to induce pharyngeal swallowing (adapted with permission from Doty [1951]).
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Critical Reviews in Oral Biology & Medicine, Vol. 13, No. 5,
409-425 (2002)
DOI: 10.1177/154411130201300505
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