Cns

The central pathways which control the various laryngeal functions are numerous and complex. These communications have been investigated in various animal models. In the region of the upper medulla and lower pons, the nucleus ambiguus and the nucleus tractus solitarius are the primary ganglia for the efferent and afferent pathways to the larynx, respectively.

Yoshida et al. [4] studied the motoneuron distribution in the monkey brainstem. Motoneurons to the recurrent laryngeal nerve were distributed in the rostral part of the ipsilateral nucleus ambiguus with a more compact distribution laterally, and a more scattered array in the dorsomedial region. The motoneurons to the cricothyroid muscle originate in the ipsilateral nucleus ambiguus and retrofacial nucleus and are scattered around the outer, compact cell group. Yajima and Hayashi [5] described physiological evidence of branching of motoneurons originating in the nucleus ambiguus. They demonstrated that a single motoneuron could have one axonal branch coursing with the SLN and another branch coursing with the RLN. This simultaneous innervation of two distinct muscles supports the idea that central control of coordinated movements of distinct muscles contributes to specific tasks performed by the larynx, such as respiration or swallowing [5]. This task-specific motor control was supported by later work from Yajima and Larson [6], They concluded that there are subsets of medullary neurons (motor and nonmotor) which are involved in specific laryngeal behaviors. These neurons are active in one or more specific laryngeal tasks. For example, some neurons may be active in voicing, some in respiration, some in swallowing, and some in a combination of these tasks; thus, specific behaviors of the larynx are controlled by subgroups ofneurons [6],

The afferent pathway of SLN and RLN were studied in the rat by Patrickson et al. [7]. The SLN had terminal fields bilaterally in the interstitial and medial subnuclei of the nucleus tractus solitarius, with greater density in the ipsilateral projections compared with the contralateral projections. The RLN distribution was similar except it was only ipsilateral, and there were significantly less dense terminal projections [7]. The convergence of these afferents to the interstitial and dorsomedial subnuclei support these areas as sensory integrators of the larynx [7].

Sympathetic innervation to the larynx arises from the superior cervical ganglion and travels among the laryngeal nerve fibers to the laryngeal submucosa [8], The parasympathetics that travel with the vagus nerve originate in the dorsal motor nucleus of the vagus. These preganglionic axons travel through the pharyngeal plexus, the cardiac plexus, and the esophageal plexus, then synapse in their respective ganglia in the walls of the end organs. The secretomotor fibers which supply the larynx travel through the recurrent laryngeal and the superior laryngeal nerves [2].

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