Proceedings of a Workshop on - The Havemeyer Foundation

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Havemeyer Foundation Monograph Series No. 11 DEVELOPMENT AND INNERVATION OF THE LARYNX C. Hahn Neuromuscular Diagnostic Laboratory, Royal (Dick) School <strong>of</strong> Veterinary Studies, The University <strong>of</strong> Edinburgh, Easter Bush, Roslin, Midlothian EH25 9RG, UK A brief look at the evolution and development <strong>of</strong> the larynx goes some way to explain the curious anatomy <strong>of</strong> this organ. About 400 million years ago, the lungfish evolved the ability to breathe air directly from the external environment, perhaps because its watery home was periodically subject to drought (Ewings 1949). It developed a simple larynx-like slit behind the gills that allowed air into the swim-bladder when the creature was exposed to the atmosphere and that kept water out when it was submerged (Fig 1). As the descendants <strong>of</strong> the lungfish moved onto land, the swim-bladder evolved into a multi-compartment organ with a large surface area the sole function <strong>of</strong> which was gas exchange. The larynx in the meantime developed adductor and abductor muscles and lateral cartilages (such as found in the axolotl), then separate arytenoid and cricoid cartilages (newt), primitive thyroid cartilages (alligators and their feathered relatives, the birds) and finally the complex mammalian larynx. As the survival <strong>of</strong> equids once depended on running long distances to escape predators, horses evolved a larynx that when fully abducted has an aperture Fig 1: Lungfish with modified swim bladder and dilator and sphincter muscles (modified from Ewings, V. (1949). The Comparative Anatomy and Physiology <strong>of</strong> the Larynx. William Heinemann, Medical Books, London). that is larger than the trachea itself (this in sharp contrast to the human larynx, where the abducted larynx allows for speech but is only half the diameter <strong>of</strong> the trachea). An appreciation <strong>of</strong> the neuroanatomy <strong>of</strong> laryngeal innervation is a pre-requisite to understanding the pathology <strong>of</strong> recurrent laryngeal neuropathy. The main source <strong>of</strong> laryngeal innervation <strong>of</strong> the equine larynx is the ipsilateral recurrent laryngeal nerve (rln). Motor neurons <strong>of</strong> the rln are based in the nucleus ambiguus in the caudal brainstem. This nucleus was recently localised in the horse (Hackett 2000) and was found to be a loosely organised column <strong>of</strong> cells in the ventrolateral medulla oblongata (Fig 2). A somatotopic distribution <strong>of</strong> adductor and abductor motor neurons was not apparent but neurons innervating the cricoarytenoideus lateralis muscle were observed throughout the nucleus, whereas neurons innervating the cricoarytenoideus dorsalis tended to be situated more rostrally. Nucleus ambiguous axons loop around the parasympathetic nucleus <strong>of</strong> the vagus to emerge from the brainstem as axons <strong>of</strong> the internal branch <strong>of</strong> cranial nerve (CN) XI. They only join the vagus nerve (CN X) on leaving the skull through the jugular foramen and tympano-occipital fissure. Cranial movement <strong>of</strong> the head during embryogenesis, and differential degeneration <strong>of</strong> the 6th aortic arch, resulted in extremely long nerves with the left and right nerves having different pathways. The left nerve loops around the aorta while the right takes a shorter route around the right subclavian artery. Including its vagal course, the total length from neuronal cell body to larynx <strong>of</strong> the left rln can be up to 250 cm in length, making it twice as long as other motor nerves in the horse and 31 cm longer than the right rln (Cole 1946). 3
Equine Recurrent Laryngeal Neuropathy Glosopharyngeal (CN IX) Vagus Internal branch Accessory nerve (CN XI) Vagus n. CN XI Recurrent laryngeal nerve Cranial laryngeal nerve CN IX Fig 2: The recurrent laryngeal nerve is supplied by axons originating in the caudal nucleus ambiguus. (Modified from de Lahunta, A. (1983) Veterinary Neuroanatomy and Clinical Neurology, 2nd edn. Saunders, Philadelphia, pp 105). The normal rln nerve consists <strong>of</strong> medium sized myelinated fibres with only scattered, smaller diameter fibres present. Myelinated axons in the rln segregate as fascicles within the vagus nerve. However, after these fascicles separate from the vagus as the rln, the axons that are targeted to innervate a particular intrinsic laryngeal muscle are not discreetly clustered within the rln at its origin in the thorax, but instead are mixed among the fascicles throughout its length. Although the rln is thought <strong>of</strong> classically as a motor nerve, primary afferent (‘dorsal root ganglia’) rln neurons have been demonstrated in the proximal and distal vagal ganglia. The distal vagal ganglion is poorly described in the horse but has recently been identified to consist histologically <strong>of</strong> scattered neurons in the vagus nerve at its bifurcation with the cranial laryngeal nerve (I.G. Mayhew, personal communication, Fig 3). Involvement <strong>of</strong> sensory axons in horses with recurrent laryngeal neuropathy has not been established. Each nerve then courses cranially to provide motor innervation to the paired intrinsic Fig 3: Scattered neuronal cell bodies <strong>of</strong> the distal vagal ganglion in the proximal vagus nerve. laryngeal muscles, with the exception <strong>of</strong> the cricothyroideus muscles. These muscles have a different embryologic origin and are innervated by nucleus ambiguus neurons whose axons join an external branch <strong>of</strong> the paired cranial laryngeal or vagus nerves (de Lahunta 1983). It is likely that the complexity and length <strong>of</strong> this pathway underlies the pathology <strong>of</strong> recurrent laryngeal neuropathy REFERENCES Cole, C.R. (1946) Changes in the equine larynx associated with laryngeal hemiplegia. Am. J. vet. Res. 7, 69-77. de Lahunta, A. (1983) Veterinary Neuroanatomy and Clinical Neurology. Philadelphia, W.B. Saunders Company. Ewings, V. (1949) The Comparative Anatomy and Physiology <strong>of</strong> the Larynx. London, William Heinemann, Medical Books. Hackett, S. (2000) The Equine Nucleus Ambiguus: Myotopic and Neurotopic Representations <strong>of</strong> Motor and Sensory Components <strong>of</strong> the Recurrent Laryngeal Nerve. Ithaca, Cornell University. 4
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