Proceedings of a Workshop on - The Havemeyer Foundation

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).
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