Neuroanatomy of vagal bundle in the horse

The vagal bundle provides autonomic innervation to the airways and is composed of the parasympathetic (i.e. vagus nerve) and the sympathetic systems. The vagus nerve provides cholinergic innervation and the sympathetic system provides adrenergic innervation.

In the horse, the autonomic innervation of the airways is ipsilateral and more developed in the right side. The parasympathetic nervous system is anatomically larger and provides direct autonomic innervation to the upper and lower airways. The sympathetic nervous system provides direct innervation to the cardiovascular system primarily.

In the horse, the vagal bundle exhibits large connections between the sympathetic and parasympathetic trunks in the cranial thorax. These connections allow intermingling of fibers and the presence of cholinergic and adrenergic fibers in either trunk. By this means, the adrenergic fibers can reach the lungs via the vagus nerve and facilitate bronchial relaxation. The presence of adrenergic fibers in the vagal branches to the lower airways was demonstrated with the use of special stains on selected sections of the vagal bundle. Immunohistochemical staining techniques assisted in identifying the presence of neurotransmitters of the postganglionic sympathetic neurons by identifying enzymes intervening in the metabolism of the neurotransmitters of interest (i.e. tyrosine hydroxylase). In horses, other mechanisms for bronchial relaxation have been suggested (i.e. non-adrenergic non-cholinergic inhibitory system, systemically released catecholamines).

The equine larynx is likely innervated by cholinergic and adrenergic fibers intermingled in the cranial laryngeal nerve and the recurrent laryngeal nerve. The role of a newly described pararecurrent laryngeal nerve is yet to be determined.