Sympathetic nerve fibers and ganglia in canine cervical vagus nerves: Localization and quantitation

BACKGROUND Cervical vagal nerve (CVN) stimulation may improve left ventricular ejection fraction in patients with heart failure. OBJECTIVES To test the hypothesis that sympathetic structures are present in the CVN and to describe the location and quantitate these sympathetic components of the CVN. METHODS We performed immunohistochemical studies of the CVN from 11 normal dogs and simultaneously recorded stellate ganglion nerve activity, left thoracic vagal nerve activity, and subcutaneous electrocardiogram in 2 additional dogs. RESULTS A total of 28 individual nerve bundles were present in the CVNs of the first 11 dogs, with an average of 1.87 +/- 1.06 per dog. All CVNs contain tyrosine hydroxylase-positive (sympathetic) nerves, with a total cross-sectional area of 0.97 +/- 0.38 mm(2). The sympathetic nerves were nonmyelinated, typically located at the periphery of the nerve bundles and occupied 0.03%-2.80% of the CVN cross-sectional area. Cholineacetyltransferase-positive nerve fibers occupied 12.90%-42.86% of the CVN cross-sectional areas. Ten of 11 CVNs showed tyrosine hydroxylase and cholineacetyltransferase colocalization. In 2 dogs with nerve recordings, we documented heart rate acceleration during spontaneous vagal nerve activity in the absence of stellate ganglion nerve activity. CONCLUSIONS Sympathetic nerve fibers are invariably present in the CVNs of normal dogs and occupy in average up to 2.8% of the cross-sectional area. Because sympathetic nerve fibers are present in the periphery of the CVNs, they may be susceptible to activation by electrical stimulation. Spontaneous activation of the sympathetic component of the vagal nerve may accelerate the heart rate.