Quantitative Ultrastructural Measurement of Innervation Density, Neurotransmitter Vesicles and Neuromuscular Cleft Width in the Rabbit Central Ear Artery and Its Main Side Branch

Abstract

The purpose of this study was to provide anatomical information about innervation density and the neuromuscular relationship in the rabbit central ear artery and its main side branch, so that, when combined with functional observations, a better understanding of neuroeffector mechanisms in these vessels could be established. Electron-microscopic methods were used to measure number and size of axon varicosities, their content of vesicles, the sizes of dense cores in the small vesicles and the neuromuscular cleft width. For each 100 µm of the outer circumference of the tunica media, the central ear artery contained 4 varicosities with a total cross-sectional area of 2.2 µm², and the main side branch about 9 varicosities with a total cross-sectional area of 4.1 µm². On the average, the varicosities of the central and branch arteries contained 93.6 and 95.1 small, and 2.3 and 1.9 large vesicles, respectively, per micron. The diameter of dense cores in the small vesicles was significantly smaller in the main side branch (18.3 vs. 16.7 nm). In the central ear artery, the mean cleft width was 5.27 µm, and 3% of all clefts were narrower than 1 µm. In the main side branch, the mean cleft width was 2.77 µm, and 22% were narrower than 1 µm. Thus, the innervation of the main side branch is closer to the vascular smooth muscle cells and is characterized by a larger number of smaller varicosities per unit circumference. Although vesicle density is the same as in the central artery, the dense cores are smaller in diameter. These data do not provide an explanation as to why the response of the main side branch to nerve stimulation in vitro is proportionally weaker. The reasons for this apparent contradiction are not clear, but we offer some possible explanations.