COMPARISON OF DENSITY OF SYMPATHETIC VARICOSITIES AND THEIR CLOSENESSTO SMOOTH-MUSCLE CELLS IN RABBIT MIDDLE CEREBRAL AND EAR ARTERIES ANDTHEIR BRANCHES

The density and nerve varicosity-smooth muscle cell separation of rabb it cerebral and ear arterial beds were compared. The rabbit middle cer ebral artery and three of its successive branches and a comparable-siz ed ear artery and two branches were perfusion-fixed for electron micro scopy and analyzed by quantitative morphometric procedures. The purpos e was to determine if there are structural correlates to previously ob served differences in the sympathetic control of these two vascular sy stems. The in vitro contractile response of isolated artery segments t o electrical field stimulation of their intramural nerves is considera bly less in cerebral arteries compared with the similar-sized ear arte ries. Furthermore, in the cerebral but not the ear circulation, there is progressive diminution of the neurogenic response with successive b ranching. Although the total varicosity densities of the major ear and brain arteries studied are similar, and this parameter stays fairly c onstant with successive branching of the ear, it falls off considerabl y in the cerebral vessels. There is a significant difference in densit ies between the two vascular beds when ''bare'' varicosities located < 1 mu m from the medial smooth muscle are compared. The second-order br anch of the ear artery has an average of 18 bare varicosities per 500- mu m circumference, and the corresponding cerebral vessel has only 2.8 bare varicosities per 500-mu m circumference. The mean bare varicosit y-smooth muscle cell separation (mean +/- SEM) is significantly (P < . 05) less in the ear (1.18 +/- 0.06 mu m) than in the cerebral arteries (4.95 +/- 0.23 mu m). This is true of all Vessels studied. Fifty-nine percent of the bare varicosities in the ear arteries are <1 mu m from the smooth muscle cells, and 1.2% are more distant than 5 mu m. These values for cerebral vessels are 9.5% and 37%, respectively. In the ea r vessels, 25% of the bare varicosities make close neuromuscular conta ct (within 500 nm of the smooth muscle), whereas only 3% do so in cere bral vessels; in cerebral compared with ear vessels, the percentage be comes significantly less with branching. These structural features of brain vessels, taken together with the lower sensitivity to and the di minished capacity to respond to norepinephrine, probably account for t heir weak neurogenic control. The results indicate that the cerebral c irculation of the rabbit receives a sympathetic innervation that is re latively ineffective in altering cerebrovascular tone.