In recent years the presence of adrenergic nerve fibers on cerebral arteries as small as 10 to 15 μ both on the pial and within the brain substance has been confirmed morphologically. Although possible roles of the adrenergic fibers in control of cerebral circulation are fairly well clarified, there are few studies as to neural factors in the territory of the basilar artery.
In the present study, electrical stimulation of the superior cervical ganglion (SCG) was performed to define the contribution of neural mechanism to control of basilar arterial flow. Studies were also made to determine the effects of hexamethonium(C6)and chronic bilateral superior cervical ganglionectomy on autoregulation mechanism and CO2 reactivity in the territory of the basilar artery.
Fifty-two mongrel adult dogs weighing 8 to 26 kg were anesthetized with pentobarbital and mechanically ventilated. The basilar artery was exposed through the transcervical-transclival approach and the flow was measured with an electromagnetic flowmeter. The unilateral and bilateral SCG were stimulated by square-wave direct current impulses (3-5 v, 2.0 msec duration, 20 Hz). Autoregulation of basilar flow was studied under acute hypertension, which was induced by an intraaortic balloon inflation before and after intravenous administration of C6. The same study was performed in the animals, in which both SCG were removed 2 weeks to 2 months prior to the experiment. Also changes in basilar flow following 7 % CO2 inhalation were studied in the respective experiments.
The mean percentage reduction of basilar flow was 11.7% on unilateral SCG stimulation and 20.4% on bilateral, associated with a slight rise of systemic blood pressure. The effects of bilateral stimulation were greater than those of unilateral (p<0.01). Inflation of the balloon produced an increase in basilar flow. However, it returned rapidly to the resting level and remained unchanged during 3 minutes period of induced hypertension. After the administration of C6, basilar flow increased along with the rise of blood pressure. CO2 reactivity was well maintained under this condition. Two to four weeks after bilateral SCG-ectomy, fluorescence of the adrenergic fibers disappeared completely from the trunk and main superficial branches of the basilar artery. Studies using chronically bilaterally sympathectomized dogs revealed an impairment of autoregulation in response to rise of blood pressure.
Adrenergic fibers observed in abundance on the basilar artery may have an important functional role in conveying vasoconstrictive impulses via the SCG. It is assumed that the bilateral SCG contribute to neurogenic control of blood flow in the territory of the basilar artery and the neurogenic control plays an important role in autoregulation of cerebral blood flow in response to rise of blood pressure.
