Ultrastructural distribution of vasodilator and constrictor nerves in cat cerebral arteries

This study examined the ultrastructural distribution of agranular vesicle-containing nerve (AVN) and granular vesicle-containing nerve (GVN) in cat cerebral blood vessels fixed in KMnO4. The percentages of the AVN and GVN throughout the adventitial layer in the basilar, middle cerebral, and anterior cerebral arteries were 63.4 and 36.7, 53.9 and 46.1, and 60.2 and 39.8, respectively. The AVN in the cat basilar artery is approximately 37 times denser than that in the rabbit basilar artery. This morphological result, in correlation with the previous pharmacological findings, provides indirect evidence that the AVN are the dilator nerves. The lack of neurogenic vasoconstriction in the cat basilar, middle cerebral, and anterior cerebral arteries may be attributable in part to a combination of denser dilator than constrictor nerves, a possible closer synaptic cleft distance for dilator than for constrictor nerves, and the reported insensitivity of the postsynaptic alpha-receptors. The small pial vessels (branches of the middle cerebral arteries), on the other hand, contain predominantly GVN (88%), suggesting that dilator nerves relative to constrictor nerves decrease as the size of the cerebral blood vessels becomes smaller. Furthermore, the synaptic cleft distance is found not parallel to the outer diameter of the artery. Thus, the results of this study indicate that the neurogenic control of brain circulation varies with brain region, lending further support to the theory that the dual cerebral innervation varies among species.