Interrelationships among wall structure, smooth muscle orientation, and contraction in human major cerebral arteries.

Human major cerebral arteries were fixed under pressure in the brain or after excision and ligation. They were sectioned and stained for light microscopy. Sterological point counting was used on mid-plane longitudinal sections of ten segments to determine that the media occupied 52.0% +/- 6.39 (SD) of the arterial wall and that the smooth muscle comprised 72.0% +/- 4.76 (SD) of the media. In arterial segments containing nine planar bifurcations, areas of varying muscle orientation and composition were identified and mapped out. A circumferential alignment of smooth muscle was consistently found in the cerebral branching systems except for the bifurcation region proximal to the apex at major bifurcations where the regular pattern was replaced by multilayered multidirectional smooth muscle. Theoretical models relating smooth muscle contraction to arterial caliber in cylindrical segments are outlined. In the extreme case of complete closure of an artery, our prediction is that for a typical cerebral arterial media the outermost muscle cells should have to contract to 42% of their relaxed length. The possible importance of medial smooth muscle patterns in the pathogenesis of cerebrovascular lesions is discussed.