Elliott A, Bennett P M
Soc Gen Physiol Ser. 1982;37:11-28.
Molluscan adductor muscles are in general of two kinds--a slow-acting smooth muscle, which after contraction can go into a catch state, in which tension is maintained for many hours with a small turnover of ATP, and a quicker-acting muscle, usually to some extent striated, which has not the catch property in marked degree, if at all. The thick filaments of both kinds of adductors contain myosin and paramyosin. Those from catch muscles are noteworthy for their high paramyosin content(up to about 90% by weight), which may be correlated with their great length and diameter. The paramyosin forms a core with a surface layer of myosin. Recently, we have shown that the paramyosin core of catch muscles resembles a single crystal in its molecular arrangement, although the degree of regularity varies from one filament to another. The evidence is from electron microscopy both of individual filaments and of pieces of whole muscle embedded and sectioned. Individual negatively stained filaments change greatly in appearance when rotated round their long axes, in a way not compatible with a helical structure. A three-dimensional reconstruction of the filament from micrographs of a rotated filament confirms this. Transverse sections of smooth adductor muscles show no internal features within the thick filaments when viewed accurately along the filament axis. On tilting striations appear on the filaments that may be correlated with the planes of the Bear-Selby net. They are a general feature of the filaments and have been seen in the smooth adductors of P. maximus, O. edulis, Crassostrea gigas, M. mercenaria, of the anterior byssus retractor of M. edulis and in the red (obliquely striated) adductor of M. mercenaria. The arrangement of the myosin on the paramyosin core is not known.
软体动物的闭壳肌一般有两种——一种是作用缓慢的平滑肌，收缩后可进入强直收缩状态，在这种状态下，只需少量ATP周转就能维持数小时的张力；另一种是作用较快的肌肉，通常在一定程度上有横纹，即使有明显的强直收缩特性也不显著。这两种闭壳肌的粗肌丝都含有肌球蛋白和副肌球蛋白。来自强直收缩肌肉的粗肌丝因其高含量的副肌球蛋白（重量可达约90%）而值得注意，这可能与其较大的长度和直径有关。副肌球蛋白形成一个核心，表面有一层肌球蛋白。最近，我们已经表明，强直收缩肌肉的副肌球蛋白核心在分子排列上类似于单晶，尽管不同肌丝之间的规则程度有所不同。证据来自对单个肌丝以及嵌入并切片的整块肌肉的电子显微镜观察。单个经负染的肌丝绕其长轴旋转时，外观会发生很大变化，这种方式与螺旋结构不相符。从旋转肌丝的显微照片对肌丝进行的三维重建证实了这一点。沿肌丝轴精确观察时，平滑肌闭壳肌的横切面在粗肌丝内没有内部特征。当倾斜时，肌丝上会出现条纹，这些条纹可能与贝尔-塞尔比网的平面有关。它们是肌丝的一个普遍特征，在大珠母贝、食用牡蛎、太平洋牡蛎、硬壳蛤、紫贻贝的平滑肌闭壳肌、紫贻贝前足丝牵缩肌以及硬壳蛤的红色（斜纹）闭壳肌中都能看到。肌球蛋白在副肌球蛋白核心上的排列尚不清楚。
