The fine structural characteristics of cardiac muscle cells and its myofibril organization in the orb web spider N. clavata were examined by transmission electron microscopy. Although myofibril striations are not remarkable as those of skeletal muscles, muscle fibers contain multiple myofibrils, abundant mitochondria, extensive sarcoplasmic reticulum and transverse tubules (T-tubules). Myofibrils are divided into distinct sarcomeres defined by Z-lines with average length of 2.0 μm, but the distinction between the A-band and the I-bands is not clear due to uniform striations over the length of the sarcomeres. Dyadic junction which consisted of a single T-tubule paired with a terminal cisterna of the sarcoplasmic reticulum is found mainly at the A-I level of sarcomere. Each cell is arranged to form multiple connections with neighboring cells through the intercalated discs. These specialized junctions include three types of intercellular junctions: gap junctions, fascia adherens and desmosomes for heart function. Our transmission electron microscopy (TEM) observations clearly show that spider’s cardiac muscle contraction is controlled by neurogenic rather than myogenic mechanism since each cardiac muscle fiber is innervated by a branch of motor neuron through neuromuscular junctions.

中文翻译：

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球网蜘蛛 Nephila clavata 心肌细胞的精细结构

通过透射电子显微镜检查了球网蜘蛛 N. clavata 中心肌细胞的精细结构特征及其肌原纤维组织。虽然肌原纤维条纹不像骨骼肌那样显着，但肌纤维含有多个肌原纤维、丰富的​​线粒体、广泛的肌浆网和横小管（T 小管）。肌原纤维分为由平均长度为 2.0 μm 的 Z 线定义的不同肌节，但由于肌节长度上的均匀条纹，A 带和 I 带之间的区别并不明确。由单个 T 小管与肌浆网末端池组成的二元连接主要存在于肌节的 AI 水平。每个单元被布置为通过插入的圆盘与相邻单元形成多个连接。这些专门的连接包括三种类型的细胞间连接：间隙连接、筋膜粘附和心脏功能桥粒。我们的透射电子显微镜 (TEM) 观察清楚地表明，蜘蛛的心肌收缩是由神经源性而非肌源性机制控制的，因为每条心肌纤维都由运动神经元的一个分支通过神经肌肉接头支配。