Abstract
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.
Highlights
Cardiac muscle is an involuntary, striated muscle that constitutes the main tissue of the heart
We describe the fine structural characteristic of the cardiac muscles of the golden orb-web spider Nephila clavata
Each cell is arranged to form a multiple connection with neighboring cells via intercalated discs (Fig. 1a)
Summary
Cardiac muscle is an involuntary, striated muscle that constitutes the main tissue of the heart. It is organized into the basic units of striated muscle fibrils known as sarcomeres (Craig and Padrón 2004). Previous studies showed evidences that the cardiac muscle was highly adapted to resistant to fatigue (Nelesen et al 2008). It has a large number of mitochondria, enabling continuous aerobic respiration through oxidative phosphorylation and a good blood supply that provides nutrients and oxygen (Lyons et al 2006).
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