Sometimes a Z line's place is taken by a dark line across the width of the fibre - intercalated disc . The muscle thus pulls upon itself during contraction. Each cell has only one or two nuclei lying centrally , elongated, but with blunt ends. Fibres branch and anastomose and, until intercalated discs were discerned using EM, the muscle was believed to be syncytial - one huge cell. EM shows the intercalated discs to be extensive, interdigitated cell junctions with gap junctions, fasciae adhaerentes , where the myofibrils attach, and desmosomes .
Mitochondria are more numerous.
Cardiac myofilaments are not clearly aggregated into myofibrils. 2 Sarcoplasm contains glycogen granules, lipid droplets, many mitochondria and soluble proteins. A Golgi complex lies by some nuclei. Cisternae of junctional SR lie immediately adjacent to the T-system, and from these a tubular and vesicular system of free SR extends on either side along, and wrapping around, the myofibrils. The ion permeability of the sarcolemma is altered by acetylcholine to the point where it propagates an action potential.