Enrico D. Canale

The Conduction System

The sinoatrial (SA) node, within which the pacemaker normally resides, contains nodal cells, transitional cells, and intercalated clear cells arranged into a well-defined mass. The location and extent of the SA node varies slightly between mammalian species. In the rabbit, cat, mouse, rat, guinea-pig and monkey, the SA node is located in the wall of the superior vena cava (SVC), above or extending down to the crista terminalis (Viragh and Challice 1973 a; Viragh and Porte 1973 a; LEV and Thaemert 1973; Tranum-Jensen 1976). The SA node of the guinea-pig (Anderson 1972a), golden hamster (Walls 1942), and rabbit (James 1967) occupies the full thickness of the atrial wall. In most human hearts the SA node occupies a lateral position at the junction of the SVC and right atrium, but can also be draped over the junction of the SVC and the right atrial appendage (Fig. 29) (Anderson et al. 1979).

Sheep cardiac Purkinje fibers: configurational changes during the cardiac cycle.

SummaryPrevious attempts to study the cytoarchitecture of cardiac Purkinje fibers with the scanning electron microscope (SEM) have been limited by the surrounding dense connective tissue. In this study the connective tissue was removed by treatment with 8N HCl, after adult sheep hearts were fixed in diastole or systole and tissue taken for SEM and transmission electron microscopy (TEM).In SEM, Purkinje fibers freely anastomosed in false tendons and formed a subendocardial plexus. In systole, medium and small-sized Purkinje fibers formed deep clefts not observed in diastole. The clefts are thought to be due to sarcolemmal folding and fiber buckling and may therefore affect conduction. The myofibrils beneath the laterally apposed sarcolemmas of adjacent Purkinje cells when fixed in systole were often observed as tightly curved arches in series. Similar configurations with expanded arches were observed in diastole. The formation of arches by myofibrils is unique to Purkinje fibers and is interpreted as the mechanism responsible for their compliance to stretch. The significance of contraction in producing the observed geometric changes in Purkinje fibers and the implications of their cytoarchitecture with respect to conduction are discussed.

Development of the Conduction System

In the mature heart, the conduction system plays a major role in determining the normal wave patterns of the ECG. Embryos with a four-chambered heart and an immature conduction system produce similar wave patterns. Cardiac electrical activity has been recorded from chick embryos as early as stage 24 (Hoff et al. 1939; Hamburger and Hamilton 1951) and stage 16 (Seidl et al. 1981) (Fig. 85), when no morphologically definable conduction system is present and the outward appearance of the heart is different from the adult. Perhaps the most significant result of electrocardiographic studies is the demonstration of an atrio-ventricular (AV) conduction delay as early as the 20-somite stage in the chick embryo (Hoff et al. 1939). This AV delay may be related to the development of specialised ring tissue (Anderson and Taylor 1972; Wenink 1976; Anderson et al. 1976). Alternatively, conduction delays may be due not to intrinsic properties of cell membranes, but to the effects of tissue geometry and cellular arrangement, which have been demonstrated to affect impulse propagation and action potential configuration in myocardium (Van Capelle and Janse 1976; Spach 1982; Spach and Kootsey 1983). The constriction of the AV canal first appears in the embryo of 19 somites (Hamburger and Hamilton 1951) and may be implicated in providing sufficient local alterations in cell relationships to affect conduction at the 20-somite stage.

Morphometry of Cardiac Muscle

Quantification or ‘morphometry’ of cardiac muscle can be performed using a variety of indices. In recent years much attention has focused on ‘stereology’, and many excellent texts and articles describing the methodology and underlying theory are now available (see Aherne and Dunnill 1982; Loud and Anversa 1984; Weibel 1979; Williams 1977). This technique is applicable at the tissue, cellular and subcellular levels, and provides a wide range of quantitative parameters including volume fractions, surface area to volume ratios, and numerical densities.

Innervation of the Heart

The neural control of the heart, sympathetic and parasympathetic interactions, baroreceptor reflex regulation, and the role of the central nervous system as an integrator of neural influences, have been extensively reviewed by Randall (1977), Levy and Martin (1979), Downing (1979) and Korner (1979). This chapter will discuss the distribution and morphology of neural elements in the heart and their relationships to myocardial tissue as revealed by chemical, histochemical, and ultrastructural techniques.

Morphology of Cardiac Muscle

Cardiac muscle is made up of cross-striated fibres, quasi-cylindrical in shape, which bifurcate and connect with adjacent fibres to form a complex three-dimensional network (Figs. 2, 6, 8). Each fibre is a linear unit composed of several cardiac muscle cells joined end-to-end or end-to-side, in an interdigitating rectangular step-like manner, by specialized junctional complexes called intercalated discs (Figs. 6, 8, 12). Since individual muscle cells (myocytes) rarely branch, the bifurcation of fibres is mainly due to end-to-side interdigitations (see Sommer 1982).

Cardiac Muscle Cells in Culture

Cardiac muscle has been grown in culture since 1910 when the independent spontaneous contraction of small clumps of tissue was observed (Burrows 1910, 1912). Until the 1950’s, most of the studies involved explant culture and the modification of nutrient medium to maintain the cells in a differentiated, contractile state (see Murray 1965). In 1955 Cavanaugh enzymatically dispersed cardiac muscle into single cells and systematically observed their behaviour.

Development of Cardiac Muscle

Developmental studies require an accurate assessment of the progress of development within embryos. Two commonly used indices are embryo size and age.