The spread of excitation

A heart would be ineffective if the rhythmic action potentials and their associated contractions in any one fibre were independent of those of its fellows: it is obviously essential that the contractions of the fibres should be synchronized. This is brought about by direct current flow from one cell to the next via the gap junction channels in the intercalated discs. Hence local circuits set up by an action potential in one cell can cross into the next cell and so excite it.

The excitation sequence in mammals is modified by the existence of some cells, the Purkinje fibres, which have become specialized for conduction rather

Atrioventrici

Sinuatria! node node

Atrioventrici

Sinuatria! node node

Purkinje tissue

Bundle of His

Fig. 11.5. Pacemaker and specialized conductile regions of the mammalian heart. From Scher (1965).

Purkinje tissue

Bundle of His

Fig. 11.5. Pacemaker and specialized conductile regions of the mammalian heart. From Scher (1965).

than contraction. They are longer and more elongated than other cardiac cells and contain little contractile material.

Excitation begins at the pacemaker region in the sinuatrial node (Fig. 11.5); the fibres here are spontaneously active and their rhythmic production of action potentials determines the rate of beating of the whole heart. These then excite their neighbours by local current flow, and so a wave of depolarization spreads across the atria, resulting in a contraction of each atrium so that blood is forced into the ventricles. The atria are connected to the ventricles at the atrioventricular node. This consists of fibres which are small in diameter so that conduction at this point is slow; this ensures that there is an appreciable delay between the excitation of the atria and the ventricles. The atrioventricular node is connected to the large Purkinje fibres in the bundle of His. These serve to carry the excitation on to the main mass of the ventricular muscle, beginning in the septum and then spreading from the apex of the ventricle up to its base.

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