Chapter 10: Rhythmical Excitation of the Heart Unit Three: The Heart Chapter 10: Rhythmical Excitation of the Heart Guyton and Hall, Textbook of Medical Physiology, 12th edition

Rhythmic Excitation Specialized Excitatory and Conductive System Sinus (Sinoatrial node)-located in the wall of the right atrium and has almost no contractile muscle filaments Automatic rhythmicity-cardiac fibers with the ability of self-excitation

Fig. 10.1 Sinus node and the Purkinje system of the heart

Rhythmic Excitation (cont.) Mechanism of Sinus Nodal Rhythmicity Fig. 10.2 Rhythmical discharge of a sinus node fiber

Rhythmic Excitation (cont.) Lesser negativity since the cell membranes are naturally leaky to Na+ and Ca++ Have the three types of channels Fast sodium Slow sodium-calcium Potassium At -55 mV the fast Na+ channels may be inactivated Only the slow channels open The AP is therefore slower to develop

Self-Excitation of Sinus Node Fibers Internodal Pathways and Transmisson of the Impulse AV Node and Delay of Impulse Cause of slow conduction Ventricular Purkinje System and Distribution of Fibers One-way Conduction Through the AV Bundle (His)

Fig. 10.4 Transmission of the cardiac impulse through the heart, showing the time of appearance in different parts of the heart.

Control of Excitation and Conduction in the Heart Sinus Node as the Pacemaker of the Heart Abnormal Pacemakers (Ectopic) Role of the Purkinje System Parasympathetic and Sympathetic Control Vagal effects Sympathetic stimulation Mechanism of the sympathetic effect