Transcript Unit One: Introduction to Physiology: The Cell and

Chapter 10: Rhythmical Excitation of the Heart
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Rhythmic Excitation
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Specialized Excitatory and Conductive System
a. Sinus (Sinoatrial node)-located in the wall of the
right atrium and has almost no contractile muscle
filaments
b. 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.)
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Mechanism of Sinus Nodal Rhythmicity
Fig. 10.2 Rhythmical discharge of a sinus node fiber
Rhythmic Excitation (cont.)
a. Lesser negativity since the cell membranes are
naturally leaky to Na+ and Ca++
b. Have the three types of channels
1. Fast sodium
2. Slow sodium-calcium
3. Potassium
c. At -55 mV the fast Na+ channels may be inactivated
d. Only the slow channels open
e. The AP is therefore slower to develop
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Self-Excitation of Sinus Node Fibers
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Internodal Pathways and Transmisson of the Impulse
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AV Node and Delay of Impulse
a. Cause of slow conduction
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Ventricular Purkinje System and Distribution of
Fibers
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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
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Sinus Node as the Pacemaker of the Heart
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Abnormal Pacemakers (Ectopic)
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Role of the Purkinje System
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Parasympathetic and Sympathetic Control
a. Vagal effects
b. Sympathetic stimulation
c. Mechanism of the sympathetic effect