The document summarizes key aspects of cardiac muscle physiology and the function of the heart as a pump. It describes the structure of cardiac muscle fibers arranged in a syncytium connected by intercalated discs. It explains that the cardiac muscle action potential is caused by the opening of fast sodium and slow calcium channels, resulting in a long plateau phase. The refractory period prevents premature stimulation from causing contraction. Calcium influx through slow calcium channels triggers further calcium release from the sarcoplasmic reticulum, initiating muscle contraction.

1. Chapter 9: Cardiac Muscle; The Heart
As A Pump and Function of the Heart Valves
AFSHAN ARZOO
LECTURER, RCCT
Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. Physiology of Cardiac Muscle
• Physiologic Anatomy
a. Muscle fibers arranged in a latticework
b. Striated and Involuntary
c. Actin and myosin, typical myofibrils
d. Sliding filament mechanism

3. Fig. 9.1 Structure of the heart
and blood flow through
it.

4. Physiology (cont.)
• Cardiac Muscle as a Synctium-cardiac muscle fibers
are made up of many individual cells connected in
series and in parallel (intercalated discs)
Fig. 9.2 Synctium

5. Physiology (cont.)
a. Composed of two synctiums: atrial and ventricular
b. Disks allow the action potential to travel easily from
cell to cell
• Action Potentials in Cardiac Muscle
a. What causes the long action potential and plateau?
b. The AP is caused by the opening of two channel types
1. The same fast Na+ channels as in skeletal muscle
2. Slow Ca++ channels (slower to open and close)
3. Longer period of depolarization leads to plateau

6. Fig. 9.3 Rhythmic AP+s from Purkinje fibers and a ventricular muscle.

7. Physiology (cont.)
c. Permeability for K+ decreases about 5-fold
d. Prevents an early return to depolarized state
• Velocity of Signal Conduction
• Refractory Period of Cardiac Muscle
a. Refractory to restimulation during the AP
b. Ventricle; 0.25-0.30 sec. which is the duration
of the plateau
c. There is an additional relative refractory period

8. Fig. 9.4 Force of ventricular heart muscle contraction, showing the duration of the refractory period
and relative refractory period, plus the effect of premature contraction. Note that there is no
summation as occurs in skeletal muscle.

9. 9
The importance of calcium influx through the slow
voltage gated calcium channels

10. Mechanism of Cardiac Muscle Excitation,
Contraction & Relaxation

11. Physiology (cont.)
• Excitation-Contraction Coupling
Fig. 9.5

12. • Excitation-Contraction Coupling (cont.)
a. Calcium enters from extracellular fluid
b. Triggers calcium release from SR
c. At the end of the plateau, calcium flow stops and
calcium is reabsorbed by the SR