CHAPTER 20 Cardiovascular System: The Heart
687
1
Action potentials originate in the
sinoatrial (SA) node (the pacemaker)and travel across the wall of the atrium(
arrows
) from the SA node to theatrioventricular (AV) node.
2
Action potentials pass through the AV
Atrioventricular(AV) node
node and along the atrioventricular (AV)bundle, which extends from the AVnode, through the fibrous skeleton, intothe interventricular septum.
3
The AV bundle divides into right and left
bundle branches, and action potentialsdescend to the apex of each ventriclealong the bundle branches.
4
Action potentials are carried by the
ventricle
Atrioventricular(AV)
Purkinje fibers from the bundlebranches to the ventricular wallsand papillary muscles.
Left andbundle
Purkinjefibers
PROCESS FIGURE 20.13
Conducting System of the Heart
ASSESS
YOUR PROGRESS
16.
What is the heart skeleton composed of? What are itsfunctions?
17.
Compare and contrast cardiac muscle and skeletal muscle.
18.
Why does cardiac muscle have slow onset of contractionand prolonged contraction?
19.
What anatomical features are responsible for the ability ofcardiac muscle cells to contract as a unit?
20.
Identify the parts of the conducting system of theheart. Explain how the conducting system coordinatescontraction of the atria and ventricles.
21.
Explain why Purkinje fibers conduct action potentials morerapidly than other cardiac muscle cells.
22.
Relate why the SA node is the pacemaker of the heart.
C.
Explain the importance of a long refractory period incardiac muscle.
D.
Describe the waves and intervals of an electrocardiogram.
In chapter 3, we defined the membrane potential of a cell as theelectrical charge difference across the plasma membrane. Thischarge difference is the result of a cell’s regulation of ion move-ment into and out of the cell. Cardiac muscle cells—like otherelectrically excitable cells, such as neurons and skeletal musclefibers—have a
resting membrane potential,
the membranepotential when the cell is relaxed. The resting membrane poten-tial depends on a low permeability of the plasma membrane toNa
+
and Ca
2 +
and a higher permeability to K
+
. When neurons,skeletal muscle fibers, and cardiac muscle cells are depolarizedto their threshold level, action potentials result (see chapter 11).
20.6
Electrical Properties
LEARNING OUTCOMES
Action Potentials
Like action potentials in skeletal muscle, those in cardiac muscleexhibit depolarization followed by repolarization of the restingmembrane potential. Alterations in membrane channels areresponsible for the changes in the permeability of the plasmamembrane that produce the action potentials. Action potentials incardiac muscle last longer than those in skeletal muscle, and themembrane channels differ somewhat from those in skeletalmuscle. In contrast to action potentials in skeletal muscle, whichtake less than 2 milliseconds (ms) to complete, action potentials in
After reading this section, you should be able to
A.
Summarize the characteristics of action potentials incardiac muscle.
B.
Explain what is meant by the autorhythmicity of cardiacmuscle and relate it to the pacemaker potential.