Summary Guyton and Hall Chapter 9 Notes Cardiac Muscle - The Heart as a Pump and Function of the Heart Valves
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Course
Physiology (PHYSIOLOGY)
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Cebu Doctor\'s University
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Guyton and Hall Textbook of Medical Physiology
This is a condensed summary of the Chapter 9 - Cardiac Muscle - The Heart as a Pump and Function of the Heart Valves from Guyton and Hall Textbook of Medical Physiology. It contains pictures from the book as well. Use these notes to study for your upcoming exam or quiz.
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Guyton and Hall Textbook of Medical Physiology 14th Edition Test Bank by John E. Hall, Michael E. Hall
Test Bank For Guyton and Hall Textbook of Medical Physiology 14th Edition By John E. Hall; Michael E. Hall All Chapters 1-86 ||Complete A+ Guide
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CHAPTER 9 Causes of long action potential and plateau in cardiac muscle
are attributed to 2 major membrane differences between cardiac
CARDIAC MUSCLE; THE HEART AS A PUMP AND and skeletal muscle:
FUNCTION OF THE HEART VALVES
1. Action potential of skeletal muscle is caused by
opening of large numbers of fast sodium channels that
only open for a few seconds then abruptly close then
repolarization will occur.
In cardiac muscle, action potential is caused by
opening 2 types of channels:
i. Same voltage-activated fast sodium channels as
those in skeletal muscle
ii. L-type calcium channels aka slow calcium
channels aka calcium-sodium channels. These are
slower to open and remain open for several tenths
of a second wherein large quantities of calcium
and sodium ions flow to the interior of the cardiac
muscle fiber. This maintains a prolonged period of
depolarization, causing the plateau in the action
PHYSIOLOGY OF THE CARDIAC MUSCLE potential. The calcium ions that enter during the
plateau activate the muscle contractile process.
3 major types of cardiac muscle: 2. Permeability of cardiac muscle membrane for
potassium ions after action potential decreases about
1. Atrial muscle 5x, and this does not occur in skeletal muscle. This will
2. Ventricular muscle greatly decrease the efflux of positively charged K ions
3. Excitatory and conductive muscle fibers during the AP plateau thereby preventing early return of
the AP voltage to its resting level.
Atrial and ventricular – contract the same way as skeletal
muscle, only with longer duration of contraction Phases of Cardiac Muscle
Action Potential
Specialized excitatory – contract feebly; exhibit automatic
rhythmical electrical discharge in the form of action potentials or 0. Depolarization
conduction of action potentials through the heart. 1. Initial repolarization
2. Plateau phase
CARDIAC MUSCLE ANATOMY
3. Rapid polarization
A cardiac muscle is a 4. Resting membrane
syncytium of many heart potential
muscle cells in which the
Phase 0 – Depolarization
cardiac cells are so
interconnected that when one - Membrane potential becomes more positive
cell becomes excited, the - Fast sodium channels open (voltage-gated sodium
action potential rapidly channels)
spreads to all of them. - Sodium rapidly flows to the cell and depolarizes it
- Membrane potential reaches up to +20mV before the
This is due to the intercalated
sodium channels close
discs of the cardiac muscle
fibers that fuse to each other to form gap junctions that allow Phase 1 – Initial Repolarization
rapid infusion of ions causing action potentials to move easily
through cardiac muscles. - Fast sodium channels close
- Cell begins to repolarize
2 syncytia: - K ions leave the cell through open potassium channels
1. Atrial syncytium – walls of the 2 atrium Phase 2 – Plateau
2. Ventricular syncytium – walls of the 2 ventricles
- Calcium channels open
*This division of the heart’s muscle allows the atria to contract a - Fast potassium channels close
short time ahead of ventricular contraction, important for the - Brief initial repolarization
effectiveness of heart pumping. - Combination of decreased potassium ions efflux and
increased calcium ion influx causes the plateau
ACTION POTENTIALS IN CARDIAC MUSCLE
Phase 3 – Rapid Repolarization
In a ventricular muscle fiber,
action potential is about 105 - Calcium channels close
millivolts (a rise from -85mV to - Slow potassium channels open
+20mV during each beat). - End of plateau
- Returns the cell membrane potential to its resting level
Plateau – 0.2 second
depolarization after initial Phase 4 – Resting Membrane Potential
spike; causes ventricular
contraction to last as much as - -80 to -90 millivolts
15 times longer than skeletal
muscle Velocity of signal conduction is about 0.3 to 0.5m/s, about 1/250
the velocity in large nerve fibers. In Purkinje fibers, it is as high
Followed by abrupt repolarization. as 4m/s.
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