This document discusses electrical events in cardiac tissue. There are two types of cardiac muscle cells (fast and slow fibers) that have distinct action potentials. The cardiac muscle cells are internally negative and externally positive at rest. When stimulated by the SA node, the cells depolarize and become internally positive and externally negative. Depolarization causes muscle contraction, spreading a wave through the tissue. As the stimulus dies away, the cells repolarize and relax. Repolarization returns the cells to their polarized resting state. The cells of the conducting system are autorhythmic, with a pacemaker potential instead of a resting membrane potential, caused by slow sodium channels.
2. THE MYOCARDIUM
• Two specialized types of cardiac
muscle cells:
• Each of these 2 types of cells has a
distinctive action potential.
• Fast fibers
• Slow fibers
CARDIAC
MUSCLE
Contractile
99%
Autorrythmic
1%
10. What causes the
muscle resting
membrane
potential to change
initially?
What would be
happening with a
skeletal muscle at
this point?
11.
12. Why are fibers of the conducting system autorhythmic?
Membrane potential of SA nodal cells
13. Autorhythmic Cells
• The auto rhythmic cells do not have a stable resting membrane potential
like the nerve and the skeletal muscles.
• Because the membrane potential never rests at a constant value, it is called
a Pacemaker Potential rather than a resting membrane potential.
• Pacemaker potentials slow Na+ channels
• At threshold, Ca2+ channels open
• Explosive Ca2+ influx produces the rising phase of the action potential
• Repolarization results from inactivation of Ca2+ channels and opening of
voltage-gated K+ channels
14. Figure 18.13
1 2 3Pacemaker potential
This slow depolarization is
due to both opening of Na+
channels and closing of K+
channels. Notice that the
membrane potential is
never a flat line.
Depolarization The
action potential begins when
the pacemaker potential
reaches threshold.
Depolarization is due to Ca2+
influx through Ca2+ channels.
Repolarization is due to
Ca2+ channels inactivating and
K+ channels opening. This
allows K+ efflux, which brings
the membrane potential back
to its most negative voltage.
Action
potential
Threshold
Pacemaker
potential
1 1
2 2
3
17. Terminologies
• Chronotropism change in heart rate
• Ionotropism change in force of contraction
• Dromotropism change in conduction velocity
• Bathmotropism change in degree of excitability
1. Contractile cells, which are 99% of the cardiac muscle cells, do the mechanical work of pumping. These working cells normally do not initiate their own action potentials.
2. In contrast, the small but extremely important remainder of the cardiac cells, the auto rhythmic cells, do not contract but instead are specialized for initiating and conducting the action potentials responsible for contraction of the working cells..
