Animal Physiology for Msc And BS Zoology students.

1. Electrical Activity of the Heart
Abuzar Tabassum
(M.Phil Zoology)
(PhD Scholor)

2. Electrical Activity of the Heart
• Neurogenic and myogenic pacemakers
• Cardiac pacemaker potentials
• Cardiac Action potentials (Aps)
• Transmission of excitation over the heart

3. Cardiac Action Potentials
• The action potentials in all vertebrate cardiac muscle cells
are of longer duration than those in skeletal muscle.
• The AP in skeletal muscle is completed and the membrane is
in a non-refractory stage before the onset of contraction;
hence, repetitive stimulation and tetanic contraction are
possible (Figure A).

5. • In cardiac muscle, by contrast, the action potential plateaus and the
membrane remains in a refractory state until the heart has returned to
a relaxed state (Figure B).
• Thus, summation of contractions cannot occur in cardiac muscle.

7. Depolarization
• Cardiac APs begin with a rapid depolarization that results from a
large and rapid increase in sodium conductance.
• This differs from the slow depolarization of the pacemaker potential,
which is marked by a stable sodium conductance and decreasing
potassium conductance.

8. • Repolarization of the cell membrane is delayed while the membrane
remains depolarized in a so-called plateau phase for hundreds of
milliseconds (Figure B).
• The long duration of the cardiac AP produces a prolonged
contraction, so that an entire chamber can fully contract before any
portion begins to relax, a process that is essential for efficient
pumping of blood.

9. Plateau Phase
• The prolonged plateau of the cardiac AP results from maintenance of
a high calcium conductance and a delay in the increase in potassium
conductance.
• The high calcium conductance during the plateau phase allows Ca2+
ions to flow into the cell, because the equilibrium potential for
calcium is directed strongly inwards.

10. Repolarization
• A rapid repolarization terminates the plateau phase, due to a fall in
calcium conductance and an increase in potassium conductance.

11. • The duration of the plateau and the rates of depolarization and
repolarization vary in different cells of the same heart.
• The summation of these changes are recorded as the electrocardiogram
(Figure 12-8).
• Atrial cells generally have an AP of shorter duration than ventricular
cells.
• The duration of the AP in atrial or ventricular fibers from hearts of
different species also varies.
• In smaller mammals, the duration of the ventricular AP is shorter, thus
heart rates generally are higher than in larger mammals.

12. Transmission of excitation over
the heart
• Electrical activity initiated in the pacemaker region is conducted
over the entire heart.
• Depolarization in one cell resulting in the depolarization of
neighboring cells by virtue of current flow through gap junctions.
• Gap junctions are regions of low resistance between cells and allow
current flow from one cell to the next across intercalated disks.
• These junctions between cells are located in regions of close
apposition between neighboring myocardial cells, termed the
intercalated disk.

14. • Adhesion of cells at intercalated disks is strengthened by the
presence of desmosomes (Desmosomes are specialized adhesive
protein complexes that localize to intercellular junctions).
• The area of contact is increased by folding and interdigitation of
membranes (Figure12-9).

15. • Although the junctions between myocardial cells can conduct in
both directions, transmission is usually unidirectional because the
impulse is initiated in and spreads only from the pacemaker region.
• There are usually several pathways for excitation of any single
cardiac muscle fiber, since intercellular connections are numerous.
• If a portion of the heart becomes nonfunctional, the wave of
excitation can easily flow around that portion, so that the remainder
of the heart can still be excited.

16. • The prolonged nature of cardiac APs ensures that multiple
connections do not result in multiple stimulation and a
reverberation of activity in cardiac muscle.
• An AP initiated in the pacemaker region results in a single AP being
conducted through all the myocardial cells
• Another AP from the pacemaker region is required for the next
wave of excitation.

17. • In the mammalian heart, the wave of excitation spreads from the
sinoatrial node (S.A Node) over both atria in a concentric fashion at
a velocity of about 0.8 m.s-1
• The atria are connected electrically to the ventricles only through the
atrioventricular (A.V) node
• In other regions the atria and ventricles are joined by connective
tissue that does not conduct the wave of excitation from the atria to
the ventricles (Figure).

19. • Excitation spreads to the ventricle through small junctional fibers,
in which the velocity of the wave of excitation is slowed to about 0.05
m.s-1
• The junctional fibers are connected to nodal fibers, which in turn are
connected via transitional fibers to the bundle of His
• This structure branches into right and left bundles, which subdivide
into Purkinje fibers that extend into the myocardium of the two
ventricles.

21. • Conduction is slow through the nodal fibers (about 0.1 m.s-1) but
rapid through the bundle of His (4-5 m.s-l).
• The bundle of His and the Purkinje fibers deliver the wave of
excitation to all regions of the ventricular myocardium very rapidly,
causing all the ventricular muscle fibers to contract together.
• As each wave of excitation arrives, the ventricular myocardial cells
contract almost immediately, with the wave of excitation passing at a
velocity of 0.5 m .s-1 from the internal lining of the heart wall
(endocardium) to the external lining (epicardium).

22. • The functional significance of the electrical organization of the
myocardium is its ability to generate separate, synchronous
contractions of the atria and the ventricles.
• Thus, slow conduction through the atrioventricular node (A.V)
allows atrial contractions to precede ventricular contractions and
also allows time for blood to move from the atria into the ventricles.

23. Electrocardiogram
• Because of the large number of cells involved, the currents that flow
during the synchronous activity of cardiac cells can be detected as
small changes in potential from points all over the body.
• These potential changes recorded as the electrocardiogram are a
reflection of electrical activity in the heart and can be easily
monitored and then analyzed.

25. • The P-wave is associated with depolarization of the atrium
• The QRS complex with depolarization of the ventricle.
• And the T-wave with repolarization of the ventricle.
• The electrical activity associated with atrial repolarization is covered
by the much larger QRS complex.

26. • The exact form of the electrocardiogram varies with the species in
question and is affected by the nature and position of recording
electrodes, as well as by the nature of cardiac contraction.
• The electrocardiogram is valuable medically because it can be used to
diagnose cardiac abnormalities.