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‫االسراء اية ‪58‬‬
‫‪Dra abdelaziz Hussein, Mansoura‬‬
‫‪Faculty of Medicine‬‬
By
Dr. Abdel Aziz M. Hussein
Lecturer of Medical Physiology
Member of American Society of Physiology
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
Cardiac Properties
Excitability
Conductivity
Rhythmicity
Contractility
Central
Pump
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
Cardiac Properties
Impulses originate
regularly at a
frequency of 60-100
beat/ min
SAN
AVN
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
• Def.
• Rhythmicity is the ability of heart to beat
regularly
• Automaticity is the ability of heart to generate
impulses without external stimuli
• Autorhythmicity is the ability of cardiac ms to
generate impulses without external stimuli at
regular rate
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
• Origin
• Myogenic not neurogenic.
• The nerves control the rate but do not initiate the
beat
cocaine
• Evidences:
a) Local anesthetics e.g. cocaine block
the nerve but not stop the heart
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
• Evidences:
b)Transplanted
heart (no nerve supply)
continues to beat.
c) Heart of human fetus start to beat before
development of nerves
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
• Nature:
• Most cardiac fibers have the
ability of self excitation especially
nodal and conducting fibers
110 b/min
• SAN
has
the
greatest rhythm,
so it is called
"pace maker of
the heart
90 b/min
45 b/min
35 b/min
25 b/min
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
Mechanism of Autorhythmicity of SAN:
•RMP of SAN is low about -55mv to -60 mv.
•This is due to natural leakiness to Na ions
Action Potential (AP):
1. Prepotential or pacemaker
potential (phase 4)
2. Upstroke (phase 0)
3. Repolarization (phase 3)
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
It is a gradual rise of membrane potential from the
resting level of -55 mv to the firing level or the
threshold voltage of -40 mv
It occurs during diastole, so it is also called the
diastolic depolarization (DD)
Mechanism:
1. Na influx through funny (slow)
Na channels
2. Ca influx through T (=transient)
-type Ca channel
3. Decreased K+ efflux
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
Importance of Pacemaker potential:
1. It is the cause of Rhythmicity.
2. The rate of slope of the prepotential determines the
heart rate. The more rapid is the slope, the more will be
the heart rate.
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
2. Upstroke (phase 0):
•From membrane potential (– 40 m.v) to (+ 10 m.v)
Mechanism:
• Inward Ca++ current
through L (long lasting )
Ca++ channels
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
3. Repolarization (phase 3):
•From membrane potential (+10 m.v) to (-60 m.v)
Mechanism:
• increase K+ efflux
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
0
-10
-20
-40
Phase 0
Repolarization
+10
Depolarization
mv
Phase 3
Phase 4
(only in
pacemaker cells
R.M.P
-60
+
Na
Na
Na++++
Na
Na+
Na
m
Na+
h
++ ++
caca
ca++
K+
+++
+K
KKK
ca++
++
ca ++
ca++
ca
++
ca
++ ca
++ ++
caca
Factors affecting Rhythmicity
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
Factors Affecting Rhythmicity
• Chronotropism means an influence on the heart rate
• +ve chronotropic → ↑ heart rate
• -ve chronotropic → ↓ heart rate
Factors
Nervous
Factors
sympath
etic
Parasym
pathetic
Physical
Warming
and
cooling
Chemical
Drugs
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
Gases
Toxins
Ions
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
Autonomic N. System
Sympathetic stimulation:
• has a +ve chronotropic effect
•Release noradrenalin → increases fiber membrane
permeability to Na+ & Ca++→ rapid slope of
Prepotential → threshold is reached rapidly → increase
Rhythmicity
Figure 14-17: Modulation of heart rate by the nervous system
Autonomic N. System
Parasympathetic stimulation:
• has a -ve chronotropic effect
•Acetylcholine released at the vagal endings →increase
the permeability of fiber membrane to K+ → rapid K+
efflux → hyperpolarization→ decrease Rhythmicity
Figure 14-17: Modulation of heart rate by the nervous system
Physical Factors
Warming:
• Moderate warming : increases Rhythmicity due to;
a)↓permeability of the membrane to K+ ions during the
pacemaker potential, i.e. rapid slope of prepotential
b)↑speed of ionic fluxes across the membrane during the
action potential
•Excessive warming (45 ) denatures the intracellular
proteins and produces cardiac damage
•Moderate cooling decreases Rhythmicity
•Excessive cooling stop Rhythmicity
Figure 14-17: Modulation of heart rate by the nervous system
Chemical Factors
1. Drugs and hormones:
•Catecholamines have +ve chronotropic effect.
•Cholinergic drugs (as methacholine) have –ve
chronotropic effect
•Thyroxine has a +ve chronotropic effect as it stimulates
the metabolism of the SAN.
•Digitalis depresses nodal tissue. It increases K+ efflux
→ hyperpolarization
Figure 14-17: Modulation of heart rate by the nervous system
Chemical Factors
2. Blood Gases:
•O2 lack (hypoxia):
•Mild hypoxia increases rhythmicity
•Severe hypoxia decreases and stop rhythmicity
•Hypercapnia: has a -ve chronotropic effect through
acidemia
•H ion concentration :
•Acidosis decreases rhythmicity
•Alkalosis increases rhythmicity
•Severe acidosis and alkalosis decreases rhythmicity .
Figure 14-17: Modulation of heart rate by the nervous system
Chemical Factors
3. Inorganic ions :
•K ions :
• mild hyperkalemia have -ve chronotropic effect.
(because excess K in ECF decreases K efflux during
repolarization)
•Mild hypokalemia has opposite effect
•Ca ions:
• Mild increase of Ca++ (hypercalcemia) decreases
rhythmicity by activating K+ channels → hyperpolarization
→ so longer time is needed to reach threshold potential.
•Mild hypocalemia has the opposite
Figure 14-17: Modulation of heart rate by the nervous system
Chemical Factors
3. Inorganic ions :
•K ions :
• mild hyperkalemia have -ve chronotropic effect.
(because excess K in ECF decreases K efflux during
repolarization)
•Mild hypokalemia has opposite effect
•Ca ions:
• Mild increase of Ca++ (hypercalcemia) decreases
rhythmicity by activating K+ channels → hyperpolarization
→ so longer time is needed to reach threshold potential.
•Mild hypocalemia has the opposite
Figure 14-17: Modulation of heart rate by the nervous system
Toxins
•Typhoid or diphtheria toxins have a –ve chronotropic
effect, due to direct inhibitory action on nodal tissues
Figure 14-17: Modulation of heart rate by the nervous system
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine
THANKS
Dra abdelaziz Hussein, Mansoura
Faculty of Medicine