Download Contractile cells have a long refractory period during which they can

Auto-rhythmic
cells
Contractile cells
have a long
refractory period
during which
they can not
reach threshold
1
+ 30
Plateau
phase of
action
potential
Na+ in fast
0
PCa2+, L; PK+
(ordinary
voltage-gated)
K+ out fast
Membrane potential (mV)
Ca2+ in slow
Threshold
potential
– 70
– 90
250
Time (msec)
Fig. 9-9, p. 239
Action potentials in the SA and AV Nodes are small, slowly rising, and so conduct slowly. Action potentials in the atria, AV bundle, bundle branches, His‐Purkinje system, and ventricles are large, rapidly rising, and conduct rapidly.
Structure
Conduction Velocity
SA Node
Very slow

Atrial Myocardium
Fast

AV Node
Very slow

AV Bundle
Very fast

Bundle branches
Very, very fast

His-Purkinje system
Very, very fast

Moderate
Ventricular Myocardium
2
Normal activation sequence.
The action potentials are not the same in all regions of the heart.
Action potentials in the SA and AV Nodes are small, slowly rising, and so conduct slowly. The SA and AV Nodes depolarize spontaneously during diastole (“pacemaker activity”).
Action potentials in the atria, AV bundle, bundle branches, His‐Purkinje system, and ventricles are large, rapidly rising, and conduct rapidly.
Structure
SA Node

Atrial Myocardium

AV Node

AV Bundle

Bundle branches

His-Purkinje system

Ventricular Myocardium
Pacemaker activity is fastest in the SA Node; slow in the AV Node; and very slow and unreliable in the AV bundle, bundle branches, and His‐Purkinje system.
Structure
Conduction Velocity
SA Node

Atrial Myocardium

AV Node

AV Bundle

Bundle branches

His-Purkinje system

Ventricular Myocardium.
Rate of Pacemaker Firing (/min)
Very slow
60-100
Fast
None
Very slow
40-55
Very fast
25-40
Very, very fast
25-40
Very, very fast
25-40
Moderate
None
3
The Electrocardiogram (ECG): A record of potential differences generated during depolarization and repolarization of the heart recorded from the body surface. Naming of the waves and deflections of the electrocardiogram follows conventions established by Einthoven, a physicist who invented the string galvanometer in 1902, which made possible the high fidelity recording of these small potential differences, and for which Einthoven won the Nobel Prize for Physiology/Medicine in 1924
4
The Electrocardiogram (ECG): A record of potential differences generated during depolarization and repolarization of the heart recorded from the body surface.
ACROSS = TIME 1 mm across = 1/25th second = 0.04 seconds
UP and DOWN equal VOLTAGE (AMPLITUDE, POTENTIAL DIFFERENCE) 1 mm up = 0.1 mv 10mm = 1mv, baseline = 0 mv = isoelectric
Time
Potential
Difference
The Electrocardiogram (ECG): A record of potential differences generated during
depolarization and repolarization of the heart recorded from the body surface.
The “Grid”
0.04 sec
1 mV
0.20 sec
Potential difference between
right arm and left arm (Lead I)
Potential difference between
left arm and left leg (Lead III)
5
The Electrocardiogram (ECG): A record of potential differences generated during
depolarization and repolarization of the heart recorded from the body surface.
The “Grid”
0.04 sec
1 mV
Potential difference between
right arm and left arm (Lead I)
0.20 sec
QRS
P
Potential difference between
left arm and left leg (Lead III)
P wave- Atrial depolarization
T
QRS complex - Ventricular depolarization
T wave- Ventricular depolarization
Why do the waves recorded in the ECG go up in some leads and down in others?
6
Lead
• an organized method of placing a positive and negative electrode on the surface of the body to detect the electrical activity of the heart (which is constant)
• as positive and negative positions change, the recording of the same event changes • each leads appearance is different
• the AXIS of a LEAD by definition goes from ‐ to +
Fig. 9‐12, p. 241
7
8
9
10
The Normal 12 lead Electrocardiogram
11
Interatrial
pathway
SA node
AV node
Right atrium
Left atrium
Interatrial
pathway
Bundle
of His
Electrically
nonconductive
fibrous tissue
Left ventricle
Right ventricle
Purkinje
fibers
(b) Spread of cardiac excitation
Fig. 9-7b, p. 235
List in order the steps by which the heart’
List in order the steps by which the heart’s conduction system initiates normal ventricular systole
1.
2.
3.
4.
5.
6.
7.
Cardiac impulse originates at SA node
Action potential spreads throughout right and left atria
Impulse passes from atria into ventricles through AV node (only point of electrical contact between chambers)
Action potential briefly delayed at AV node (ensures atrial contraction precedes ventricular contraction to allow complete ventricular filling)
Impulse travels rapidly down interventricular septum by means of bundle of His
Impulse rapidly disperses throughout myocardium by means of Purkinje fibers
Rest of ventricular cells activated by cell‐to‐cell spread of impulse through gap junctions. This is ventricular systole
12
Electrocardiogram (ECG)
• Record of overall spread of electrical activity through heart
• Represents
– Recording part of electrical activity induced in body fluids by cardiac electrical impulse that reaches body surface
• Not direct recording of actual electrical activity of heart
– Recording of overall spread of activity throughout heart during depolarization and repolarization
• Not a recording of a single action potential in a single cell at a single point in time
– Comparisons in voltage detected by electrodes at two different points on body surface, not the actual potential
• The heart as a DIPOLE
SA node
fires
TP interval =
Time during which
ventricles are
relaxing and filling
Recorded potential
P wave =
Atrial depolarization
R
200 msec
T
P
Q
PR
segment
P
PR segment =
AV nodal delay
S
ST
segment
TP
interval
T wave =
Ventricular
repolarization
ST segment =
Time during which
ventricles are contracting
and emptying
QRS complex =
Ventricular depolarization
atria repolarizing
simultaneously)
Fig. 9-13, p. 242
13