Download ANPS 020 Black 01-30

ANPS 020
January 30, 2012
THE CONDUCTING SYSTEM OF THE HEART
Pacemaker cells are leaky, and cannot maintain a stable resting potential
They slowly depolarize to threshold, then fire an action potential
The rate of spontaneous depolarization determines the heart rate
SA node generates 80-100 action potentials per minute
-fastest rate of firing, so drives all the other cells of conducting system at this rate
AV generates 40-60 action potentials per minute
DISORDERS OF THE CONDUCTING SYSTEM
Abnormal Pacemaker Function
Bradycardia: abnormally slow heart rate
Tachycardia: abnormally fast heart rate
Ectopic pacemaker
Abnormal cells generate high rate of action potentials
Bypass conducting system: don’t wait for signals through the regular pathway
Disrupt ventricular contractions; ventricle may no contract bottom-to-top
A pacemaker device may be implanted to regulate abnormal heart activity
-helps resynchronize the heart
THE CARDIAC CYCLE AND THE ECG
Pumping blood: a mechanical event initiated by electrical events
The electrical event is initiated by the pacemaker cells of the conductive systems
The conductive system distributes the stimulus to the contractile cells of the heart wall, initiating
contraction of the myocardium and movement of blood.
CHARACTERISTICS OF CARDIAC MUSCLE CELLS
Small size
Single, central nucleus
Branching interconnections between cells
Intercalated disc
STRUCTURE OF CARDIAC MUSCLE CELLS
Intercalated dics interconnect cardiac muscle cells
-thickening on each side of disc
Intercalated discs contain two types of cell-cell junctions
Desmosomes physically tie cells together
Gap junctions connect cytoplasm
Allow ion flow between cells
“electrical coupling” of cells
THE ACTION POTENTIAL OF A SINGLE CONTRACTILE CARDIAC MUSCLE
The resting membrane potential of contractile cells is stable (unlike that of the conductive cells like
those of the SA node)
The absolute refractory period is very long: cardiac muscles cells cannot be stimulated again until this
period is over
THE ROLE OF CALCIUM IONS IN CARDIAC CONTRACTIONS
Contraction of cardiac muscle cell is produced by an increase in calcium ion concentration around
myofibrils
-2 sources: extra cellular and intra cellular
THE ACTION POTENTIONAL IN A CARDIAC MUSCLE CELL CAUSES CONTRACTION OF CELL
In skeletal muscle, the action potential was brief relative to the contraction. A second action potential
soon after the first increased cytoplasm calcium levels and increased the strength and duration of
contraction.
In cardiac muscle, the action potential lasts longer than the contraction. On contraction is over before
another can begin, preventing summation of contraction and tetany. This ensures time for the heart to
fill between contractions.
Tetany: maintaining contraction
THE CARDIAC CYCLE
Heartbeat: a single contraction of the heart
Begins with action potential at SA node
-transmitted through conducting system (conducting cardiac cells)
-Produces action potentials in cardiac muscles cells (contractile cardiac cells)
The entire heart contracts in series
SA node generates electrical signal which spreads through atria, so atria contract first, which completes
the filling of ventricles
The electrical activity slows at AV node, giving times for ventricles to fill
Electrical signal finally passes into ventricles, contracting them from the apex toward the base, pushing
blood toward the arteries leading out of the heart
Right and left sides contract at same time, pumping approximately equal volumes of blood from right
and left sides simultaneously
MONITORING HEART ACTIVITY: ECG
Electrocardiogram
A recording of electrical events in the heart, representing all the action potentials form all the cardiac
cells-conduction and contractile
Obtained by placing electrodes at specific body locations
Abnormal patterns diagnose damage
FEATURES
P wave: atria depolarize
QRS complex: ventricles depolarize
T wave: ventricles repolarize
Time intervals between ECG waves:
P-R interval: time from start f atrial depolarization to start of QRS complex
QT interval: time from ventricular depolarization to ventricular repolarization
ABNORMAL ECG RECORDINDS
Atria not pumping
P waves absent
SA node nonfunctional
Slower rate now driven by AV node
Ventricles not pumping
P waves not always followed by QRS wave
A form of “heart bloc” problem within conduction system between SA node and rest of system
Chaotic deflections
“Ventricular fibrillation”
Bad. Very Bad.
Doesn’t give ventricle time to fill.
DEFIBRILLATORS SHOCK THE HEART BACK INTO A NORMAL RHYTHM
THE CARDIAC CYCLE
The period between the start of one heartbeat and the beginning of the next
-fells with blood and pushed out
Includes both contraction and relaxation
Phases of the cardiac cycle within ay on chamber
-systole: muscle wall contracting
Diastole: muscle wall is relaxed – a time for filling
Blood pressure:
In any chamber, pressure rises during systole and falls during diastole
Blood flows form area of high pressure to low pressure
-movement is controlled by timing of contractions
-directions of movement is regulated by one-way valves
Cardiac cycle and heart rate
At 75 beats per minute cardiac cycle lasts about 800 msecs
When heart rate increases all phases of cardiac cycle shorten, particularly diastole.