Myocardial Rhythmicity (automaticity)
It is the ability of
cardiac muscle to generate excitation wave (action potential)
spontaneously and regularly
voluntary muscles, the cardiac rhythmic activity does not depend upon its
connection with the CNS. For example, whereas the rhythmic contraction of the
diaphragm immediately stops when phrenic nerves are cut; a completely isolated
heart continues to beat when it is perfused with suitable fluid. In other words
the cardiac rhythmicity is myogenic in nature.
Rhythmicity in different cardiac fibers:
is not possessed to the same extent by the different cardiac fibers. The S-A
node possesses the greatest rhythmicity and initiates the beat of the whole
heart. The tissues of the human heart can be arranged in descending order as
regards their rhythmicity: (S-A node l05/m),(A-V node 45-60/m),purkinje
system and ventricles (25-40/m) i.e. idioventricular rhythm.
focus which generates the highest frequency of impulses is the one which drives
the whole heart and is called the pacemaker of the heart.
normal pacemaker of the human heart is the S-A node. If not influenced
by any extrinsic nervous or chemical stimulation, the S-A node discharges
spontaneously at a rate
about 105 impulse/min "intrinsic heart rate".
node initiates the excitation wave and makes the pace (=speed) for the whole
cardiac muscle. If the S-A node is damaged, the A-V node becomes the new pace-maker
and the heart follows it, but at a slower rate (A-V nodal rhythm). Any
pace-maker other than S-A node is called an ectopic pacemaker.
Factors affecting myocardial rhythmicity:
a) Innervation: Vagus
nerve inhibits rhythmicity of the whole heart except that of the
ventricles because they are not supplied by the vagus. Sympathetic stimulation
increases rhythmicity of the whole heart including the ventricles. There is a
resting sympathetic tone which tends to increase the heart rate up to
120 beat/min; but, this tone is masked by the strong inhibitory vagal tone
which decreases the rate down to 75 beats/ min during rest.
increasing the body temperature 1oC, the heart rate raises about 15
beats/ min and vice versa.
Effect of ions: If calcium is added in excess it would
progressively increase the systole until the heart stops in a contracted state (Calcium
rigor).On the other hand if excess potassium is added, the diastole is
enhanced until the heart stops in diastole.
complete absence of O2 the heart stops after several beats.