Nervous Control of the Heart
Internal Control of Heart Rate
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The heart or cardiac muscle beats involuntarily.
The beat of each cell of heart muscle is in
synchrony or in pace with the beat of every
other heart muscle cell.
Nervous impulses from a specialized bundle
of nervous tissue called the Sinoatrial (SA)
Node control the rate of the heart beat and
also synchronize the beating of all cardiac cells.
Internal Control of Heart Rate
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The sinoatrial or SA
node is known as the
pacemaker.
The SA node causes a
wave of contraction to
start in both the and
This wave of contraction
causes the
Atrioventricular (AV)
node to initiate
contractions of the
ventricles.
Internal Control of Heart Rate
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The bundle of HIS carries
the nervous stimuli to all
parts of the ventricle via
Purkinje Fibres which brings
the impulse to each cardiac
cell.
The SA node paces the heart
at 72 beats/min which can be
considered a “normal”
resting heart rate.
External Control of Heart Rate
The Medulla Oblangata in the brain controls the
nerves that influence heart rate.
O2 and CO2 concentration levels in the blood are
measured by chemoreceptors that stimulate one of
two nerves to the heart in order to maintain
homeostasis.
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1.
2.
Increased or accelerated heart rate is caused by stimulation
of the sympathetic nerve.
Decreased or slowing of the heart rate is caused by the
stimulation of the parasympathetic nerve.
Pressure sensitive baroreceptors in the arteries respond to
changes in the blood pressure.
Factors Affecting Heart Rate

Increase Physical Activity =
Increase in Heart Rate

Increase in Emotions =
Increase in Heart Rate
Chemicals that Increase
Heart Rate
1.
2.
3.
4.
Nicotine
Adrenaline
Thyroxin
Caffeine
Chemicals that Decrease Heart Rate
1.
2.
Carbon Monoxide
Alchohol
Tachycardia
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When your heart beats more than 72 beats per
minute, such as during exercise or from the
consumption of caffeine or nicotine.
Bradycardia
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When the heart beats very slowly, less than 72
beats per minute, such as athletes who have an
elevated cardiac output even during rest.
Heartbeat Sounds
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If you listen to your heartbeat through a stethoscope you hear 2
characteristic sounds that repeat over and over as the heart
continues to beat: Lub-dub, lub-dub…
Lub
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The sound which is a result of the
closing of both atrio-ventricular
valves (tricuspid/bicuspid)
Occurs during Systole, or
ventricular contraction (i.e.
atricular relaxation)
Is louder and longer of the 2
sounds
Heartbeat Sounds
Dub
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The sound which is the
result of the closing of
both semi-lunar valves
(pulmonary/aortic)
Occurs during Diastole,
or Ventricular relaxation
(or atrial contraction)
Is the softer and shorter
of the 2 sounds.