Control of the Heart Beat
IB Assessment Statement
• Outline the control of the heartbeat in terms of myogenıc
muscle contraction, the role of the pacemaker, nerves,
the medulla of the brain and epinephrine (adrenaline)
Animations
EKG Tutorial:
http://library.med.utah.edu/kw/pharm/hyper_
heart1.html
Control of the heart beat
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/
chapter22/animation__conducting_system
_of_the_heart.html
Control of the heartbeat
• Cardiac cycle
– The events which occur in the heart from the
end of one beat until the end of the next beat.
– Assuming a heart rate of 70 beats per minute,
each beat of the heart takes 0.8 s.
Diastole – Heart Muscles Relax
•
The steps of the cycle:
– The cycle might be considered to begin as
the heart muscle relaxes after a contraction.
This relaxation is called _diastole__
•
Both ventricles begin to contract.
•
This contraction is called ___Systole_.
•
.
Heart Sounds
• Heart Sounds
– Described as lub-dub. Each lub-dub
represents one beat of the heart.
• Lub is caused by the _Closure_ of the AV valves
• Dub is caused by the __Closure_____ of the
semi-lunar valves.
Control of the heartbeat
SELF REGULATING (intrinsic or selfcontrol)
– The heart's contraction is MYOGENIC (myo,
muscle; genic, origin), that is, it is able to beat
on its own without any stimulus from outside.
Control of the heartbeat
• Each contraction begins in the sinoatrial (SA)
node in the right atrium.
• Because these cells start the wave of muscle
contraction through the heart, they are called the
pacemaker.
Control of the heartbeat
Origin of the heart beat:
• The pacemaker (and
thus the heartbeat) is
controlled by ınvoluntary
nerve ımpulses that
come from the medulla
part of the braın( the
hind braın).
Control of the heartbeat
Origin of the heart beat:
• Hormones lıke ephinephrine (adrenaline) which
is secreted by adrenalıne gland causes the
pacemaker to ıncrease heart rate.
The steps of the process are:
1. The contraction is coordinated by a
patch of tissue located in the wall of
the right atrium, called the pacemaker
or sinoatrial (SA) node.
The steps of the
process are:
2. Once an impulse is
generated by the
pacemaker, the impulse
(message to contract) is
passed from one
branched myocardial
cell to another,
causing the atria to
contract
Pacemaker causes atria to contract
The steps of the process are:
3. There is a specialized bundle of fibers
which carry the impulse to the
atrioventricular (AV) node.
The steps of the process are:
The atrioventricular (AV) node is located in
the base of the right atrium, very near
the wall between the ventricles (septum).
The steps of the process are:
4) After a short delay,
the atrioventricular
(AV) node transmits
the impulse to a tract
of conducting fibers
called the
atrioventricular (AV)
bundle.
The steps of the process are:
The AV bundle consists
of two branches which
carry the impulse
through the septum to
the conducting
fibers.
• The impulse spreads from the pacemaker (SA node)
to a network of fibers in the atria.
Sinoatrial (SA)
node
Conducting fibers
• The impulse is picked up by a bundle of fibers called
the atrioventricular (AV) node and carried to the
network of fibers in the ventricles.
Conducting fibers
Atrioventricular
(AV) node
5. Conducting fibers are
branches of the AV
bundle which radiate
through the walls of the
ventricles, transmitting
an impulse to the cells of
the ventricles, causing
them to contract
simultaneously.
Controlling the Cardiac Cycle
• The following slides review the control of
the cardiac cycle in more detail.
Control of the Heart Beat More Info:
Step one
• Myogenic muscle contraction describes
the way the heart generates its own
impulse to contract. It does not require
external nerve input.
• In the wall of the right atrium there are a
group of specialised cells(SAN).
• Cells of the Sino-Atrial Node generate an
impulse that can spread across the
muscle cell of both atria (red pathway).
• The impulse causes a contraction of both
atria together.
• The impulse cannot spread to the muscle
cells of the ventricles.
• The impulse is picked up by a sensory
ending called the atrio-ventricular node
(AVN).
Control of the Heart Beat More Info:
Step two
•
•
•
•
•
•
•
•
The atria have already contracted sending
blood down into the ventricles.
The ventricles are stretched and full of blood.
(A) The impulse to contract (generated in the
SAN) is picked up by the AVN .
(B) The impulse to 'contract' travels down the
septum of the heart, insulated from ventricle
muscle fibres
(C) The impulse emerges first at the apex of
the heart. This causes this region to contract
first.
(D) The impulse now emerges higher up
causing this region to contract.
(E). This region contract last.
The effect is to spread the contraction from
the apex upwards, pushing blood towards the
semi-lunar valves.
Modification of Heartbeat
•
•
•
•
•
Modification of myogenic
contraction
The basic myogenic contraction can
be accelerated or slowed by nerve
input form the brain stem or medulla.
There are two nerves:
Decelerator nerve (parasympathetic)
which decreases the rate of
depolarisation at the SAN. Note that
the synapse releases acetyl choline.
Accelerator nerve (sympathetic) which
accelerates the rate of depolarisation
at the SAN. Synapse releases noradrenaline.
Adrenaline and Heartrate
Epinephrine (adrenaline) and
heart rate
•The hormone epinephrine is
produced in the adrenal glands (an
endocrine gland).
•The hormone travels through the
blood to its target tissue, the sinoatrial node(SAN).
•Epinephrine increases the rate of
depolarisation of the SAN.
•This accelerates heart rate.
•This reaction is associated with the
'fight or flight response'.
•
Nervous Control Of the
Heart Beat
– Nerve control (extrinsic control):
• The cardiac center in the medulla oblongata
(located at the base of the brain) controls cardiac
output (both heart rate and stroke volume).
– stroke volume (SV) is the volume of blood
pumped from one ventricle of the heart with
each beat
• Opposing sympathetic (stimulatory) and
parasympathetic (inhibitory) impulses control
the pacemaker.
•
Parasympathetic
"Rest and Digest" responses
Sympathetic
Fight or Flight" responses
Slowing Down the Heart Rate
Vagus nerve is a cranial nerve that is part of the
parasympathetic division of the autonomic
(automatic) system.
» It arises from the medulla oblongata.
» Carries impulses to the heart continuously.
» Secretes a chemical called acetylcholine
on the heart.
» This chemical inhibits the pacemaker,
causing the heart rate to slow and the
contractile strength of the muscle to
weaken.
Speeding up the Heart Rate
• Accelerator nerve is part of the
sympathetic division of the autonomic
nervous system.
• It arises from the medulla oblongata
• The nerve secretes epinephrine (also called
adrenalin).
• Epinephrine stimulates the pacemaker, causing
the heart rate to increase, and contraction strength
to increase.
Regulation of Circulation
• Systems of specific reflexes (automatic
responses) also help regulate circulation.
• Arterial pressure reflex
» An increase in pressure in the major arteries in the
chest and neck (carotid sinus and aorta) stimulates
stretch receptors (also called pressure receptors or
baroreceptors).
» These stretch receptors cause impulses to be sent
along sensory nerves to the medulla oblongata.
» From the medulla oblongata, reflex signals are sent
back to the heart to slow the heart.
Regulation of Circulation
Regulation of Blood Volume
– Blood volume reflex
• When blood volume increases, the volume of
blood in the superior vena cava and in the right
and left atria of the heart also increases.
• This stretches the atria and large veins, exciting
stretch receptors.
• These stretch receptors send messages along
sensory nerves to the medulla oblongata.
• The medulla oblongata sends appropriate
messages to various structures to reduce blood
volume.
Regulation of Blood Volume
Chemical Control
– _Adrenaline___ (epinephrine) produced by the
adrenal glands in response to sympathetic stimulation
will stimulate the pacemaker, thus increasing heart
rate.
– Elevated levels of Epinephirine_and norepiephrine
will increase the heart rate and strength of
contraction.
– Elevated levels of acetylcholine (ACh ) will decrease
heart rate.
Temperature Control
Temperature control: increased
temperature _increase_ the rate of
impulses generated by the SA node while
_Low____ temperature decreases the rate.
(That's why the heart is cooled during heart
surgery to stop the heart beating).
Emotional Control
• Emotional control: Heart rate is controlled
by higher brain centers.
– Emotional fainting results from the higher brain
centers causing increased blood flow to the muscles
and intense vagus nerve stimulation of the heart,
causing the heart to slow markedly.
– Arterial pressure falls instantly, which reduces blood
flow to the brain and causes the person to lose
consciousness.
Electrocardiogram (EKG or
ECG)
• An electrocardiogram (EKG or ECG) is a
measure of the electrical currents from the
heart, measured on the surface of the
body.
EKG
http://library.med.utah.edu/kw/pharm/hyper
_heart1.html)