Cardiac Conducting System
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Transcript Cardiac Conducting System
Higher Human Biology
CARDIAC CONDUCTING SYSTEM
Cardiac Conducting System
The heart beat originates in the heart itself.
Heart muscle cells are self contractile
They are able to contract and produce an
electrochemical signal which in turn causes
other cells to contract
This ensures that the heart beats in a
coordinated manner.
This sequence of events is brought about by
the Pacemaker (SAN) and the conducting
system.
Pacemaker
Also known as Sino-atrial node (SAN)
Located in the wall of the right atrium
Small region of specialised tissue
Autorhythmic cells
Exhibits spontaneous excitation
The pacemaker initiates electrical impulses
that make cardiac muscle cells contract at a
certain rate.
Pacemaker cont.
Works automatically
Would continue to function even in the
absence of nerve connections to the rest of
the body
Refer to fig.12.4 (page 167)
Electrical impulses originating in the SAN
spread through muscles cells in the atrial
walls making them contract simultaneously
Atrial systole
This impulse is then picked up by the Atrio-
ventricular node (AVN)
This is located centrally near the base of the
atria
Connecting Fibres
The impulse passes from the AVN into a
bundle of connecting fibres
These divide into left and right branches
They then divide into a dense network of
conducting fibres in the ventricular walls
Stimulating of these fibres causes ventricular
systole
This starts from the heart apex and works
upwards
This coordination of heartbeat means that;
many muscle cells contract together in systole
Ventricular systole happen slightly later than atrial
systole to allow ventricles to completely fill with
blood before contracting
Regulation
The pacemaker initiates each heartbeat BUT
Heart rate is not fixed
It is altered by:
Nervous activity
Hormonal activity
Autonomic Nervous Control
Heart is supplied with branches of the
Autonomic Nervous System
An area in the medulla of the brain regulates
heart rate
Medulla
Cardio-accelerator centre (increase heart
rate)
Impulse carried by sympathetic nerve
Releases neurotransmitter – norepinephrine
(noradrenaline)
Cardio-inhibitor centre (decrease heart rate)
Impulse carried by parasympathetic nerve
Release neurotransmitter - acetylcholine
Hormonal Control
During exercise or at times of stress
Adrenal glands release the hormone
epinephrine (adrenaline) into bloodstream
This makes the SAN generate impulses at a
higher rate
Increase in heart rate
ECGs
Electrical activity of the heart
Picked up by electrodes on the skin
Amplified
Displayed on oscilloscope screen
Electrocardiogram (ECG)
P wave = electrical impulses spreading from
SAN over the atria
QRS complex = impulses passing through the
ventricles
T wave = Electrical recovery of ventricles
towards the end of ventricular systole