Download 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