Higher Human Biology
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 atriaQRS complex = impulses passing through the ventriclesT wave = Electrical recovery of ventricles towards the end of ventricular systole