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