Cardiac Conducting System

1. Higher Human Biology Cardiac Conducting System

2. 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.

3. 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.

4. Pacemaker cont. • Works automatically • Would continue to function even in the absence of nerve connections to the rest of the body

5. 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

6. This impulse is then picked up by the Atrio-ventricular node (AVN) • This is located centrally near the base of the atria

7. 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

8. 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

9. Regulation • The pacemaker initiates each heartbeat BUT • Heart rate is not fixed • It is altered by: • Nervous activity • Hormonal activity

10. Autonomic Nervous Control • Heart is supplied with branches of the Autonomic Nervous System • An area in the medulla of the brain regulates heart rate

11. 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

12. 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

13. ECGs • Electrical activity of the heart • Picked up by electrodes on the skin • Amplified • Displayed on oscilloscope screen • Electrocardiogram (ECG)

14. 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