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 atria QRS complex = impulses passing through the ventricles T wave = Electrical recovery of ventricles towards the end of ventricular systole