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Explain electrical activity within the heart that co-ordinates the cardiac cycle

Answers
Electric Activity in the Heart The sinoatrial node (also commonly spelled sinuatrial node) is the impulse-generating (pacemaker) tissue located in the right atrium of the heart: i.e., generator of normal sinus rhythm.The sinoatrial node (also commonly spelled sinuatrial node, abbreviated SA node) is the impulse-generating (pacemaker) tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm.These cells form the atrioventricular node (AV node), which is an area between the atria and ventricles, within the atrial septum.If the AV node also fails, Purkinje fibers are capable of acting as the pacemaker.The reason Purkinje cells do not normally control the heart rate is that they generate action potentials at a lower frequency than the AV or SA nodes.
njoroge
08 April 2016
The heart is an autonomic muscle, this means that it doesn't need innervation (a nerve from the brain) to tell it when to beat - it can beat on its own (which is why a heart transplant can happen). In order for the heart to beat, it needs to have two qualities: automatic (no need for the nerve from the brain) & rhythmic (like in the first aid course, they tell you to do CPR at the song 'staying alive'). All the cells in heart are 'automatic' however there are bundles of cells in specific areas that stand out and kind of instruct the rest:  SAN cells - they spontaneously depolarise & they have the highest rhythmic rate - makes them the pacemaker in the normal heart – 72/minAVN cells - they have the second highest rhythm (40-60 beats/min)(these cells would be pacemakers if the SAN doesn't work) Bundle of His + Purkinje system — 25-.‐30/min — even if they aren’t specialised to be automatic
Ioana C.
12 April 2016
The answers so far have focused on the different areas in the heart that can initiate electrical activity. While this is certainly correct I want to shed some light on the journey of electrical activity through the heart during the heart beat.The sino atrial node, located in the right atrium, is the primary centre of electrical activity. As mentioned in the other answers, the atrio-ventricular node and the purkinje fibres can also produce this electrical activity however because they discharge at a slower rate they do not normally provide this function, nor is it their primary function. The electrical activity from the sino-atrial node spreads throughout the atria and causes them to contract. When the electrical activity reaches the atrio-ventricular node it is slowed down- an important function of the AVN that allows the atria to fully contract before the ventricles contract. The impulse then leaves the AVN and travels down the purkinje fibres to the bottom of the heart. The purkinje fibres are non-contractile and therefore this allows the ventricles to contract upwards, from the base, pushing blood towards the exit of the ventricles. 
cwinfield
15 April 2016
The electrical activity in the heart is coordinated by the sinoatrial node located in the right atrium. This then spreads through the myogenic cells of the atria causing it the contract and squeeze the blood through into the ventricles.The electrical activity then reaches the atrioventricular node where it slows down. This gives the atria time to completely contract and empty before the activity spreads down the bundle of His (in the septum) and up either side of the ventricles via the purkinje fibres. This allows the ventricles to contract from the bottom upwards in order to push blood into the aorta and pulmonary vessels, completing the cycle.
Isabelle H.
24 April 2016
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