Know anything about the heart.....?

Question

When the heart is externally stimulated just after the start of the contraction cycle, why does this stimulation have no affect on heart rate?

Answer 1

the heart electrical system of the heart has a set level of beats base don which aspect the contract/electrical impulse begins at the SA, AV and perkije fiber are the main components. These are preset to beat at specific levels as a fallback mechanism and at times pacemakers must be inserted to send the electrical impulse itself. When the heart is contracting there is an extreme amount of cellular level activity going on based on that electrical impulse that occured. The potassium and sodium changes that occur result in a massive depolarization or the contraction phase. Following these phases there is a refractory period in which the potassium go back to the correct side of the cell membrane. The impulse during this refractory period would have little if any effect on the heart because all potassium ions are being transported back acrossed the membrane not allowing another change from say an external force. The heart rhythms you see when you look at a PQRST complex is basically the polarization and depolarization of the atriums and ventricles of the heart. If you want to know the exact portions that relate to the ion movement for cardiac rhythms email me. Hope that helps

Answer 2

The valves are essentially passive.  They allow the blood to move in one direction and return closed position in response to changes of pressure in the heart.

The external stimulation would not affect them directly.