control of heart rate

• Cardiac muscle is myogenic - does not need stimulus to contract. The muscle cells are coordinated so they don’t contract out of sequence. 
• If the coordination of the excitation of the heart muscle goes wrong, the heart contracts uncontrollably and enters fibrillation - which can be fatal. Defibs shock the heart and stop it allowing the SAN to restart the cycle. 
• Coordination action:

  • Sinoatrial node initiates the wave of contraction
  • A wave of activity spreads over the atria 
  • The tissue between the atria and ventricles does not conduct the impulse
  • The impulse passes through the atrio-ventricular node in the septum
  • There is a pause in the transmission of the impulse to make sure the ventricles fill with blood before contraction. 
  • The impulse passes to the purkyne tissue in the bundle of his - transmitting the impulse to the base of the ventricles. 
  • It then spreads up the ventricle walls. 
• Heart rate is controlled by the medulla oblongata in the brain. It has two centres:

  1. Increases the heart rate - sympathetic 
  2. Decreases the heart rate - parasympathetic 
• Chemoreceptors are found in the walls of the carotid artery, they monitor pH which can indicate the levels of CO2 and lactate in blood. Baroreceptors are also found which detect changes in blood pressure. 
• The medulla oblongata is stimulated by chemical and pressure changes in the blood. 
• Responses to CO2 and pressure:

  Increase in CO2 and a decrease in blood pressure are detected by receptors in the aorta and carotid walls. This stimulates the acceleratory centre in the medulla oblongata and sends impulses through sympathetic nerves. The SAN is stimulated and the heart rate increases. 

   
• Responses to CO2 and pressure:

  Decrease in  CO2 and increase in blood pressure is detected by receptors, which are not stimulated as much - the inhibitory centre in the medulla sends impulses through parasympathetic nerves. The SAN is stimulated and heart rate decreases.