A2-LEVEL AQA BIOLOGY NOTES 
TOPIC 3: control of heart rate

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Cardiac (heart) muscle is myogenic. This means it can initiate its own contractions. It achieves this with its own pacemaker: the sinoatrial node (SAN):

• SAN initiates action potential independently of cranial input
  
• Sends a wave of excitation over the atrial walls and through the AVN
  
• Bundle of His contains Purkyne fibres which conduct excitation wave from AVN to purkinje fibres
  
• Conducted down the Purkinje fibres to the ventricles
  
• Causes ventricles to contract
  

Supplied with nerves from the medulla oblongata

• Found at the base of the brain
  
• Region of the brain that co-ordinates the unconscious functions of the body
  
• Nerves connect to the SAN
  
• Nerves can affect the frequency of the contractions
  

Heart muscle responds to the presence of the hormone adrenaline

• Beats faster
  
• Beats stronger: myocytes contract with greater contractile force
  

Nodes

• The heart has 2 main nodes that contain nerve cells
  
  • Sinoatrial node
    
  • Atrioventricular node
    

Controlling Heart Beat

• SAN sends an electrical impulse over the atrial walls
  
• Causes atria to contract
  
• Conducted to AVN, where the electrical wave of excitation is conducted down the Purkinje fibres
  
• Causes the ventricles to contract
  
• Layer of insulation exists between the atria and ventricles to prevent the first wave of impulse from the SAN from travelling all the way down
  
• This layer causes a pause at the AVN
  

Rate of Initiation

• Controlled by:
  
  • Nerves that run from the brain
    
  • Hormones in the blood
    
    • E.g. – Adrenaline
      

Nerves

• Vagus Nerve
  
  • Sends signals to decrease heart rate
    
• Acceleratory Nerve
  
  • Sends signals to increase heart rate
    
• Both nerves connect to the medulla oblongata in the brain
  

Medulla Oblongata

• Involved in many unconscious functions of the brain including breathing and heart rate
  
• Specific regions of the medulla oblongata sense factors that require a change in heart rate
  
• Cardiovascular centre can sense things like changes in carbon dioxide levels
  

Interaction between control mechanisms

• Resting conditions
  
  • Heart rate controlled by SAN
    
  • Set frequency
    
  • Typically 60-80 beats per minute
    
  • Frequency of excitation waves can be controlled by the cardiovascular centre in the medulla oblongata
    

Factors affecting heart rate

• Movement
  
  • Limb movement is detected by stretch receptors in muscles
    
  • Impulses sent to the cardiovascular centre
    
  • Informs that oxygen is needed
    
  • Increases heart rate
    

• Exercise
  
  • Muscles produce more carbon dioxide
    
  • Carbon dioxide reacts with water in blood plasma, lowering the pH
    
  • Drop in pH detected by chemoreceptors
    
  • Chemoreceptors send impulses to the cardiovascular centre
    
  • APs fired from chemoreceptors are passed to cardiovascular centre of the medulla oblongata
    
  • Signals sent down the accelerator nerve to increase heart rate
    
  • Heart rate increases
    
  • When we stop exercising concentration of carbon dioxide falls
    
  • Reduces activity of the accelerator pathway
    
  • Heart rate declines
    

• Adrenaline
  
  • Secreted in response to stress, shock or excitement
    
  • Presence increases heart rate
    
  • Adrenaline binds to specific receptors on the membranes of cells in the SAN
    
  • Helps to prepare body for activity
    

• Blood Pressure
  
  • Monitored by stretch/pressure receptors in the walls of the carotid sinus
    
  • If blood pressure rises too high, signals are sent to the cardiovascular centre
    
  • Signals sent through the vagus nerve to decrease the heart rate
    
  • Heart rate is reduced