A-level Biology - Heart rate control

Hi. Once receptors detect, e.g. higher acidity of blood indicating high levels of CO2, they send impulses along the sensory neurones to medulla. Then the medulla sends impulses along the parasympathetic neurone to the SAN. The parasympathetic neurone can secrete acetycholine which can bind to receptors on the SAN to decrease the frequency of impulses sent to the AVN. I understand this, but I am still confused about the frequency of impulses along the parasympathetic neurone. How does this change? Does it? Surely higher freq. of impulses would cause the heart to contract more? But also secrete more acetylcholine? I know when increasing heart rate, the medulla sends impulses to the SAN at a higher frequency, but how is it different when decreasing heart rate? Thanks.

Hi, I think you might have gotten some of the details mixed up.

When you have a high levels of CO2, impulses are sent via sensory neurones and then along the medulla, along the sympathetic nervous system. noradrenaline is secreted, which binds to SAN receptors to increase the heart rate.

When you decrease the heart rate, impulses are sent via sensory neurones and the medulla, via the parasympathetic nervous system. Acetylchloline is secreted which binds to SAN and decreases the frequency of impulses.

Hope this helps

Hi thanks for your response, that does clear some parts up, however how does the frequency if impulses change? In a markscheme about increasing heart rate, it says that there is a 'higher frequency of impulses sent to the SAN', but the equivalent isn't stated in the markscheme for decreasing heart rate. For increasing heart rate it says: send impulses to medulla (1) more impulses to SAN (1) by sympathetic neurone. But for decreasing, it says: (1) send impulses to medulla, impulses via parasympathetic neurone (1) to SAN (1), release of ACh inhibits SAN (1), decreases impulses to AVN (1). It doesn't mention frequency, which I am still not sure about.