T/F: ACh has a negative chronotropic effect on the cardiac rate by hyperpolarizing the SA node and thus..?
decreasing the rate of its spontaneous firing.
- Anonymous1 decade agoFavorite Answer
Not sure what your question is - you have just made a statement? However if you want the statement explained, the SA node of the heart is called that because of it's location: it is in the wall of the right atrium near the coronary sinus (venous entry point for blood from the heart's own circulation) - hence sinoatrial (SA) node. It's better known as the heart pacemaker, as the heart cells in this area leak ions. After a contraction the cells are returned to a charged (polarised) state by the distribution of ions across the membrane, and then the ions will slowly leak through, reducing the charge (level of polarisation) until a firing point (threshold) is reached, and will spontaneously trigger a contraction. This occurs faster in the SA node or pacemaker than anywhere else in the heart, hence it's ability to contract first, pass the stimulus on to surrounding cells and act as a pacemaker (contraction starter) for the heart. Nerves going to the pacemaker cells release chemicals onto them, which will affect the permeability of the cell membranes to different ions so as to speed up or slow the rate of ion leakage. Noradrenaline (norepinephrine in the US) will increase permeability to ions, they leak faster and trigger a contraction faster - the heart speeds up. Acetyl Choline (ACh) on the other hand is secreted onto the heart pacemaker cells and causes ions to leak in the wrong direction - creating more of a charge over the cell membrane than usual (hyperpolarisation) and so it will take longer for the usual leakage to reach the firing point, and the heart rate is slowed. In this ways the nervous system can speed up or slow down the heart rate.