the sinoatrial node is the part of the heart that normally controls the heart rate.
the sinoatrial node is the part of the heart that normally controls the heart rate.
which part of the heart normally controls the heart rate?
the sinoatrial node
the basal or resting heart rate (hr) is defined as the heart rate when a person is awake, in a neutrally temperate environment, and has not been subject to any recent exertion or stimulation, such as stress or surprise. a large body of evidence indicates that the normal range is 60-100 beats per minute. this resting heart rate is often correlated with mortality. for example, all-cause mortality is increased by 1.22 (hazard ratio) when heart rate exceeds 90 beats per minute. the mortality rate of patients with myocardial infarction increased from 15% to 41% if their admission heart rate was greater than 90 beats per minute. ecg of 46,129 individuals with low risk for cardiovascular disease revealed that 96% had resting heart rates ranging from 48-98 beats per minute. finally, expert consensus reveals that 98% of cardiologists believe that the "60 to 100" range is too high, with a vast majority of them agreeing that 50 to 90 beats per minute is more appropriate. the normal resting heart rate is based on the at-rest firing rate of the heart's sinoatrial node, where the faster pacemaker cells driving the self-generated rhythmic firing and responsible for the heart's autorhythmicity are located. for endurance athletes at the elite level, it is not unusual to have a resting heart rate between 33 and 50 bpm.
heart rate
loewi performed a very simple yet elegant experiment. using an isolated frog heart he had previously found that stimulation of the vagus nerve resulted in a slowing of the heart rate, while stimulation of the sympathetic nerve caused the heart rate to speed up (figure 1). he reasoned that stimulation of either the vagus or sympathetic nerve would cause the nerve terminal to release a substance which would either slow or accelerate the heart rate. to prove this, he took a frog heart, which had been cannulated in order to perfuse the fluid surrounding the heart, and electrically stimulated the vagus nerve until the heart rate slowed. he then collected the fluid surrounding the heart and added it to a second frog heart which had been stripped of its vagal and sympathetic nerves. by adding the fluid surrounding the first heart to the second heart, he caused the heart rate of the second heart to slow down. this proved that stimulation of the vagus nerve caused the release of a substance which acted upon the heart tissue and directly caused the heart rate to slow down. (figure 2) this substance was called "vagusstoff". "vagustoff" was later confirmed to be acetylcholine and was found to be the principal neurotransmitter in the parasympathetic nervous system.
vagusstoff