Posted on 30 July 2021
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When a the heart is removed from the body, it continues to beat without any electrical input from the brain. The rate at which the heart beats without receiving any electrical signals is called the heart’s intrinsic rhythm. This is the result of the heart’s own natural pacemaker called the sinoatrial node, which contains cells that spontaneously produce electrical signals in a regular manner. The heart rate of a healthy person who is not exercising will actually be lower than the heart’s intrinsic rhythm. This is because the brain constantly sends inhibitory electrical signals (parasympathetic inhibition) to slow the heart down, only removing these signals when the heart needs to beat faster.
During ageing, the heart’s intrinsic rhythm declines by an average of 0.8 beats per minute per year, from around 130 bpm at birth to around 70 bpm at age 80. However, inhibitory (parasympathetic) signals from the brain also decrease throughout adulthood while stimulatory (sympathetic) signals increase, and so resting heart rate does not change significantly.
The regular electrical signals produced by cells in the sinoatrial node are the result of the opening and closing of a range of protein channels located in their cellular membranes. The slowing with age of the heart’s intrinsic rhythm is due in part to changes in the expression and function of these protein channels, and may also be partly caused by structural changes such as the growth of non-conductive fibrous tissue. However, we still don’t have a detailed understanding of why the sinoatrial node slows down with age.
Slowing down as we age: aging of the cardiac pacemaker’s neural control: https://doi.org/10.1007/s11357-021-00420-3
Cardiac Pacemaker Activity and Aging: https://dx.doi.org/10.1146%2Fannurev-physiol-021119-034453
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