|MadSci Network: Anatomy|
You have asked a very interesting and puzzling question. The reason why heart rate is decreased in older, as compared to young, humans and animals is probably related to changes in the brain regions that control the heart, rather than to changes in the heart itself. The brain controls heart rate via nerves innervating the heart “pacemaker” such asthe vagus nerve. The nerve cells for this nerve are located in medulla of the brain, in a cluster called the motor nucleus of the vagus. These nerve cells in turn are affected by nearby neurons in the nucleus of the tractus solitarius (NTS) and in the hypothalamus. Both of these brain regions show damage and altered function in older rats and humans,for reasons that are not precisely known (see a paper by H. Itoh and RD Bunag in Experimental Gerontology vol. 27, p. 309, 1992)(You can look this up at a Web sitecalled Pubmed that allows you to search medical articles in the National Library of Medicine in the USA). I am studying cells in the NTS and hypothalamusthat destroy their own mitochondria as they get older, for unknown reasons (see my paper in a journal called Glia, vol. 16, p. 218, 1996).
Other influences upon heart rate include hormones like thyroid hormone. Thyroid hormone also decreases with age, once again, probably due to changes in the hypothalamus (See an article by Y. Touitou in a journal called Hormone Research, vol.43, p. 12, 1995). These things probably cause an age- related slowing of the heart, but the exact reasons why these things come about are still mysterious.
Try the links in the MadSci Library for more information on Anatomy.