Biomedical Engineering Reference
In-Depth Information
Figure 4
. Center of pressure displacements (top graphs) and their corresponding log-
transformed power spectra (bottom graphs) for a 30-year-old healthy woman (left) and a
69-year-old woman with previous falls (right), obtained while the subjects stood on a force-
plate for 30 seconds. The slope of the regression line through the power spectra represents
the scaling exponent C, which is greater in the elderly faller than in the young subject,
indicating a loss of complexity. Reprinted with permission from (2).
4.
MECHANISMS OF PHYSIOLOGIC COMPLEXITY
A variety of mechanisms probably underlie the complexity of physiologic
systems, including neuronal networks in the nervous system, biochemical path-
ways in metabolic control systems, signaling pathways within and between cells,
genetic switches, and transcription control elements. Two experiments highlight
the importance of the autonomic nervous system in generating the complexity of
heart rate dynamics. As shown in Figure 5, when autonomic nervous system
influences on the heart are eliminated through the administration of the mus-
carinic receptor blocker atropine and the beta-receptor blocker propranolol, the
complex dynamics observed under control conditions are lost.
In another study, baby pigs were shown to develop increasing heart rate
complexity as they matured from 8 to 33 days after birth (17) (Figure 6). During
this period of time, the heart becomes innervated by sympathetic nerves from
the right stellate ganglion. When the right stellate ganglion is denervated at
birth, heart rate complexity does not develop. Thus, during healthy development,
complexity appears to emerge in physiologic systems such as heart rate, and
with senescence system complexity is lost.
