Biomedical Engineering Reference
In-Depth Information
person is a carrier of some 40 billion fat cells all in pursuit of a collective (hips,
thighs) or a community (organs, abdomen) to inhabit. These too are self-
sufficient part-whole cellular entities. The total power consumption of an adult
human is that of a 100-watt light bulb. Each individual cell is an intricate self-
contained chemical computer that can perform over 10 million chemical reac-
tions per second. Correspondingly, the cerebral cortex of the human brain con-
tains nearly 20 billion neurons, each with over 2,000 synapses, ready to
communicate and exchange signals with each other and the rest of the body. The
power consumption of this subsystem is surprisingly high (~33% of body total),
i.e., 1000 times greater energy utilization than other cell types. (Our mothers
were quite insightful, insisting that we cover our heads on a cold day.) The
unlikely comparison of neurons to their electronic equivalent translates into the
sum total of transistors comprising 500 Pentium-4 microprocessors. The corre-
sponding processing power of the brain is estimated to be 50 terabits per second
(compared to ~25 gigabits per second for a Pentium-4). For the brain the emer-
gent complex systems properties manifest attributes such as consciences, mem-
ory, and ability to learn. These system-derived properties cannot be understood
by studying the neurons or their topological distribution.
Clearly, organismic processes are deliberately ordered to maintain and pre-
serve the integrity of the system. In contrast, the physicochemical processes oc-
curring in an organism that has been impaired (by disease, pathologic condition)
still follow the conventional laws of physics but differ profoundly in terms of
principles of relational organization and order from the identifiably normal
(healthy) system. Molecular biology is not going to give us all the information
we need. The information about the whole (collective behavior) is larger than
the sum of the information about the parts, i.e., the missing link in the pervasive
reductionism practiced today. What is especially needed is a coherent picture of
how this information is being used to carry out biological functions.
4.1. Distributed and Shared Regulation
The classical concept of cardiac neural regulation presumes that the neural
efferent signals originate from extracardiac centers and, in particular, from the
central nervous system (CNS). A byproduct of this supposition is that the
cardiac afferent information is considered relevant and meaningful only if it is
transmitted directly to the cardiovascular regulatory centers residing within the
CNS. In this view, information processing is delegated exclusively to the CNS,
whereas the intracardiac ganglia are assigned the passive role of a relay station.
This limiting perspective of cardiac neural control is no longer tenable,
particularly because it does not make allowances for the existence of the intrin-
sic components of neural regulation. In studies of patients undergoing heart
