The Suprachiasmatic Nucleus and the Circadian Timing System - Progress in Molecular Biology and Translational Science - page 22

resonance imaging, has required that localization of function be viewed on a
network, or system, level. 125,126 Advances in imaging technology are bring-
ing the resolution of functional imaging to a level that would permit study-
ing the human CTS (Refs. 127,128 ) , so we have much to look forward to.
6. REPRISE
In the early years after discovery of SCN involvement in circadian reg-
ulation, the simplest hypothesis was to assign it a role as “master pacemaker.”
After the unraveling of the molecular basis of circadian function, however, it
became evident rapidly that the molecular machinery of circadian function is
widely distributed among living organisms and in cells and tissues through-
out mammalians (cf. Ref. 129 ) . Further, the entrainment pathway was found
to have specialized photoreceptor neurons that are the beginning of the
RHT. These advances have required revision of ideas about the neurobiol-
ogy of circadian timing. Gone is the “master pacemaker.” In its place is a
network of brain structures and peripheral tissues with the SCN, a special-
ized brain oscillator, sitting at the interface between the circadian photic
world and the brain to transduce light information into neural information
( Fig. 1.7 ) . This process is clearly complex, and “understanding” the SCN
will be more difficult than we imagine. The problems will be centered
not only at the molecular level, or even the cellular level, but will involve
explanations at the network and systems levels. As the scientific focus of cir-
cadian neurobiology moves increasingly to cellular and molecular studies,
how will elegant explanations of the parts lead to a comprehension of
CTS? This is an old problem in biology, discussed often but less often
resolved (cf. Ref. 130 , for review). It was stated well by the great systems
biologist Ernst Mayr in a paper on emergent properties in biology, “When
two entities are combined at a higher level of integration, not all the prop-
erties of the new entity are necessarily a logical or predictable consequence
of the properties of the components.” ( Ref. 131 , p 1505). This has been
noted as a problem in the study of circadian function, 132 and one we should
remember as circadian neurobiology moves on.
REFERENCES
1. Pittendrigh CS. Temporal organization: reflections of a Darwinian clock-watcher.
Annu Rev Physiol . 1993;55:17-54.
2. Crosby EC, Woodburne RT. The comparative anatomy of the preoptic area and the
hypothalamus. In: Fulton JF, ed. The Hypothalamus . Baltimore: Williams and Wilkins;
1939:52-169.