The Suprachiasmatic Nucleus and the Circadian Timing System - Progress in Molecular Biology and Translational Science - page 21

SCN

Retina

Brain
oscillators

Peripheral
oscillators

Behavioral state
Physiological
processes

Figure 1.7 Schematic representation of a mammalian circadian timing system (CTS).

Retinal input to the SCN provides a primary entraining stimulus. Secondary entraining
stimuli are contributed by brain oscillators, peripheral oscillators, physiological pro-
cesses of the internal milieu, and behavioral state. Specific behaviors such as activity
also contribute.

of data obtained. In Section 1 , I described the historical localization of func-
tion formulation for understanding brain function. Early studies relied on
lesion effects, and it took many years for the full realization to emerge that
localized destruction of the brain, either experimental or with disease or
injury, informed the investigator about what the brain could do in the
absence of the part but not necessarily what the part did. Neurophysiology
was often more informative but was limited in that recordings were most
often made from anesthetized animals with evident limitations. Invasive
studies are not possible with the potentially most informative species, Homo
sapiens . In circadian neurobiology, a predominant paradigm has been use of
the brain slice preparation, either acutely or in organotypic culture. These
in vitro preparations can provide much insight into molecular and cellular
events and intrinsic organization, but they have limitations, 124 and we need
to investigate the SCN as a component of the CTS, and the CTS as a com-
ponent of regulatory systems in the brain in vivo . Perhaps, a model for this
can be derived from recent developments in cognitive neuroscience. The
recent evolution of functional imaging, particularly functional magnetic