The Suprachiasmatic Nucleus and the Circadian Timing System - Progress in Molecular Biology and Translational Science - page 2

presents an analysis of the functional organization of the SCN as a component of a neu-
ral network, the CTS. This network functions as a coordinator of hypothalamic regulatory
systems imposing a temporal organization of physiological processes and behavioral
state to promote environmental adaptation.
1. INTRODUCTION
“Even when current knowledge emphasizes the importance of a single
program-pacemaker, like the suprachiasmatic nucleus in mammals, it seems
likely its control of the overall program reduces to entrainment of slave oscil-
lations inherent in those systems it times” ( Ref. 1 , p 48).
Circadian timing is a fundamental adaptation of living organisms. 1 In
mammals, a circadian timing system (CTS) regulates the timing of behav-
ioral state coordinated with the timing of physiological and cellular processes
throughout the organism. In this chapter, I focus on the functional organi-
zation of the suprachiasmatic nucleus (SCN) of the hypothalamus, a princi-
pal feature of the CTS. The SCN is a prominent component of the anterior
hypothalamus lying in the ventral periventricular zone, dorsal to the optic
chiasm, lateral to the third ventricle, and medial to the anterior hypothalamic
area. In most mammals, it lies caudal to the medial preoptic area and extends
to the retrochiasmatic area. The SCN was described as a distinct nucleus in
the 1880s and subsequently was recognized in a number of species. Com-
prehensive comparative studies of the hypothalamus by Crosby and
Woodburne 2 found the SCN present in a wide range of metatherian and
eutherian mammalian brains. Despite the anatomical prominence of the
SCN, it was nearly 100 years from its discovery to recognition of a function.
An analysis of the functional organization of the SCN can best be evaluated
in three contexts: (1) the concept of localization of function; (2) the neuronal
organization of the SCN as a mini network; and (3) the SCN as a component
of a functional network, the CTS. Localization of function in the brain was
first presented as a coherent theory by Gall 3 who posited it in three questions:
(1) what are the functions of the nervous system? (2) where are these functions
localized? and (3) how do we understand the relationship? These were reason-
able, even prescient, questions particularly for the time. However, Gall's inter-
est in complex human behaviors influenced him to put forth four additional
premises: (1) the brain is the organ of the mind; (2) the mind is comprised of a
set of definable faculties or functions; (3) the size of brain components medi-
ating these faculties corresponds with the efficiency of each; and (4) the devel-
opment of these brain areas is reflected in the size and shape of the overlying