The Suprachiasmatic Nucleus and the Circadian Timing System - Progress in Molecular Biology and Translational Science - page 3

skull. The latter was his scientific downfall as he applied it to the development
of phrenology that was then used to make predictions of individual cognitive
abilities based on palpation of the skull. This was quite popular during the
nineteenth century but became, as characterized by the famous British neu-
rologist McDonald Critchley, “a theory of brain function that began as a her-
esy and ended as a superstition.” 4
Nonetheless, Critchley and others noted that the clarity of Gall's formu-
lation of localization of function provided a framework for investigating
functional localization clinically and experimentally. Stated in a more mod-
ern form, it is “the doctrine that various parts of the brain have relatively
distinct mental, behavioral and/or physiological functions” ( Ref. 5 , p 10).
To discuss this further, it is important to review brain organization briefly.
The adult brain is made up of individual neurons that are specialized dur-
ing development for their function in brain areas in which they are generated.
Brain neurons are grouped in two fundamental patterns, laminated groups
(cortices) and nonlaminated groups (nuclei and fields). Nuclei may be made
up of a homogeneous group of neurons or have divisions. The designation of a
neuronal group as a nucleus, or a division of a nucleus, was originally based on
appearance in gross anatomical preparations or in stained tissue sections to
determine cytoarchitecture. More recently, the precision of such designations
has been improved by the addition of other information from analysis of
chemoarchitecture, patterns of gene expression, connections, and physiology.
We also recognize that the definition of functionmust be made in the con-
text of levels of organization. For the nervous system, these are generally con-
sideredtobeasshownin Table 1.1 . Reductionism is a dominant theme in
modern biology, and much of the work in circadian neurobiology in the pre-
sent era is at the cellular and molecular levels. Nevertheless, this chapter is
directed to the structural level with functional correlations as they are under-
stood and begins with a general description of brain organization. The brain
has five subdivisions from rostral to caudal: telencephalon, diencephalon, mes-
encephalon, metencephalon, and myelencephalon (cf. Ref. 7 , forreview).
The hypothalamus is the ventral longitudinal cell group of the diencephalon.
The hypothalamus began to be recognized as a distinct subdivision of the
diencephalon in the mid-nineteenth century. The fundamental organization
of the diencephalon as comprising four longitudinal cell columns was
established early in the twentieth century. 8,9 The hypothalamus has three
rostrocaudal subdivisions: the anterior, or chiasmal, hypothalamus lying pre-
dominantly above the optic chiasm; the tuberal hypothalamus situated above
the pituitary stalk; and, behind this, the posterior hypothalamus including the
mammillary bodies. The structure and functions of the hypothalamus have