FoundationalConceptsNeuroscience

research have revealed that the PER gene is transcribed and translated
into protein, after which the protein enters the cell nucleus and inter-
acts with transcription factors to suppress transcription of the PER
gene. The resulting reduced production of the associated PER protein
then leads to reduced suppression of PER gene transcription, and the
cycle begins anew. This process of feedback inhibition of the tran-
scription of the PER gene by its own gene product produces a rhythm
of PER gene expression that then propagates out to other aspects of
cell physiology.
Other genes in Drosophila have also been identified in this oscilla-
tory time-keeping cycle: TIM (timeless), CLOCK, and CYCLE, to name a
few. And in the human SCN, analogous genes and gene products have
been discovered that appear to generate and maintain the oscillations
of the circadian clock in the human body. Through the neural cir-
cuitry of the brain, the oscillations of the SCN exert regulatory actions
on many other body processes. And the SCN is also the place where
the overall circadian period of the biological clock is brought into syn-
chrony with the environmental light-dark cycle of day and night.
The retina, via the optic nerve, makes most of its connections with
the thalamic lateral geniculate nucleus and thence the visual cortex
(V1) of the occipital lobe, and with the superior colliculus in the mid-
brain (see Chapter 14). Of the million or so axons in each optic nerve,
close to 90 percent go to the lateral geniculate nucleus, and close
to 10 percent go to the midbrain. However, about 1 percent of the
optic nerve axons, perhaps ten thousand or so nerve fibers from each
eye, emerge from a distinct population of retinal ganglion cells and
connect with the SCN. These particular ganglion cells are intrinsically
photosensitive; that is, rather than activation via input originating
from rods and cones, they contain their own (intrinsic) rhodopsin-like
photoreceptor protein, called melanopsin (see Chapter 14). The axon