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More About Human SCN Anatomy
Even in the absence of external time cues, humans maintain
a sleep-wake rhythm very close to 24 hours. Typically an organism's circadian
system is made up of components that receive environmental input, that
generate the 24-hour rhythm, and that mediate rhythmic output to all the
tissues of the body. In mammals, the controlling clock component that
generates a 24-hour rhythm is the suprachiasmatic nucleus (SCN), located
in a part of the brain called the hypothalamus. The SCN produces a signal
that can keep the rest of the body on an approximately 24-hour schedule.
However, because the internal clock's period is not exactly 24 hours,
environmental cues—most importantly, light—are required to reset
the clock each morning and keep the organism in sync with the external
world. Watch this animation to see how sunlight is turned into a signal
that can reset neurons in the SCN.
Light enters the eye and activates neurons in the retina that convert
photons (light particles) to electrical signals. The retinal neurons transmit
the electrical signals from the retina via long axons in the optic nerve.
Along the way is the optic chiasm, where the optic nerves from the left
and right eye meet and cross. At the optic chiasm, visual information
continues toward the back of the brain, where it is processed into images
that we can consciously perceive. The neurons carrying information to
the SCN, however, take a different path. They exit the optic chiasm and
turn upward, toward the SCN (suprachiasmatic means "above the chiasm").
The SCN is a small, paired, wing-shaped structure in the hypothalamus,
located at the base of the brain. The animation shows the isolated left
SCN, optic nerve, and eye, while the right SCN is shown embedded within
the hypothalamus in the brain. Within each side of the SCN is a network
of up to several thousand neurons. Experiments with individual isolated
SCN neurons suggest that each SCN cell is a functional clock, normally
synchronized with the activity of its neighbors.
Inside a single SCN neuron, the protein product of a biological clock
gene turns off production of more protein, forming a negative feedback
loop. Go to the animation entitled
"The Mammalian Molecular Model" to see how these molecular oscillations
result in circadian rhythms.
Even in the absence of external time cues, humans maintain a sleep-wake
rhythm very close to 24 hours. Typically an organism's circadian system
is made up of components that receive environmental input, that generate
the 24-hour rhythm, and that mediate rhythmic output to all the tissues
of the body. In mammals, the controlling clock component that generates
a 24-hour rhythm is the suprachiasmatic nucleus (SCN), located in a part
of the brain called the hypothalamus. The SCN produces a signal that can
keep the rest of the body on an approximately 24-hour schedule. However,
because the internal clock's period is not exactly 24 hours, environmental
cues—most importantly, light—are required to reset the clock
each morning and keep the organism in sync with the external world. Watch
this animation to see how sunlight is turned into a signal that can reset
neurons in the SCN.
Light enters the eye and activates neurons in the retina that convert
photons (light particles) to electrical signals. The retinal neurons transmit
the electrical signals from the retina via long axons in the optic nerve.
Along the way is the optic chiasm, where the optic nerves from the left
and right eye meet and cross. At the optic chiasm, visual information
continues toward the back of the brain, where it is processed into images
that we can consciously perceive. The neurons carrying information to
the SCN, however, take a different path. They exit the optic chiasm and
turn upward, toward the SCN (suprachiasmatic means "above the chiasm").
The SCN is a small, paired, wing-shaped structure in the hypothalamus,
located at the base of the brain. The animation shows the isolated left
SCN, optic nerve, and eye, while the right SCN is shown embedded within
the hypothalamus in the brain. Within each side of the SCN is a network
of up to several thousand neurons. Experiments with individual isolated
SCN neurons suggest that each SCN cell is a functional clock, normally
synchronized with the activity of its neighbors.
Inside a single SCN neuron, the protein product of a biological clock
gene turns off production of more protein, forming a negative feedback
loop. Go to the animation entitled "The
Mammalian Molecular Model" to see how these molecular oscillations
result in circadian rhythms.
Human SCN Anatomy Background
Living organisms have evolved internal timekeeping mechanisms to synchronize
behavior and physiology with the cycles of day and night. These biological
clocks have been found in organisms as diverse as fungi, fruit flies,
hamsters, and humans. The biological clock of humans is found deep within
the brain. This animation takes the viewer on a three-dimensional tour
following the path of light input to the suprachiasmatic nucleus (SCN),
a collection of neurons that regulates our circadian rhythms.
This animation was designed in conjunction with HHMI's 2000 Holiday Lectures
on Science series Clockwork Genes: Discoveries in Biological Time.
Human SCN Anatomy Teaching Tips
The animations in this section have a wide variety of classroom applications.
Use the tips below to get started but look for more specific teaching
tips in the near future. Please tell us how you are using the animations
in your classroom by sending e-mail to biointeractive@hhmi.org.
-
Use the animations to make abstract scientific ideas visible and
concrete.
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Explain important scientific principles through the animations. For
example, the biological clocks animations can be used to demonstrate
the fundamentals of transcription and translation.
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Make sure that students learn the material by repeating sections
of the animations as often as you think necessary to reinforce underlying
scientific principles. You can start, restart, and play back sections
of the animations.
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Urge students to use the animations in accordance with their own
learning styles. Students who are more visually oriented can watch
the animations first and read the text later, while others might prefer
to read the explanations first and then view the graphics.
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Incorporate the animations into Web-based learning modules that you
create to supplement your classroom curricula.
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Encourage students to incorporate the animations into their own Web-based
projects.
Human SCN Anatomy Resources
References:
1. Bear, MF, Connors, BW, and Paradiso, MA. Neuroscience: exploring
the brain. Baltimore: Williams and Wilkins, 1996.
2. Herzog, ED, Takahashi, JS, and Block, GD. Clock controls circadian
period in isolated suprachiasmatic nucleus neurons. Nature Neuroscience
1:708-713
3. Lydic, R, Albers, HE, Tepper, B, and Moore-Ede, MC. Three-dimensional
structure of the mammalian suprachiasmatic nuclei: a comparative study
of five species. J. Comp. Neurol. 204: 225-237, 1982.
4. van den Pol, A. Hypothalamic suprachiasmatic nucleus: intrinsic anatomy.
J. Comp. Neurol. 191: 661-702, 1980.
Human SCN Anatomy Credits
Director: Dennis Liu, Ph.D.
Scientific Direction: Joseph Takahashi, Ph.D.
Scientific Content: Donna Messersmith, Ph.D.
Animator: Eric Keller |
