The Human Suprachiasmatic Nucleus

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 [email protected].

1. Use the animations to make abstract scientific ideas visible and concrete.

2. Explain important scientific principles through the animations. For example, the biological clocks animations can be used to demonstrate the fundamentals of transcription and translation.

3. 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.

4. 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.

5. Incorporate the animations into Web-based learning modules that you create to supplement your classroom curricula.

6. 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