Circadian rhythms
In this lesson, we'll learn about circadian rhythms, the 24-hour cycles that govern many of our body's physiological and behavioral processes.
1) First, we'll learn about two structures that are critical to maintaining circadian rhythms: the suprachiasmatic nucleus and the pineal gland. Watch these videos and read the associated articles:
Article: Know Your Brain - Suprachiasmatic Nucleus
Article: Know Your Brain - Pineal Gland
2) Next, watch this video about melatonin:
3) Finally, answer these questions to test your understanding:
1. Which structure is primarily responsible for secreting melatonin into the bloodstream in a circadian pattern to regulate the sleep–wake cycle?
- A. adrenal glands
- B. pituitary gland
- C. pineal gland
- D. hippocampus
Show Answer
Correct answer: C
Although melatonin is produced in several parts of the body, the pineal gland is the main source of melatonin released into the bloodstream in a circadian pattern. It receives signals from the suprachiasmatic nucleus (SCN) of the hypothalamus and increases melatonin production in darkness, helping to regulate the sleep–wake cycle.
2. Cells in this nucleus can be considered the main "timekeepers" of the brain, as they generate and maintain rhythmic activity on a 24-hour cycle to help regulate our circadian rhythms:
- A. nucleus accumbens
- B. subthalamic nucleus
- C. suprachiasmatic nucleus
- D. red nucleus
Show Answer
Correct answer: C
The suprachiasmatic nucleus is a nucleus in the hypothalamus that contains cells that act as the circadian pacemaker of mammals. The cells "keep time" through a process of gene transcription, protein production, and protein degradation that takes approximately 24 hours to complete.
3. In the mammalian suprachiasmatic nucleus, the proteins CLOCK and BMAL1 bind together and promote the expression of which two genes central to the circadian negative-feedback loop?
- A. Per and Cry
- B. Tim and Dbt
- C. Clock and Bmal1
- D. Rev-erb and Ror
Show Answer
Correct answer: A
CLOCK and BMAL1 work together to turn on the Per and Cry genes. As PER and CRY proteins build up, they feed back and shut down CLOCK–BMAL1 activity, creating the daily on–off cycle that drives circadian rhythms in the suprachiasmatic nucleus.
4. Where does the pineal gland get its name?
- A. it was first discovered by a scientist with the last name Pineal
- B. it resembles a pine cone in shape
- C. it is located near the pineal nerve
- D. it releases pinealocytes, which are shaped like pine needles
Show Answer
Correct answer: B
The pineal gland is named for its pine cone-like shape.
5. The highest level of melatonin secretion typically occurs at what time?
- A. mid-morning
- B. in the afternoon
- C. early evening
- D. in the middle of the night
Show Answer
Correct answer: D
Melatonin secretion from the pineal gland occurs in the dark phase of the night. Melatonin secretion is inhibited by exposure to light, and the highest levels of secretion typically occur after midnight. Secretion from the pineal gland during the day is negligible.
