What Is the Suprachiasmatic Nucleus?

The suprachiasmatic nucleus is a tiny cluster of about 20,000 neurons located in the hypothalamus, just above the optic chiasm. Despite its small size, the SCN acts as the principal circadian pacemaker in mammals, orchestrating the timing of countless biological processes to align with the day-night cycle.

How Does the SCN Control Circadian Rhythms?

• Receives Light Input: The SCN receives direct signals from the eyes about environmental light, allowing it to synchronize the body’s internal clock with external day and night.

• Coordinates Body Clocks: The SCN sends timing signals to other brain regions and peripheral organs, ensuring that physiological activities occur at optimal times.

• Regulates Sleep and Hormones: It is responsible for the timing of sleep-wake cycles, hormone secretion (such as cortisol and melatonin), body temperature, and even heart rate.

Scientific Evidence

Research shows that damaging or removing the SCN disrupts all circadian rhythms, leading to irregular sleep, hormone cycles, and behavior patterns. The SCN’s unique network of neurons and neurotransmitters, including vasoactive intestinal polypeptide (VIP) and arginine-vasopressin (AVP), is crucial for generating and synchronizing these rhythms.

Why Not Other Brain Regions?

• Medulla: Primarily controls autonomic functions like breathing and heart rate, not circadian rhythms.

• Cortex: Responsible for higher cognitive functions.

• Pituitary gland: Regulates hormone release but is itself under circadian control from the SCN.

Conclusion

Circadian rhythms in humans are controlled by the suprachiasmatic nucleus (SCN) of the hypothalamus. This master clock ensures our bodies stay in sync with the environment, supporting healthy sleep and overall well-being.

Correct answer: (4) Suprachiasmatic nucleus