Week 11 (Rhythms, SCN, Melatonin)

Endogenous Rhythms

Up until the mid-1900s 1900s people thought sleep was only controlled by external stimuli.

  • Like light cues or temperature

We now know that the body can generate its own cycle of activity and inactivity.

  • Known as endogenous rhythms

Circannual Rhythms

Migratory birds generate an endogenous circannual rhythm

  • Circannual about a year

  • Every year, they fly north and south at the right times

    • They don’t need to wait until it’s cold or warm to know when to fly

Animals produce endogenous circadian rhythms

  • About a day

  • Some of these cycles can be slightly shorter or longer than 24 hours

  • Even in constant darkness, these rhythms persist

Circadian rhythms affect more than just wakefulness and sleep

  • Eating and drinking

  • Urination

  • Hormone secretion

  • Metabolism

  • Sensitivity to drugs

  • Temperature

  • Mood

Biological Clock

Some people refer to the endogenous circadian rhythm as a biological clock

Human circadian rhythms are generated over 24 hours

  • Circadian rhythms can persist without light

  • However when we change our sleep schedule, it takes time to reset

Light is critical for resetting circadian rhythms in land animals

  • It acts as a zeitgeber

    • A zeitgeber is a stimulus that resets circadian rhythem

  • Over hald of all blind people report frequent sleep problems

  • When their biological clocks are out of phase with external time

  • Marine animals rely on the tides as their zeitgeber

Other less strong zeitgebers exist in humans

  • Mostly modify the effects of light, have a weak effect

    • Exercise

    • Arousal of any kind

    • Meals

    • Environmental temperature

Jet Lag

Refers to the disruption of circadian rhythms due to crossing time zones

  • Stem from a mismatch between our internal circadian clock and external time

  • Adjusting to time zones going west is easier than adjusting to going east for most people

    • Going west leads to a phase-delay

    • Going east leads to a phase-advance

    • Most people find it harder to go to bed earlier than it is to stay up late

Biological Clock Mechanisms

The suprachiasmatic nucleus (SCN) is the main driver of sleep and body temperature rhythms

  • Part of the hypothalamus

  • Named for it’s location supra (above) the optic chaism

Suprachiasmatic Nucleus (SCN)

The SCN is sometimes referred to as the circadian clock

  • Due to its important role in sleep

Generates circadian rhythms in a genetically controller manner

  • Even if SCN neurons are removed, they still produce action potentials in a circadian rhythm

  • A mutation in one gene (Tau) causes the SCN to produce a 20-hr instead of a 24hr rhythm

    • Showing that the rhythm comes from the SCN

Light and the Retina

The retina contains retinal ganglion cells that are responsive to light

  • Called intrinsically photosensitive retinal ganglion cells (ipRGCs)

  • They have their own photopigmant called melanopsin

  • Located