Chapter 10: Biological Rhythms

Biological Rhythms

Chronobiology

the study of biological clocks and their associated rhythms. ‘biochronometry"‘

Physiological rhythms

Examples include the peaking of LH in hamsters every 30-90 minutes in a pulsatile manner, every evening, every breeding season.

Behavioral rhythms

Examples include the patterns of hamsters sleeping during the day and being active at night.

Biological rhythms

Synchronize the internal environment with the external environment.
Synchronize processes within internal environment.

Evolved to synchronize behavior/physiology with changes in environment (ex. night/winter)

Endocrine function shows rhythmic variation over time

Assessments every ten minutes: LH released in 30-90 min pulses
Assessments every 6 hrs: programmed increased in the 30-90 min pulse frequency and pulse amplitude (elevated blood concentrations of LH)

Assessments every month: annual pattern —> LH higher during breeding season in spring/summer vs autumn/winter

Rhythm

recurrent event characterized by its period, frequency, and phase

Period

The length of time required to complete one cycle of a rhythm, such as the amount of time between peaks in a cycle.

time periods/cycle (if 2 cycles in a month, period = 0.5 months)

Frequency

The number of completed cycles per unit of time; for example, two cycles per month.

cycles/time period

Amplitude

In biological rhythms, the amount of change in the rhythm above (to the peak) or below (to the nadir) the average value.

Phase

A point in a rhythm relative to some objective time point during the cycle, or during the cycle of another rhythm.

Objective Time Point

Ex. In Hamster’s 24-hour activity-rest cycle, the phase of onset of activity corresponds with the onset of dark

Circadian rhythm

A biological rhythm with a period of about 24 hours.

Circatidal rhythm

A biological rhythm with a period of about 12.4 hours that is closely tied to changes in tides.

Circalunar rhythm

A biological rhythm with a period of about 29.5 days that is closely tied to phases of the moon.

Circannual rhythm

A biological rhythm with a period of about 12 months.

Antlion behavior exhibits

Circalunar behavioral rhythm

Antlion larger pits; Antlion smaller pits

Full moon; new moon

Male Syrian Hamsters

Critical Day length = 12-12.5 hours of light per day to maintain full gonadal size and function

Female ground squirrels

Circannual control of LH concentrations

Ovariectomy at beginning of breeding season = classic castration response (increased LH)

Ovariectomy at after breeding season = no LH increase until the next breeding season

Ovariectomy in Female Ground Squirrels

Steroid negative feedback activated only at certain times

Exogenous

Relating to a substance or process outside the organism, such as light.

Endogenous

Relating to a substance or process within the organism, such as genes.

Evidence that rhythms are synchronized by the environment NOT driven by it

1. Animals in spacecraft kept in constant conditions free of Earth’s geophysical cues displayed rhythms similar to those on Earth

2. Animals that are caged next to each other and have the same geophysical cues have slightly different rhythms

3. the period (and phase) of biological rhythms are heritable and depend on identified genes

4. the period (and phase) of a biological rhythm of one individual can be transferred to another via tissue transplant

Wildtype Hamsters

~24-hr free-running periods
Received SCN lesions and SCN transplants of other genotype
Took on free-running periods of the other genotype

Tau Hamsters (20h free-running periods)

~20-hour free-running periods

Received SCN lesions and SCN transplants of other genotype
Took on free-running periods of the other genotype

Diurnal

Active during the day, e.g., horses, ground squirrels, chickens.

Nocturnal

Active at night, e.g., raccoons, hamsters, owls.

Crepuscular

Active at dawn and dusk, e.g., rabbits, deer, mosquitoes.

What are elevated at night?

Melatonin + Growth Hormone

Zeitgeber

a potent environmental time cue, or temporal synchronizer
External stimulus that synchronizes endogenous rhythms to environmental cues

Nonphotic zeitgebers

food, drugs, social interactions, exercise, and temperature

Free-running Rhythm

biological rhythm not synchronized to own natural zeitgeber
expresses own endogenous rhythm

Entrainment

The synchronization of biological rhythms to a periodic environmental cue

Hamsters in total darkness

sleep/wake phases begin to drift or free-run

Phase delay in Hamsters (in total darkness)

Exposure to light early in the night leads to phase delay for the hamster

Phase advance in Hamsters (in total darkness)

Exposure to light late in the night leads to phase advance

Photoperiod

Day length, or the amount of light per day

SCN (Suprachiasmatic nucleus)

The master clock in mammals that synchronizes circadian rhythms and responds to photoperiod

The SCN has connections to retina,

access to environmental light allows internal time to be synchronized with external/environmental time

SCN serves as a master_____

clock and calendar because it responds to photoperiod

Circadian clocks in non-mammals

eyes of amphibians, pineal glands of fishes/reptiles/birds

Individual SCN neurons _______

maintain rhythmicity

the mean firing rates of 2 individual SCN cells in culture differ with different periods and phases as well

Disruption of biological rhythms

Has been linked with negative mental and physical outcomes

Jet Lag

Phase shifts in all zeitgebers at once

Jet Lag causes disruptions in

sleep, digestion, attention, and motivation

What eases jet lag?

appropriately-timed melatonin

What is an effect of Night Shift work?

Interruption of melatonin rhythms by night-shift work has been linked with an elevated risk of breast cancer

Resynchronization

Takes longer on west-to-east travel than vice versa when 4+ time zones are crossed