7. Biological Rhythms

biological rhythms

the rhythms that animals and people have evolved to follow environmental rhythms. day/night, tides, seasons, temperature

can be behavioral, physiological, biochemical

Chronobiology

field that studies biological blocks and rhythms within an organism

Types of biological rhythms (ICU)

Ultradian

Circadian

Infradian

Ultradian

rhyms that last less than 24 hours, can repeat in cycles several times in a day: like eating/hunger,

sleep stages

Circadian rhythms

rhythms that is 24 hrs long: when we are awake and when we are asleep

Infradian

rhythms that are longer than 24 hrs can be days, months, years. ex. reproductive cycle for some species, circalunar days 29 days, hibernation

why do almost all land animals follow circadian rhythm (earth rotation, internal body)

animals sync their behavior according to light and day cycles from the earths spin

physical and biochemical processes in animals body follow daily rhythm (whether dirunal or nocturnal):

temp, urine, bloodflow, hormone, hair growth, metabolism

Diurnal

species that are active during the light

Nocturnal

species that are active in the dark

what is the adaptive value for circadian rhythms (PPO)

allows animals to predict get ready for daily events, instead of reacting them

we physically, behaviourally prepare before event

ex. food being available at certain time in day and insulin starts rising

organizes the behavior of different animals throughout the day

Zeltgeber (time giver)

any cue that can tell animal to synchronize with their environment

Phase shift

when the animal shifts their activity to changes/syns to a stimulus ex. stops sleeping, wakes up when its light out

Entrainment

the act of synchronizing to the environment

Period

the length/round of a full rhythm

Endogenous clock experiment

in this experiment a rodent is given a hamster wheel, lighting is controlled to convince rodent its day or night, and their activity is measured.

results

hamsters are nocturnal: will run when it is subjective nightthey will rest when it is subjective day

but daily patterns of activity and inactivity still continue even when deprived of cues (continuous light or dark)

differed periods between subejcts.

meaning they must have internal mechanism that is not dependent on light, and it is not as precise as following the light some variation

subjective time

the animals internal estimate time of day

active slightly before the light changes to dark: shows body has predictability

the running of light was shifted so was activity of rats (phase shift)

how are animals synchronized to themselves (endogenous clock)

the superchiasmatic nucleus (SCN)

the superchiasmatic nucleus (SCN) overview

region responsible for the endogenous clock

is part of the hypothalamus above the optic chiasm

lesions here will lead to disrupted circadian rhythms and hormone secretion

Effects of SCN lesions (hamster)

dont have internal clockj so they follow external cues. their activity is still linked to lights on/off

when placed in continuous dim light activity becomes random. we normally use both internal clock, and environment. when both are taken activity will be random

so SCN is needed to generate a circadian rhythm Internally)

Tau

hamster mutation so they naturally keep shorter free running period / rhythm 20 hrs., in constant conditions there period is 20 not 24

only noticeable in constant condition

SCN hamster transplant experiment

hamsters with scn lesions were given constant conditions so no circadian rhythm / random activity

took tissue from transplant of hamster with tau mutation

and their circadian rhythms were reinstated but matched the shorter period of the donor

so it does restore entrainment of dark light cycle just to donor’s period

How does the SCN detect light

we entrain our circadian rhythms to dark light cycle differently depending on the species

some species have thin skulls that light penetrates

birds, amphibians pineal gland is light sensitive

mammals: light info goes from eye to SCN through the retinohypothalamic pathway

Retinohypothalamic pathway

specific ganglion cells (not relying on rods and cones) have large fields that only detect light or no light

they do this with melanopsin a pigment that makes ganglion cells light sensitive

these ganglion cells project to brainstem and send light signals to brainstem. needed to control pupil size

Melanopsin

photopigment for specific ganglion cells. makes them sensitive to light especially blue light

research to confirm melanopsin ganglion cells diff from reg ganglion cells

experiments show if rodent doesn’t have eyes they cant use light, but if they have these cells intact they will still be able to entrain to light

what is SCN (structure, location, cell structure)

Super chiasmic nuclei a paired structure found in hypothalamus by optic chiasm. follows a 24 hr cycle

SCN cells make two proteins:

clock and cycle

How does clock and cycle proteins work (SCN cells)

1. they bind together (dimer)

1. help transcription of Per and cry genes: the dimer binds to DNA. and enhances the transcription of genes for Period and Cryptochrome

1. once built: Per and Cry proteins bind together into a complex that will stop the activity of the clock / cycle dimer changing transcription to make per and cry, thus slowing production of per and cry proteins

1. complex is modified or breaks down and this allows cycle of gene transcription to continue (whole thing was 24 hrs)

1. specific ganglion cells detect light with melanopsin, and messages retinohypothalamus, to make adjustment: if light → activates SCN neurons with glutamate. promoting transcription of per gene, and leads to sync with day-night cycle

if constant light or darkness because of SCN cells they can still have internal clock and keep time without external cues. but cues help synchronize it better

Light induced phase shifts
(what, types: MEL)

how much the internal clock is adjusted depends on when the light hits us

midday subjective day

early subjective night

late subjective night

midday subjective day + (light exposure)

dead zone no effect because the brain already thinks its day extra light isn’t gonna change anything

early subjective night + (light exposure)

give light to early in the night: it shifts activity later, we go to sleep later

late subjective night + (light exposure)

exposure to light when its late at night will shift the clock forward to start of the next day

Infradian rhythms types, theory

menstrual cycle (28 days)

seasonal breeding

hibernation cycle

migration timing

(some driven by environmental factors (food, availability and temp)

annual rhythms will persist when put in constant condition which suggests there are other endogenous clock systems (cos how would they know without external cues) and is sperate from SCN cos lesioned doesn’t effect,

maybe connected to melatonin changes to match shorter, longer nights