Mimosa
an internal ‘circadian’ clock drives rhythms in leaf opening
Circadian rhythms
- 24hr variations in physiology and behaviour
- persist in the absence of any cyclic cue from the environment
- but somehow retain synchrony with external time e.g. through light intensity day/night
in mammals - circadian clock
- in hypothalamus (suprachiasmatic nuclei)
what is SCN innervated by
optic nerve (lies on retina to measure light)
rods ambient light detection range
- really sensitive to light
- dimmest night to twilight active
what happens with rods with too much light
- become fully hyper-polarised or ‘saturated’ and stop responding
cones light detection
- cable under any light intensity
- from brightest sun to twilight
cones in too bright light
don’t stop responding but become fully hyper-polarized and can no longer detect a range of brighter light intensities
what is cGMP produced by
Guanylyl cyclase + GCAP (guanylyl cyclase activating protein)
GTP -> cGMP
calcium GCAP feedback loop in dark
High Ca²⁺ (in dark): Inhibits GCAP -> GC is less active -> Less cGMP is produced -> Channels stay open (depolarized state).
- levels are high production is low for cGMP
calcium GCAP feedback loop in light
Low Ca²⁺ (in light): Activates GCAP -> GC is more active -> More cGMP is produced -> Channels can re-open when light conditions change.
- PDE breaks down cGMP causing closing of channels and lower Ca levels, activity of PDE must be higher than GC.
- GCAP activated to restore ccGMP levels (higher production)
summary
- measuring light to tell time of day
- Ca drives sensitivity adaptation
- sensitivity adaptation allows cones to be responsive at all background light levels but makes them bad at distinguishing ambient light
If a circadian clock runs longer than 24 hours and they are in complete darkness
get up later every day
opposite for shorter clock
Bilateral Enucleation
- removal of eyes
- still responsive to light cycle and able to synchronizing behaviour to light dark cycle
black ink
injected under the scalp of blind animal
- prevents light reaching brain (animals resume non light schedule)
- photoreceptors synchronizing clock must be in the brain
- in house sparrow / bird
extra-retinal photoreceptors and opsins - in birdy frizardish
- e.g. pineal & parapineal eye, deep brain, intestines (ll around body)
- rely on a group of proteins related to rod and cone opsins
mammals - bilateral enucleation
abolishes all responses to light
- time of day responses originate in retina
pupil response in rodless + coneless mice
- in light cone still restricts
- biological rhythms still synced to day-night
- other photoreceptor drives non-visual responses
retinal ganglion cell excited by light (depolarises)
ON RGC
ON RGCs
have a direct mechanism for detecting light
- even when not getting input from bipolar cells
what fraction of RGCs respond directly to light
- ones that project to hypothalamus are also photo receptors themselves
- depolarize to light
opsin like proteins in vertebrate genomes - light activated
- have an amino acid sequence which indicates a shared history with rod and cone opsins
- membrane associated and 7 TMDs
- GPCRs
- able to blind retinaldehyde (what absorbs light)
non-rod/cone photoreceptors present in both non-mammalian and mammalian retina
melanopsin
melanopsin
- in retinal ganglion cells projects to hypothalamus
