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|Title:||Light sensitivity of the photoperiodic response system in higher vertebrates: Wavelength and intensity effects|
use daily light in one way or the other in regulation of their short and/ or long
term activities. Light is perceived by pigment(s) present in the retinal (RP)
and/ or extra-retinal photoreceptors (ERPs). ERPs may be located at
in the body but in non-mammalian vertebrates they are found predominantly in the
pineal body and hypothalamic region of the brain , Light radiations directly penetrate
brain tissues to reach and stimulate the hypothalamic (deepbrain) photoreceptors.
How does light information finally reach to the clock is not fully understood in
many vertebrate groups? In mammals, however, the light information from the retina
to the clock (the hypothalamic suprachiasmatic nuclei, SCN) is relayed through
the retino-hypothalamic tract (RHT) which originates from the retinal ganglion
cells, and through the geniculo-hypothalamic tract (GHT) which originates from the
photically responsive cells of a portion of the lateral geniculate nucleus (LGN),
called the intergeniculate leaflet (IGL). |
A response to light (the photoperiodic response) is the result of the interpretation of light information by the photoperiodic system. Apart from the duration, the animals use the gradual shifts in the intensity and wavelength of daily light to regulate their photoperiodic clock system. The wavelengths to which photoreceptors are maximally sensitive or the wavelengths which have greater access to the photoreceptors can induce a maximal response. There can also be differential effects of wavelength and intensity of light on circadian process (es) involved in the entrainment and induction of the photoperiodic clock. This may have some adaptive implications. Entrainment to daily light-dark (LD) cycle may be achieved at dawn or dusk, depending whether the animal is day- or night-active, when there is relatively low intensity of light. By contrast, photoperiodic induction in many species occurs during long days of spring and summer when plenty of daylight at higher intensity is available later in the day.
|ISSN:||0975-1009 (Online); 0019-5189 (Print)|
|Appears in Collections:||IJEB Vol.37(11) [November 1999]|
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