Phototeception

1. PHOTORECEPTION

2. Photoreception : Refers to mechanism of light detection that
leads to vision and depends on specialised called
photoreceptors
The quality of vision provided by photoreceptors varies enormously among animals.
Flatworms have so-called cup-shaped eyes which face in opposite directions and are
used essentially to obtain directional information. The animal will tend to move to darker
regions, thereby minimizing the risk of attack by predators.
 The arthropods (insects" crustaceans and spiders) have compound eyes .Each
compound eye is made from many smaller optical units called ommatidia. Compound
eyes produce mosaic images
In mammals, birds, and other vertebrates, the retina contains several types of cells that
are interconnected into a network. The visual receptor cells themselves fall into two
classes, rods and cones, Cones function best in bright light whereas rods function best in
dim light.

3. Diversity of eyes
Simple eye – Flatworms
Pinhole eye – Nautilus
Single chamered eye
with spherical lens – fishes
Mollusk.
 Multiple lens –
copepod pontella
Compound eye – insects
Vertebrate eye.

4. PHOTORECEPTORS
Photoreceptors: Photoreceptors are the cells in the retina that respond
to light. Their distinguishing feature is the presence of large amounts of tightly
packed membrane that contains the photopigment rhodopsin .
INVERTREBRATE : the photoreceptors are know as rhadom ,containing lessly
dense packed photopigments.
In reptiles and birds the receptors may also contain coloured oil droplets that
modify the spectrum of the light absorbed by the photopigment,
thereby enhancing colour vision.

5. PHOTOPIGMENTS
1. The photopigments that absorb light all have a similar structure, which consists of
a protein called an opsin and a small attached molecule known as the chromophore.
2. The chromophore absorbs photons of light, that involves a change in its configuration. In
vertebrate rods the chromophore is retinal, the aldehyde of vitamin A1.
3. When retinal absorbs a photon, the double bond between the 11th and
12th carbon atoms flips, thus reconfiguring the molecule from the 11-cis to the all-
trans form.
4. This in turn triggers a molecular transduction cascade, resulting in the closure
of sodium channels in the membrane and hyperpolarization (increase in negativity) of the
cell.
5. Retinal then detaches from opsin, is regenerated to the 11-cis state in the cells
of the pigment epithelium that surround the rods, and is reattached to an opsin
molecule.

6. NEURAL TRANSMISSION
VERTREBRATE
Dark condition : cGMP binds to sodium channels and keeping the channel open thus
allowing the sodium ion to enter thus depolarizing it .
In this case neurotarnsmitter is released.
Light condition : cGMP do not binds to sodium channels and keeping the channel closed
thus hperpolarizing it . In this case neurotarnsmitter is not released.
INVERTREBRATE
The response is opposite of that of vertebrate receptors.

7. REFERENCES
1. Animal physiology by Ian Kay.
2. https://www.britannica.com/science/photoreception/Adaptive-
mechanisms-of-vision
3. https://www.britannica.com/science/photoreception/Compound-
eyes.
4. Animal physiology: Mechanism and adaptation by Eckert and
Randall.