Photochemistry of Vision Physiology of Vision Photochemistry of Vision Dr. Sumera Gul

Learning Objectives At the end of the lecture the students should be able to: Discuss Light and Dark adaptation Explain the mechanism of signal transmission in retinal neurons Discuss the neurotransmitters released by retinal neurons Explain the functions of various cells in retinal layers

Review of Previous Lecture

Rhodopsin-Retinal Visual Cycle After exposure to light Retinal (retinene) and Scotopsin separate

Color Vision Blue sensitive pigment Red sensitive pigment Green sensitive pigment Only one pigment is present in a cone

Color Blindness Protanope Deuteranope Red-green color blindness X-linked recessive 8% of all women are carriers

Light and Dark Adaptation

Light Adaptation What will happen if a person has been in bright light for hours?

Light Adaptation Large portions of the photochemicals in both the rods and the cones will have been reduced to retinal and opsins. Furthermore, much of the retinal of both the rods and the cones will have been converted into vitamin A. Low concentrations of the photosensitive chemicals in the rods and cones sensitivity of the eye to light is correspondingly reduced.

Dark Adaptation What will happen if a person remains in darkness for a long time?

Dark Adaptation The retinal and opsins in the rods and cones are converted back into the light-sensitive pigments. Vitamin A is converted back into retinal to increase light-sensitive pigments.

Dark Adaptation After remaining is bright light when a person is exposed to complete darkness Sensitivity if retina is very low initially After 1 min increases 10 fold

Dark Adaptation Dark Adaptation Curve Early portion is due to adaptation of cones (4 times rapid) Rods are slowly adapting More sensitive 100 or more rods converge on a single ganglion

Light Dark Adaptation Change in pupillary size Neural Adaptation

Neural Functions of Retina

Neurotransmitters of Retina Glutamate is released from Rods and Cones at their synapse with bipolar cells Amacrine cells release at least 8 neurotransmitters including GABA, glycine, Ach, Dopamine, Indolamine

Horizontal Cells Horizontal cells are inhibitory Responsible for lateral inhibition It enhances the visual contrast and allows high visual accuracy

Bipolar Cells Can be depolarizing or hyperpolarizing cells Can be due to different types of cells altogether Or due to direct stimulation by Rods/Cones or via horizontal cells

Amacrine Cells 30 different types Rods vision Onset and offset of visual signals Light turned on or off Respond to movement of spot

Ganglion Cells 1.6 million ganglion cells in each retina 100 million rods 3 million cones

Ganglion Cells Peripheral retina is more sensitive to light As there are more rods there which are 30-300 times more sensitive to light. And around 200 rods converge on a single optic nerve fibre.

W type Ganglion Cells Transmit signals at lower velocity mostly from rods Sensitive to detect directional movement in peripheral retina Important for vision in dark

X type Ganglion Cells Small fields Dendrites do not spread widely Final and discrete visual image Probably responsible for color vision

Y type Ganglion Cells Largest of all cells Fast Broad dendritic fields Respond rapidly to changes in visual image Less discrete

P and M cells Magnocellular cells Parvocellular cells

Magnocellular Cells Alpha cells Parasol cells To the magnocellular layer of lateral geniculate body Larger receptive field Faster Not generally sensitive to color More sensitive to contrast black and white Sensitive to rapid movements

Parvocellular cells Beta cells Midget ganglion cells Parvocellular layer of LGN Small receptive field Slower Sustained response to color Fine details

Questions?