1. Physiology-II PHL-226 Physiology of vision
By Majid Ahmad Ganaie M. Pharm., Ph.D. Associate Professor Department of Pharmacology E mail: 1

2. Vision

3. The eye can only perceive a small portion of the spectrum of electromagnetic waves

4. Three layers
1. Outer fibrous tunic
Sclera, cornea, limbus
2. Middle vascular tunic
Iris, ciliary body, choroid
3. Inner nervous tunic
Retina

5. In order to see an object:
1- The pattern of the object must fall on the vision receptors (rods and cones in the retina)  accommodation
2- The amount of light entering the eye must be regulated (too much light will “bleach out” the signals)
3- The energy from the waves of photons must be transduced into electrical signals
4- The brain must receive and interpret the signals

6. Accommodation
It is the process of adjusting the shape of the lens so that the external image fall exactly on the retina

7. Accommodation Object is far  the lens flattens
Object is near  the lens rounds
Figure 10.25

8. Accommodation abnormalities
Myopia
Hyperopia
Astigmatism: the cornea is irregular  irregular pattern of vision
Presbyopia: stiffening of the lens occurring with aging  increased difficulty with near vision

9. Figure 10.27a–b

10. Figure 10.27c

11. Regulation of the amount of light entering the eye
The iris controls the amount of light entering the eye cavities
The contraction of radial or circular smooth muscles located within the iris permit changes in the pupil diameter

12. Figure 10.28a

13. The Pupillary Muscles

14. Phototransduction - General
Rods – respond to almost any photon and are sensitive to light and black and white – can’t distinguish colors.
Cones – allow for colour vision.
cell body
cone cell
rod cell
nucleus
inner segment
outer segment
synaptic endings
membrane of disk
retinal
opsin
synaptic
vesicles
ion channels
in plasma
membrane
light
rays
of disk
Rhodopsin molecule
(opsin + retinal)
3 types of cones – red, blue, green – colour blindness usually results from a deficiency in one or more of the cones (red/green colour blindness is common)

15. The Visual Pathway: Photons hit the pigment of a photoreceptor    enzymes are activated in the cell which modify its state of polarization  the signals are sent to visual area of the occipital lobe of the brain through the optic nerve

16. Neural processing
The bipolar neurons and ganglion cells process the signal
In the fovea where the acuity is the highest: 1 cone  1 bipolar cell  1 ganglion cell
At the periphery: many rods  1 bipolar cell … acuity is much decreased
Other cells in the retina participate in signal processing

17. The Visual Pathways Visual Circadian Rhythm
Input to suprachiasmic nucleus affects the function of the brainstem
Circadian rhythm ties to day-night cycle, and affects metabolic rates

18. Colour blindness
Colour blindness results when there is a deficiency in one or more of the cone colours (i.e red/green deficiency). The patient will have difficulty differentiating between red and green colours and tend to see them all as something in between.
Colour blindness is a genetic condition and affects boys much more frequently than girls, because it is a disorder carried on the “X” chromosome. Females are XX and therefore need 2 defective Xs to have colour blindness. If a female has 1 defective X chromosome, they are a carrier. Males have XY, therefore only 1 X chromosome needs to be affected to have colour blindness.
Try this simple colour blindness test to see whether or not you have this deficiency.

19. This is the CONTROL

24. Thank you