Eye physiology from guyton and halls physiology Part 1

1. BY Muhammad Ramzan Ul Rehman Nishtar Ken 1

2. BY Muhammad Ramzan Ul Rehman Nishtar Ken
2

3. BY Muhammad Ramzan Ul Rehman Nishtar Ken
3

4. BY Muhammad Ramzan Ul Rehman Nishtar Ken 4

5. Injury to the eyeball
Fluid or blood may be collected between the neural
retina and the pigment epithelium.
Cause: Contracture of fine collagenous fibrils in the
vitreous humor
These fibrils pull the retina toward the interior of the globe.
The detached retina can resist degeneration for days because of:
1.Diffusion across the detachment gap
2.Independent blood supply by the Retinal artery
(Early surgical placement may save the permanent loss of vision)
BY Muhammad Ramzan Ul Rehman Nishtar Ken 5

6. BY Muhammad Ramzan Ul Rehman Nishtar Ken 6

7.  The fovea lies slightly below and to one side of the
optic disc. It is found in the centre of a shallow
depression or pit (the macula lutea).
 The fovea is a minute area in the center of the retina
occupying a total area a little more than 1 square
millimeter
 It is especially capable of acute and detailed vision.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 7

8.  The central fovea, only 0.3 millimeter in diameter, is composed
almost entirely of cones
 These cones have a special structure that gives a clear detail of
the image.
 The foveal cones have especially long and slender bodies, in the
foveal region, ( Peripheral retina has cones with fat bodies)
 Fovea also has blood vessels, ganglion cells and inner nuclear
layer of cells all arranged in manner that light passes unimpeded
to reach the cones.
 Only cones are present at the fovea which have individual
connections with the bipolar and ganglion cells, hence the fovea
gives us our most sensitive and acute vision
BY Muhammad Ramzan Ul Rehman Nishtar Ken 8

9.  There are hardly any cones in the peripheral retina, but many rods.
 The rods here are also shorter and wider than in the central retina.
 Receptive fields at the periphery are very large with many rods converging onto one
ganglion cell.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 9

10. Photo receptors present in the outer nuclear layer or
Receptor Layer of Retina
The human receptor layer consists
of approximately 120 million rods and
6 million cones arranged side by side.
The distribution of these
photoreceptors varies across the
surface of the retina. There are no
rods at all in the fovea, and very few
cones are found at the periphery,
where rods predominate.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 10

11.  Rods and cones (the names
reflect their respective shapes)
contain light sensitive pigments.
 Each photoreceptor consists of an
outer segment which contains
hundreds of thin plates of
membrane (lamellae or discs).
 The outer segment is connected
by a cilium to an inner segment
which contains a nucleus.
 Rods are about 500 times more
sensitive to light than cones, but
cones give us colour vision.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 11

12. The light-sensitive photochemical is
found in the outer segment.
 In rods, this is Rhodopsin
 In cones, it is one of three “color”
photochemicals, (color pigments)
that function almost exactly the same
as rhodopsin
except for differences in spectral
sensitivity.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 12

13.  Large numbers of discs are present in the outer
segments of the rods and cones.
 Each of the discs is an infolded shelf of cell
membrane.
 There are as many as 1000 discs in each rod or
cone.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 13

14. BY Muhammad Ramzan Ul Rehman Nishtar Ken 14

15. Rhodopsin and Colour Pigments
 Conjugated proteins.
 They are present in the membranes
of the discs in the form of
transmembrane proteins.
These Proteins ( Rhodopsin and
Colour Pigments) constitute about 40
per cent of the entire mass of the outer
segment.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 15

16.  Black pigment melanin.
Prevents light reflection through out the globe of the eye ball.
Clear vision.
 It stores large amount of vit A that is an important precursor of
photosensitive chemicals of rods and cones.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 16

17.  This is the point at which axons leave the eyeball and join
the optic nerve. Also, arteries enter and veins leave the
retina at the optic disc.
 There are no photoreceptors here, hence it is known as
the 'blind spot'. It is a pinky-yellow oval, approximately
2mm in diameter
BY Muhammad Ramzan Ul Rehman Nishtar Ken 17

18.  Melanin pigment layer is absent in albino.
 Light reflected in all directions inside the eye ball by unpigmented surfaces of
the retina & sclera.
 Light excites many receptors
 Visual acuity of albinos badly affected 20/100 to 20/200.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 18

19. BY Muhammad Ramzan Ul Rehman Nishtar Ken
19

20. BY Muhammad Ramzan Ul Rehman Nishtar Ken 20

21. BY Muhammad Ramzan Ul Rehman Nishtar Ken 21

22. In the dark an inward current
(the dark current) carried by
the Na+ ions flows into the
outer segment of the rod.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 22
Figure 50-6;
Guyton & Hall

23.  Normally about -40 mV
Normally the outer segment of the rod is very
permeable to Na+ ions.
 In the dark an inward current (the dark current)
carried by the Na+ ions flows into the outer
segment of the rod.
 The current flows out of the cell, through the
efflux of Na+, ions in the inner segment of the
rod.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 23

24. When rhodopsin decomposes it causes a
hyperpolarization of the rod by decreasing
Na+ permeability of the outer segment.
 The Na+ pump in the inner segment keeps
pumping Na+ out of the cell causing the
membrane potential to become more negative
(hyperpolarization).
 The greater the amount of light the greater the
electronegativity.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 24

25.  It occurs by severe vitamin A deficiency.
 Retinal & rhodopsin formed in the absence of Vitamin A
are severely depressed and insufficient.
 Amount of light at night is too little to permit adequate
vision in vitamin A deficient person.
 Dietary deficiency of Vitamin A occurs in months.
 Recovery in Night Blindness takes place in Less than one
hour by I/V Vitamin A
BY Muhammad Ramzan Ul Rehman Nishtar Ken 25

26. Adaptation
 In light conditions most of the rhodopsin has been
reduced to retinal so the level of photosensitive
chemicals is low. In dark conditions retinal is
converted back to rhodopsin. Therefore, the
sensitivity of the retinal automatically adjusts to the
light level.
 Opening and closing of the pupil also contributes to
adaptation because it can adjust the amount
entering the eye.
 Neuronal inhibition or excitation is also involved in
the adaptaion process
BY Muhammad Ramzan Ul Rehman Nishtar Ken 26

27.  The detection of images on the retina is a function
of discriminating between dark and light spots.
 It is important that the sensitivity of the retina be
adjusted to detect the dark and light spots on the
image.
 Enter the sun from a movie theater, even the dark
spots appear bright leaving little contrast.
 Enter darkness from light, the light spots are not
light enough to register.
BY Muhammad Ramzan Ul Rehman Nishtar Ken 27

28. BY Muhammad Ramzan Ul Rehman Nishtar Ken 28

29. By Muhammad Ramzan Ul Rehman
BY Muhammad Ramzan Ul Rehman Nishtar Ken
29