The autonomic nervous system controls involuntary body functions and is divided into the sympathetic and parasympathetic nervous systems. The sympathetic system prepares the body for "fight or flight" through effects like increased heart rate, while the parasympathetic system calms the body and focuses on functions like digestion. Both work in opposition to maintain homeostasis. Cholinergic fibers are associated with parasympathetic effects while adrenergic fibers produce sympathetic responses. Receptors for these neurotransmitters are located throughout the body and allow for autonomic control of various organs.

2. Introduction
Retina is a multilayered sensory tissue that lines the
back of the eye.
It contains millions of light receptors that captures
light rays and convert them into electrical impulses.
These impulses travel along the optic nerve to the
brain where they are turned to images.

3. Structure of Fovea Centralis
In this area, there are no rods.
Cones are tightly packed and it is the most sensitive
part of retina.
It’s central part is called foveola.
All other retinal layers are absent in this region.

4. Histological Layers of the Retina

5. retinal layers of physiological significance

7. Photoreceptor Layer
Rods and Cones are the end organs of vision and are
also known as photoreceptors.
 Rods(120 million) contain a photosensitive
substance rhodopsin(visual purple) and helps in
peripheral vision and vision of low illumination
(scotopic vision)
 Cones (6.5 million) also contain a photosensitive
substance and helps in highly discriminatory central
vision(photopic vision) and colour vision.

8. 8
Visual Pigments
• Rhodopsin
• Light-sensitive pigment in rods
• Decomposes in presence of light
• Triggers a complex series of
reactions that initiate nerve
impulses
• Impulses travel along optic nerve
• Pigments on cones
• Each set contains different light-
sensitive pigment
• Each set is sensitive to different
wavelengths
• Color perceived depends on which
sets of cones are stimulated
• Erythrolabe – responds to red
• Chlorolabe – responds to green
• Cyanolabe – responds to blue

9. Rods and Cones
cones
rods

10. Rods Cones
Number
Distribution
~ 120 millions in
each retina
More in periphery
Non in Fovea
~ 6 millions in
each retina
More in centre
Present
Photosensitive
pigment
Rhodopsin 3 types (iodopsin)
Connection Convergence
(300:1 connection)
No convergence
(1:1; direct private
line)
Function ↑light sensitivity
↓ visual acuity
- colour vision
Night vision
↓ light sensitivity
↑ visual acuity
+ colour vision
Day vision

11. Why Fovea is the most sensitive spot in retina?

12. Why Fovea is the most sensitive spot in retina?
 All layers are shifted aside leaving outer segments of
photosensors to be hit directly by light
 High density of small diameter Cones with long outer
segments
 1:1 convergence (cone-BC-GC)
 Wide presentation in occipital primary visual area

13. 1)- Retinal Pigmented Epithelium
 Absorption of light (due to presence of black pigment
melanin) → reduction of glare (Albinos!!)
 Production of extracellular matrix → keeping outer
segments of photoreceptors straight
 Storage of vitamin A (precursor of 11-cis retinal) for
regeneration of photosensitive pigments
 Phagocytosis of the tips of outer segments after their
shedding off by the photoreceptors → continual renewal of
outer segments

14. Photochemistry of Vision

15. What Happens in Retina
The light rays are focused directly onto the retina,
the light sensitive tissue lining the back of the eye.
Light energy is converted into neural signal
Through visual pathway, these signals reach brain.

17. Action potentials
 excitation of rods causes
 hyperpolarisation rather than depolarisation
 increased negativity of the membrane
 this is due to decreased permeability to Na
 inner segment pumps Na out
 outer segment is very leaky to Na
 normally membrane is -40mV (inside)
 when excited outer segment prevents Na influx
 inner segment continually pumps Na out
 increased negativity inside -> hyperpolarisation
 inside becomes -80mV

18. Physiology of vision
The main mechanisms are
[ ]Initiation of vision (Phototransduction)
[ ]Processing and transmission of visual sensations
[ ]Visual perception

19. Phototransduction
(Initiation of Vision)
The whole phenomenon of conversion of light energy
into nerve impulse is known as phototransduction.
Light falling upon retina cause photochemical
changes( ) which trigger a cascade of biochemical
reactions that result in generation of electrical
changes( ).

20. ( )Photochemical Changes
RHODOPSIN BLEACHING
Rhodopsin refers to the visual pigment present in the
rods-the receptors for night(scotopic) vision.
Its maximum absorption spectrum is around 500 nm.
Rhodopsin consists of a colourless protein called opsin
coupled with a carotenoid called retinine(Vit A or 11-cis-
retinal)

21. Bleaching and Regeneration of Visual Pigments

22. Visual Cycle

23. Photoreception - In More Detail

24. Photoreception

25. Effect of Light on Retinal Neurons

27. VISUAL CYCLE
In the retina of living animals , under constant lght
stimulation, a steady state must exist under which
the rate at which the photochemicals are bleached is
equal to the rate at which they are regenerated.
This equilibrium between the photo-decomposition
and regeneration of visual pigments is referred to as
visual cycle.

28. ( )Electrical Changes
 Activated rhodopsin cascade of biochemical
reactions generation of receptor potential
 Thus light energy is converted to electrical energy

29. Light Adaptation
 retinal sensitivity depends on the amount of chemical pigment
 if a person is in bright light for some time, large amount of
photochemical is reduced to retinal and opsin
 retinal converted to vitamin A
 this reduces the sensitivity of the retina
 this is known as light adaptation
 now if the person goes into a dark room
 he cannot see any object
 reason: severe reduction in retinal sensitivity

30. Dark Adaptation
Ability of the eye to adapt to decreasing illumination.
When one goes from bright sunshine into a dimly-lit
room, one cannot perceive the objects in the room
until some time has elapsed.
This is called dark adaptation time.
It is the time taken for regeneration of rhodopsin
pigment which was bleached by the bright light.

31. Night blindness
 vitamin A deficiency
 not enough quantities of
retinal to reform rhodopsin
 but in daytime cones can still
be excited

32. Autonomic Nervous System
UOB EXAM QUESTIONS

 What is Autonomic nervous system? What are its parts
and their significance?
 Compare and contrast between the sympathetic and
parasympathetic nervous system.
 Compare and contrast between the somatic and
autonomic nervous system.
 Differentiate between the adrenergic and cholinergic
fibers.
 What is the location and significance of the various
receptors of the parasympathetic nervous system.