Vision ppt

1. Special Senses
(Physiology of Visión)

2. Sense organs
• Structure in animals –
– specialized for receiving external and internal stimuli
– transmitting them in form of nerve impulses to brain
• Senses can
– Sense changes in the environment around them and in
their bodies
– so that they can respond appropriately

3. Divided in to two groups
1. General senses
1) Touch
2) Pressure
3) Pain
4) Temperature
• Evenly distributed through the skin, muscle and joints

4. 2. Special senses
1) Sight
2) Smell
3) Hearing
4) Taste
• Complex in structure

5. Vision
Major sensation
Chief apparatus – Eye
Begins when light comes into the eye

6. Eyes
 Photosensitive organs
 Designed to detect light
 Pass on information as electrical signal to brain
Eye ball
– housed comfortably in orbit within a soft atmosphere of
surrounding fat
– maintained in position by action of extra ocular muscles

7. Cont..
• Can perceive a small portion of spectrum
of electromagnetic waves
– range between about 350 to 750 nm
– See only a small portion of spectrum
– 83% of light present in our
atmosphere is in this range

8. Accessory structures
• Eyelids (palpebrae)-
• Designed to protect anterior segment of eye, especially cornea
• In most domestic animal species eyelids consist of
• Superior (upper)
• Inferior (lower)
• Nictitans (third eyelid)
• Cover eye during sleep
• Airtight seal when lids closed
• Protect eye from excessive light and foreign objects
• Assist in lubricating eye
• Stops overflow of tears
• Within upper eyelid
• Meibomian glands (sebaceous glands) lubricate eyes

9. • Eyelashes –
• Located at margin of eyelids - cilia
• Eyebrows –
• Located above each eyelid
• Both structures help keep foreign objects and direct sunlight
out of the eye
• Palpebral commissure (Canthus)
• Is corner of eye where upper and lower eyelids meet
• Caruncle –
• smooth fleshy mass containing sebaceous glands at
inner canthus

12. Anatomy of eyeball
• Consists three layers:
– (1) Fibrous tunic
– (2) Vascular tunic
– (3) Retina

13. Fibrous tunic
• External coat of eyeball
• Avascular
• Consists
1. Anterior cornea
2. Posterior sclera

14. • Cornea
– transparent layer covering the iris (colored portion of the
front of eye)
– curved and therefore helps bend light toward retina
• Sclera
– covers the entire outside surface of eye excluding cornea
– consists of dense connective tissue
– provides rigidity, helping to maintain shape of eyeball

16. Vascular tunic
• Vascular tunic (uvea) middle layer
• Vascular
• Highly pigmented
• Consists three parts
1. Choroid
2. Ciliary body
3. Iris

17. Choroid
• Highly vascularized, dark brown, posterior portion of vascular tunic
• Its brown pigment - produced by melanocytes, helps absorb light -
so that it is not scattered throughout inside of eye
• Tapetum lucidum (Eye-shine)
– an additional layer in choroids
– many species of domestic animals (cats, dogs, horses, and
ruminants)
– causes animal’s eyes to appear to glow when shined with a light
– reflects light back toward retina so that animal can see in low
light

18. Ciliary body
• In the anterior - choroid becomes Ciliary body
• That extends from ora serrata
– serrated front margin of retina, to just posterior of junction of
the sclera and cornea
• Consists
– ciliary processes
– ciliary muscles

19. • Ciliary processes
– folds of tissue containing capillaries that secrete
aqueous humor
• Ciliary muscles
– a bundle of smooth muscles - alter shape of lens in
order to allow for near or far vision
• Zonular fibers (suspensory ligaments)
– Extending from the ciliary processes to the lens

20. Iris
• Colored portion at front of eyeball
– is shaped like a disc with a hole - pupil in the center
• Color - controlled by number of pigmented cells
– A large number results in a brown color
– A low number results in a blue color
• Iris lies between cornea in front and lens in back and is
attached to ciliary processes
• Consisting circular and radial smooth muscle fibers, regulates
the amount of light entering the eye

21. Pupil
• Shape of pupil
– can be round, elliptical (slitlike)
Cats - elliptical pupil that opens and closes faster than
round pupils
• Bright light / close vision / Parasympathetic signals
stimulate - circular muscles to contract - causing the pupil
to constrict
• Dim light / distant vision / sympathetic signals stimulate -
radial muscles to contract - causing the pupil to dilate

23. Retina (sensory tunic)
• Innermost layer, lines the posterior portion of the eyeball
• Consists of two layers:
1. Outer - Pigmented layer
2. Inner - Neural layer
• Pigmented layer
– one cell- thick layer of melanin-containing epithelial
cells
– These cells also act as phagocytes and store vitamin A

24. Neural layer
• Multilayered, and grows directly out of brain during
embryonic development
• Three major layers:
1. Photoreceptor layer
2. Bipolar cell layer
3. Ganglion cell layer
Before reaching photoreceptor layer, light must first pass
through ganglion and bipolar cell layers

26. Photoreceptor Cells
• Two types :
1. Rods
2. Cones
• In Retina – 130 million rod and cone cells present
• Rods outnumber cones 20:1— except in birds, which have
more cones than rods
• Photoreceptors cells consist
– Outer segment – Photosensitive region
– Inner segment – Metabolic region

27. • Rods
– Have a low light threshold
– Are effective in dim light
– Photochemical – Rhodopsin
• Allowing for perception of only shades of gray
• Has low threshold of excitability so easily
stimulated by low-intensity of light

28. • Cones
– Have a higher threshold for light
– Provide color and high acuity vision
• Photochemical – Iodopsin
• Have higher threshold of excitability than rod cells
and require high intensity of light to be stimulated
• In fact, rod cells are about 300 time more sensitive to
light than are cone cells

29. Lens
• Biconvex
• Transparent
• Avascular structure
Can change its shape in order to focus light on the retina
• Located behind iris
• Lens is held in place by suspensory ligament attaching
it to choroid process

31. • Enclosed in a thin, elastic capsule, consists of two regions:
1. Lens epithelium
2. Lens fibers
• Lens epithelium
– Consists of cuboidal cells located on anterior surface of
lens
– Arranged like layers of an onion
• Lens fibers
– Made of folded proteins called crystallins

32. Chambers of the eye
• Lens divides - eye into
1. Anterior segments
2. Posterior segments

33. Anterior segment
• Filled with aqueous humor
– Clear, watery fluid similar in composition to plasma
– Nourishes lens and cornea
• Aqueous humor
– continually derived as a filtrate - ciliary processes
entering posterior chamber
– flows forward through the pupil of iris into anterior
chamber

34. • Iris subdivides anterior segment into
– anterior chamber - located between cornea and iris
– posterior chamber - located between iris and lens

35. Posterior segment
• Larger of the two segments
• Contains vitreous humor, a clear gel- like substance
• Vitreous humor pushes retina against pigmented layer
of choroid, allowing retina to receive a clear image
Unlike aqueous humor, vitreous humor forms during
embryonic development and lasts a lifetime

36. Physiology of vision
• Three processes are important in formation of a clear image
1. Refraction
2. Accommodation
3. Pupil diameter

37. Refraction
• When light rays pass from one medium to another of a
different density speed of light changes
• As a result - light rays bent or refracted

38. • With respect to eye
– light rays are refracted or bent at anterior and posterior
surfaces of both cornea and lens
• Approximately 75% refraction occurs at interphase with
cornea

39. Note that images are inverted, both upside down and
backward, as they are focused on retina
Brain reinterprets this image so that objects are not perceived
as inverted

40. Refraction problems
• Emmetropic
– A normal eye
• Presbyopia
– As animals age, lens loses its elasticity, and therefore its ability to
accommodate
• Myopia or nearsightedness
– When an animal can see close objects but distant objects are blurred
– Occurs because eyeball is too long relative to its focusing power, and
therefore distant objects are focused in front of the retina
• Hypermyopia or hyperopia or farsightedness
– Animal can see distant objects but is unable to focus near objects
because the eyeball is too short
– Therefore, there is not enough accommodating power to focus the light
rays of a near object, and the animal instead focuses at a point behind the
retina

42. Accommodation
• Process of increasing refractive power of lens
• In eye - angle at which light rays are bent depends on
shape of lens
– More convex lens - greater degree to which light
rays are bent

43. • As object moved closer to lens
– light rays must be bent at a greater angle to focus image
on retina
• Therefore, as object moves closer to eye, lens become
more rounded (more convex) to focus image on retina

45. • Accommodation accomplished - actions of ciliary muscle
• Ciliary muscle - relaxed
– zonular fibers surrounding lens pull on lens, thus making
it fatter or less convex
• Ciliary muscle – contracts
– decreasing tension of zonular fibers on lens- becomes
more convex, more rounded, - increases its focusing
power, causing greater bending of the light rays

46. Pupil diameter
• Amount of light - enter in eye controlled - diameter of
pupil
• Circular muscle fibers control pupil diameter

48. How light convert into image
1. Light reflects off objects and travels in a straight line to eye
2. Light passes through cornea, into pupil and through lens
3. Cornea and lens bend (refract) light to focus on retina
4. Photoreceptor on the retina convert the light into electric impulses
5. Electric impulses pass along optic nerve to brain
6. Brain process signals to create an image