Special Senses - Eyes

Question 1

Human vision

Answer 1

Dominant sense
70% of all sensory receptors in the body are in the eyes
Half of the cerebral cortex involved in processing visual information

Question 2

Accessory structures protect the eye and aid function

Answer 2

Eyebrows
Eyelids
Conjunctiva - moist membrane at front of eye
Lacrimal apparatus - tear fluid production
6 extrinsic eye muscles
4 rectus and 2 oblique

Question 3

Eye anatomy

4 things

Answer 3

Fibrous avascular outer layer: sclera and cornea
Vascular pigmented middle layer: choroid, ciliary body, iris and pupil
Inner layer: retina, pigmented layer of the retina, neural layer of the retina
Photoreceptor cells: rods, cones, bipolar cell, ganglion cells, optic disk

Question 4

Fibrous avascular outer layer

2 things

Answer 4

Sclera: tough connective tissue coat over majority of outside of eye, makes up the white of eye
Cornea: transparent structure in front of eye- no blood supply and little amount of cells, allows light to enter and removes waste

Question 5

Internal chambers and fluids

Answer 5

Anterior segment: contains aqueous humour, plasma-like fluid continuously formed, supplies nutrients and oxygen mainly to lens, cornea and retina, removes wastes
Posterior segment: produced in infant, contains vitreous humour, contributed to intraocular pressure -> structure of the eye, holds neural layer of retina firmly against pigmented layer

Question 6

Vascular pigmented middle layer

4 things

Answer 6

Choroid: layer between retina and sclera vascular, pigmented layer under sclera. Stops reflection of light that reaches back of eye
Lens: focuses light on the retina
Ciliary body: contains ciliary muscles, which attach to lens by zonular fibres, changes shape of lens to focus light
Iris: sam tissue layer as choroid, regulates amount of light entering eye by adjusting the diameter of the pupil

Question 7

Iris

Answer 7

Coloured part of the eye that lies between cornea and lens, continuous ciliary body
Regulates amount of light entering eye by adjusting diameter of pupil
Low light: increased sympathetic stimulation, dilator pupillae muscle contracts
Intense light: increased parasympathetic stimulation, sphincter pupillae muscle contracts

Question 8

Tapetum lucidum

Answer 8

Not in humans but in many vertebrates
Shiny and bright layer in choroid
Reflects light to increase ability to see in dim light: in cats 44%
For animals active at twilight/night
When light shines into the eye of an animal having a tapetum lucid, the pupil appears to glow

Question 9

Inner layer -> retina

2 things

Answer 9

Fovea: areas of greatest visual acuity, where light from centre of visual field stride retina
Optic desk: site where optic nerve leaves the eye, where optic nerve and blood vessels supplying eye pass through retina, no photoreceptor cells

Question 10

Anatomy of retina

Answer 10

Pigmented layer
Neural layer- detects light
2 types of photoreceptor: rods and cones
Bipolar cells
Ganglion cells

Question 11

Description of layer

Answer 11

Neural layer contains:
- photoreceptors that transduce light energy
- bipolar and ganglion cells
- amacrine and horizontal cells
- ganglion cell axons:
run along the inner surface of the retina, leave the eye as the optic nerve

Question 12

Rods

Answer 12

Very sensitive to light
Respond to dim light
More numerous than cones
Contain a single pigment -> vision is perceived in grey tones only
Low acute - many rods converge to one ganglion -> no sharp images

Question 13

Cones

Answer 13

Have low sensitivity 
Respond to bright light
Are concentrated in the five centralis
React more quickly than rods
Have one of 3 pigments
Have high-acquity colour vision: one cone per ganglion in fovea

Question 14

Colour blindness

Answer 14

x-linked recessive trait
Deficiency of colour vision
Most common is red-green deficiency
1:12 males, 1:200 females in populations with Northern European ancestry

Question 15

Signal pathway of light entering pathway

Answer 15

Cornea -> aqueous humour -> lens -> virtuous humor -> neural layer of retina -> photoreceptors -> grades potential -> bipolar cells -> graded potential ganglion cells -> AP along optic nerve

Question 16

Visual pigments

Answer 16

retinal: one of four opsins
Rods; opsin is rhodopsin
Cones: opsins for green, blue and red
1. retinal binds opsin in dark
2. absorb light - changes chemical structure and shape -> retinal and opsin dissociate = pigment bleaching -> initiating sequence of reactions that decrease levels of cGMP inside cells -> electric impulse
3. pigment regeneration - chemical changes revert, retinal and opsin bind
Chemical change in the rod is what makes us capable of sight

Question 17

How vision occurs

Answer 17

->Rods and cones activated by light (graded potential)
->Transmitters released from rod and cone cells communicate to bipolar cells in retina
->Varying degrees of convergence occur - determines visual acuity
->Ganglion cells generate action potentials in their axons
2 optic nerves combine at the optic chiasm, just in from of brainstem -> partial crossing over info
->Travel in optic tracts to terminate in nucleus of thalamus = lateral geniculate nucleus
->Thalamic form optic radiation, which projects to primary visual cortex in occipital lobe

Question 18

Visible spectrum in different species

Answer 18

Visible light: human eye is sensitive for wave lengths 380nm-760nm
Humans have 3 different colour sensitive cone cells
Mantis shrimp have 16 different types of receptor cells each sensitive to different regions of the UV and visible spectrum
Some insects and birds can see in UV colours

Question 19

Colour perception - different species

Dogs

Answer 19

Dogs are not completely colour blind: have a dichromatic colour perception
Doga have only 2 colour sensitive cone cells: they see blue, grey, yellow
It’s difficult for them to distinguish green and red colours
Most people who are colour blind only have 2 functioning types of cone cells: can only see around 10 000 shades

Question 20

Vision in sharks

Answer 20

• Sharks have rods and cones in their retinas that are stimulated by light
• Some sharks like cat sharks live in deep water: mostly rods in their retina predominantly nocturnal vision and probably no colour vision
  Common black tip shark and bull shark have quite high cone number but only one cone type can see well in daylight, but cannot distinguish colours: monochromacy

Question 21

Tetrachromats

Answer 21

• Some very rare humans have better than normal colour vision
• Instead of 3 types of cones, these people have a 4th cones = tetrochromatic vision
• Almost always women due to genetic reasons

Question 22

Refraction of light waves by eye

Answer 22

• Both cornea and lens have convex surfaces, causing light waves entering eye to converge onto retina
• A given point in visual field comes to focus on a single point on retina
• Passage of light waves through convex lens causes retinal image to be upside-down and reversed from left to right

Question 23

Focusing light on the retina

Answer 23

• Light is retracted 3 times along path: 1 entering cornea, 2 entering lens and 3 leaving lens
• Mammals and birds focus their eyes by changing shape of lens

Question 24

Focusing for distant vision

Answer 24

the lens flattens for distant vision: sympathetic input relaxes for ciliary muscle
Tightens the ciliary zone and flatten the lens

Question 25

Focusing for close vision

Answer 25

the lens bulges for close vision: parasympathetic input contracts the ciliary muscle. This loosens the ciliary zone and allows the lens to bulge

Question 26

Diseases affecting the eyes

4 things

Answer 26

Cataract: change in colour of usually translucent lens. Prevalent if >60 years, operation with lens replacement
Glaucoma: condition in which drainage of aqueous humour is blocked, fluid backs up and increases pressure within eye, may compress retina and optic nerve, leading to blindness
Conjunctivitis: conjunctiva becomes swollen, irritated or infected. Viral infections improve without treatment, bacterial infections require antibiotic eye drops
Ocular migraines: a migraine that involved visual disturbance, temporary visual disturbances that can affect one or both eyes. Though they can be frightening, ocular migraines typically are harmless and the visual symptoms self-resolve without medication