Sense Organs; Sight

Question 1

The eye

Answer 1

The eye is the organ of the sense of sight situated in the orbital cavity and supplied by the optic nerve (CN II).

Question 2

The eyeball; Layers

Answer 2

The eyeball is composed of three layers;
1. Sclera (white of the eye) & cornea anteriorly
2. Middle layer; Uvea (consists of iris, ciliary body and choroid – highly vascular)
3. Inner layer; Retina

Question 3

Iris

Answer 3

The coloured portion of the eye is called the iris and controls the amount of light reaching the retina by adjusting pupil size

Question 4

Lens

Answer 4

The lens is located behind the pupil and further focuses light (along with the cornea)

Question 5

Eye Accessory Structures

Answer 5

The eye has several accessory organs for protection:
Eyebrows, Eyelids / Eyelashes, Conjunctiva, Lacrimal apparatus, Blinking

Question 6

Eye Accessory Structures; Eyebrows

Answer 6

Eyebrows; prevent sweat / materials entering eye

Question 7

Eye Accessory Struct; Eyelids/ lashes

Answer 7

Eyelids / eyelashes; spreads secretions over eye, protects eye

Question 8

Eye Accessory Structures; Conjuctiva

Answer 8

Conjunctiva; Thin transparent mucous membrane of columnar epithelial cells lining the internal eyelids and anterior eyeball – protects cornea.
Eyelid margins secrete oily material to delay tear evaporation

Question 9

Eye Accessory Struct; Lacrimal apparatus

Answer 9

Lacrimal apparatus; Produces tears

Question 10

Eye Accessory Structures; Blinking

Answer 10

Blinking; spreads tears and closing the eyelid protects from injury

Question 11

Lacrimal apparatus; tears

Answer 11

Tears have an essential role in eye function. They protect the eye from infection and lubricate movements of the eye
Tears are secreted under the control of the trigeminal nerve (CN V)
Tears contain IgA and lysozymes (immune function)
Tears are produced by the lacrimal gland and are swept across the eye by ‘blinking’. They enter the lacrimal punctum and then enter the nose via the nasolacrimal duct
Tears have an emotional function – release / alert (the composition of tears changes with type of emotion)

Question 12

Light

Answer 12

The eyes are responsible for detection of visible light (part of electromagnetic spectrum)
* Wavelengths are 400-700 nano meters (nm) and exhibit colour depending on wavelength
* Objects appear the colour of wavelength that is reflected
* White light is a combination of all the colours of the visible spectrum
* To achieve clear vision light must be focused on to the retina. This involves:
- Refraction of the light rays
- Accommodation of the eyes
- Changing the size of the pupils

Question 13

Refraction

Answer 13

Refraction describes the ‘bending’ of light rays. It occurs when light moves from one substance to another that has a different density.
- The cornea and, to a lesser extent, the lens refract light rays helping to focus the image on the retina
- Images focused on the retina are inverted and left-right reversed too. The brain learns to coordinate this
- When objects are closer than 6m the light rays need to be refracted more if they are to be focused on the retina. This is achieved by ‘accommodation’

Question 14

Accommodation

Answer 14

As lens curvature becomes greater, its focusing power increases.
* The lens of the eye is convex on its anterior and posterior surfaces. The lens will refract incoming light towards each other.
* The lens can change its refractive power to project a sharp image on to the retina:
1. When an object is close, the ciliary muscle contracts. This reduces suspensory ligament tension & the lens becomes more convex = greater refraction
2. When an object is distant, the cilairy muscle relaxes, increasing the tension on the suspensory ligaments & flattening the lens = less refraction
* Looking at near objects tires the eyes (use of cilairy muscle)

Question 15

Pupil Size

Answer 15

• Contraction of the circular muscle fibres of the iris constricts the pupils
• Contraction of the radiating (radial) muscle fibres of the iris dilates pupils
• Sympathetic stimulation contracts the radial muscle fibres to dilate pupils
• Parasympathetic stimulation contracts the circular muscle fibres to constrict pupils

Question 16

Eyeball Convergence

Answer 16

Convergence describes the medial movement of two eyebalss so that both are directed towards the object
* Extra-ocular muscle move the eyes in a coordinated way under autonomic control
* The closer the object, the greater the eye rotation
* If convergence is not complete, two images are sent to the brain leading to double vision (diplopia)

Question 17

Binocular vision

Answer 17

• In humans, both eyes focus on one object which allows for the perception of depth and 3D nature
• The two images from the two eyes are fused in the cerebrum so that only one image is perceived.
• As the image highlights, the optic nerve crosses over in the brain. This location is known as the optic chiasma
• The optic chiasma is located next to the pituitary gland, which is why pituitary tumours can cause ‘tunnel vision’.

Question 18

The Retina

Answer 18

The retina is the inner layer of eye, lining the posterior ¾ of the eyeball.
* The retina is the beginning of visual pathway
* Can be viewed through an ophthalmoscope
* Only place where blood vessels and a nerve can be viewed
* Optic disc = location where the optic nerve exits the eyeball - ‘blind spot’.
* Bundled together with the central retinal artery and vein

Question 19

Retina – layers / cells

Answer 19

• The retina contains a pigmented layer of melanin-containing epithelial cells and a layer of ‘photoreceptors’ – specialised visual cells
• Rod cells = 120 million. Allow us to see in dim light but don’t provide colour. Instead rod cells provide black, white and greyscale.
• Cone cells = 6 million. Produce colour vision. There are 3 types of cone cell: Blue, red and green. Colour vision results from combinations
• The optic disc is also called the ‘blind spot’ because it contains no rod or cones

Question 20

Macula Lutea

Answer 20

The macula lutea is a yellowish spot at the exact centre of the retina
* Fovea centralis = a small depression in the centre of the macula lutea that contains only cone cells
* The fovea centralis is the area of highest visual acuity (this is why we move our eyes and head to look at things)
* Rod cells are more plentiful at the periphery of the retina

Question 21

Photopigments

Answer 21

• Rod and cone cells form invaginations of their cell membrane to create stacks of ‘discs’ (folds to increase surface area)
• Photo-pigments are transmembrane proteins within the discs of both rod and cone cells
• When light hits the photo-pigment it changes shape initiating an action potential
• These pigments are derived from vitamin A
• Once activated, a photo-pigment must then be regenerated / restored
• The pigmented layer of the retina stores a large quantity of vitamin A, which contributes to the regeneration of rod cells

Question 22

Vitamin A

Answer 22

Vitamin A derivatives are called retinoids. Retinoids include retinol, retinal and retinoic acid.
* In the retina, Vitamin A in the form of retinal binds to a protein called ‘opsin’ to produce photopigments
* Preformed vitamin A is found in foods of animal origin (i.e. liver, egg yolk)
* Carotenoids are precursors of vitamin A. They are metabolised to retinol in the intestines. Carotenoids are also powerful antioxidants; Carotenes are found in fruits/ veg such as carrots, sweet pot, leafy greens, squash, mango
* These carotinoids are not considered toxic (absorption is regulated), unlike preformed vitamin A

Question 23

Photopigments; light and dark adaptation

Answer 23

Light and dark adaptation:
* When you emerge from dark to light, the eyes adjust quickly = light adaptation. The visual system adapts in seconds by decreasing its sensitivity
* Going from light to dark sensitivity increases slowly over some minutes. Adaptation occurs quicker if abruptly moving into light
* Rods and cones must regenerate photo-pigments (cones regenerate within the first eight minutes, rod cells take much longer)