Eyes

Question 1

Special Senses

Answer 1

These are:
- Vision
- Smell
- Taste
- Hearing
- Equilibrium

Question 2

Special sensory receptors

Answer 2

• Distinct receptor cells localized in the head region
• Used by all special senses

Question 3

General receptors

Answer 3

• Modified nerves
• Mediates general senses such as the sense of touch

Question 4

Vision

Answer 4

• This is the dominant sense
• 70% of the body’s sensory receptors are found in the eye
• 30% of cerebral neurons process visual information

Question 5

Accessory structures of the eye

Answer 5

Consists of:
- Eyebrow
- Conjunctiva
- Eyelid
- Lacrimal apparatus
- Extrinsic eye muscles
- Eyeball 1/6 of the eye is visible
- most of the eye is enclosed and protected by fat cushioning and bone orbit

Question 6

Eyelids

Answer 6

• or Palpebrae
• These with eyelashes, protect the eye from physical danger and drying out

Question 7

Conjunctiva

Answer 7

• Transparent membrane
• Produces lubricating mucus to prevent drying out
• Found over the sclera

Question 8

Lacrimal apparatus

Answer 8

• Has lacrimal glands to secrete tears (dilute saline solution)
• Has small ducts to drain excess fluid to the nasolacrimal ducts

Question 9

Lacrimal fluid

Answer 9

• Contains mucus, antibodies and lysozyme to cleanse, moisten and protect the eye
• Eyeball movements are controlled by 6 extrinsic eye muscles that are innervated by the abducens and the trochlear nerves

Question 10

Eyeball wall & structure

Answer 10

The walls of the eye have 3 layers
- Fibrous layer
- Vascular layer
- Inner layer
- They enclose the internal cavity filled with humour (fluid) and maintain shape

Question 11

Fibrous layer

Answer 11

Two regions;
- Sclera: opaque white
- Cornea: clear and allows light into the eye

Question 12

Aqueous humours

Answer 12

• In the anterior cavity
• Supplies nutrients and oxygen to the lens and cornea
• Carry away waste

Question 13

Vitreous humours

Answer 13

• In the posterior cavity
• They transmit light
• Supports the posterior lens
• Holds the retina against the pigmented layer
• Maintain intraocular pressure

Question 14

Vascular layer

Answer 14

• Choroid: the vascular middle layer of tissue that nourishes the eye layers
• Ciliary body: has smooth muscle that encircles the lens, determines its shape and controls the amount of light entering the eye
• Iris: surrounds the pupil, acting reflexively to control the pupil size and the amount of light entering the eye

Question 15

Inner layer

Answer 15

• This is the retina
  Contains:
• Photoreceptors, rods, cones, bipolar cells, ganglion cells and glia.
• Lateral to the optic disc is the macula lutea with a pit in the centre (fovea centralis) which only contains cones making it the region with the best visual acuity (sharpness)

Question 16

Optic disc

Answer 16

Found in the neural layer of the retina where the optic nerve exits the eye (no room for photoreceptors)

Question 17

Rods

Answer 17

• Highly sensitive to light
• Best suitable for night vision
• Cannot resolve colour or sharp objects
• Found in the peripheral vision
• More numerous than cones

Question 18

Cones

Answer 18

• Less sensitive to light
• Best adapted to bright light
• Provides high-resolution colour vision
• Humans have 3 kinds of cones

Question 19

Glaucoma

Answer 19

• Condition where the drainage of aqueous humour is blocked
• Causes fluid backup and increase in intraocular pressure
• Pressure may increase, compress the retina and optic nerve which could lead to blindness

Question 20

Cataracts

Answer 20

• Clouding of the lens
• Causes dim or blurry vision, faded colours and trouble with bright light and seeing at night

Question 21

Distant vision

Answer 21

• Ciliary muscles are relaxed which causes the maximal flattening of the lens
• Far point of vision: this is the distance beyond which no change in lens shape is required (6m or 20ft from the eye)
• Suspensory ligaments: stretched

Question 22

Lens

Answer 22

an avascular, biconvex, transparent, flexible structure that changes shape to allow the precise focusing of light on the retina

Question 23

Close vision

Answer 23

Three adjustments for close vision:
- Accommodation of the lens to thicken and increase light refraction
- Construction of pupils to direct light to the lens
- Convergence of eyeballs allowing objects to remain focused on the fovea
- Near point of vision: Occurs a the point of maximal thickening of the lens 10cm or 4in from the eye)
- CIliary muscle contracted
- Suspensory ligaments loosened

Question 24

Myopia

Answer 24

• Nearsightedness
• Objects focus in front on the retina resulting in seeing close objects fine and distant objects are blurred
• Corrective lens: concaves with thick edges with the diverging effect

Question 25

Hyperopia

Answer 25

• Farsightedness
• Objects focus behind the retina resulting in seeing far objects fine and close objects are blurred
• Corrective lens: convexes with a thick centre and a converging effect

Question 26

Astigmatism

Answer 26

This is the uneven curvature in the cornea or lens resulting in seeing blurred images

Question 27

Retina

Answer 27

• Light passes through the cornea, aqueous humour, lens, vitreous humour, neural layer and photoreceptors to be detected
• Light is refracted entering the cornea, entering and leaving the lens
• Refraction occurs at the cornea, but can’t change focus