Eyes Flashcards
Special Senses
These are:
- Vision
- Smell
- Taste
- Hearing
- Equilibrium
Special sensory receptors
- Distinct receptor cells localized in the head region
- Used by all special senses
General receptors
- Modified nerves
- Mediates general senses such as the sense of touch
Vision
- This is the dominant sense
- 70% of the body’s sensory receptors are found in the eye
- 30% of cerebral neurons process visual information
Accessory structures of the eye
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
Eyelids
- or Palpebrae
- These with eyelashes, protect the eye from physical danger and drying out
Conjunctiva
- Transparent membrane
- Produces lubricating mucus to prevent drying out
- Found over the sclera
Lacrimal apparatus
- Has lacrimal glands to secrete tears (dilute saline solution)
- Has small ducts to drain excess fluid to the nasolacrimal ducts
Lacrimal fluid
- 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
Eyeball wall & structure
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
Fibrous layer
Two regions;
- Sclera: opaque white
- Cornea: clear and allows light into the eye
Aqueous humours
- In the anterior cavity
- Supplies nutrients and oxygen to the lens and cornea
- Carry away waste
Vitreous humours
- In the posterior cavity
- They transmit light
- Supports the posterior lens
- Holds the retina against the pigmented layer
- Maintain intraocular pressure
Vascular layer
- 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
Inner layer
- 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)
Optic disc
Found in the neural layer of the retina where the optic nerve exits the eye (no room for photoreceptors)
Rods
- Highly sensitive to light
- Best suitable for night vision
- Cannot resolve colour or sharp objects
- Found in the peripheral vision
- More numerous than cones
Cones
- Less sensitive to light
- Best adapted to bright light
- Provides high-resolution colour vision
- Humans have 3 kinds of cones
Glaucoma
- 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
Cataracts
- Clouding of the lens
- Causes dim or blurry vision, faded colours and trouble with bright light and seeing at night
Distant vision
- 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
Lens
an avascular, biconvex, transparent, flexible structure that changes shape to allow the precise focusing of light on the retina
Close vision
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
Myopia
- 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
Hyperopia
- 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
Astigmatism
This is the uneven curvature in the cornea or lens resulting in seeing blurred images
Retina
- 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
