Lecture 14 Flashcards
Vision allows humans (and animals) to…
Detect prey (in modern world = source food)
- mainly predator: eyes on front for better focus
Detect predators (in modern world = danger)
- mainly prey: eyes on sides to increase peripheral vision
Detect mates
Communicate
How much of the human cerebral cortex is involved in analyzing the visual world?
More than 1/3
What is light?
Electromagnetic radiation that is visible
What does light have?
A wavelength - distance between peaks or troughs
A frequency - number of waves per second
An amplitude - difference between peak and trough
How does light rays interact?
Reflection
Absorption
- transfer of light energy to a particle or surface
- occurs in some retinal cells
- some cells absorb light to ensure light doesn’t bounce around and produce mixed signals
Refraction
- bending of light rays as they move from one transparent material to another e.g. air to water
How does refraction form images in the retina?
Refraction occurs because the speed of light differs between mediums
e.g. slower through water than air
The greater the difference in speed in the two media, the greater the angle of refraction. Refraction occurs towards a line that is perpendicular to the border
What does the pupil do?
Lets light inside the eye
What does the iris do?
Contains muscles which control pupil size
What does the cornea do?
Glassy, transparent, covering of the pupil and iris
What does the sclera do?
Continuous with cornea, forms the tough wall of the eyeball
What do the extraocular muscles do?
Move the eyeball
- controlled by cranial nerves III, IV and VI
What does the conjunctiva do?
Membrane underneath eyelids that attaches to the sclera
What does the optic nerve?
Carries axons from retina to brain
What is a type of eye disorder?
Strabismus
- Imbalance in the extraocular muscles of the eyes
- Esotropia = convergence of the eyes, cross-eyed when muscles pulling outwards are weakened
- Exotropia = divergence of the eyes, wall-eyed when eye muscles falling inwards are weakened
What is the optic disc?
Origin of blood vessels and optic nerve, cannot sense light
What is the macula?
Region of retina for central vision, devoid of large blood vessels to improve vision quality
What is the fovea?
Retina is thinnest here where acute vision occurs (cone cells)
What is the aqueous humor?
A watery fluid that provides nutrients to the cornea and the lens
What is the lens suspended by?
zonal fibers (ligaments) which are attached to the ciliary muscle
What is the vitreous humor?
A viscous humor that keeps the eyeball attached
How is the aqueous humor produced and absorbed?
Normally produced by the ciliary body and absorbed by the canal of Schlemm (between cornea and sclera)
What does the aqueous humor contain?
Nutrients e.g. glucose
What kind of disorders can increased aqueous humor and functioning cause?
Cataracts (白内障）
- clouding of the lens
- can occur due to changes in the composition of the aqueous humor
- increased risk with age, smoking or diabetes mellitus
- can be treated by lens replacement
Glaucoma (緑内障）
- Slowed uptake of aqueous humor leads to elevated intraocular pressure
- causes compression of the optic nerve and reduced blood supply to the retina
- leads to a progressive loss of vision from periphery inwards
- - aqueous humor needs to be constantly recycled but it is no longer taken out of Schlemm
- - - pressure pushes eye out
- - - starts to affect peripheral vision but people start noticing when central vision becomes affected
Where must the light rays be focused on?
Retina
How are the light rays focused on the retina?
The cornea: 80% of total refraction
The lens: 20% of total refraction
How does refraction occur by the cornea?
Light arrives at the cornea through air but the cornea is mainly water
Light travels more slowly through water than air = refraction occurs
Light that hits the cornea directly perpendicular just moves straight through onto the retina
Why do things look blurry underwater?
Refractory index underwater is similar to the one for the cornea - the refractive power of the cornea is eliminated as light passes through water into a similar substance
What is focal distance?
Distance from the refractive substance ( the cornea), to where the parallel - light rays converge
If the distance between the cornea to the retina is 2.4 cm, what is the refractive power of the cornea?
42 diopters
What happens when light hits the cornea directly perpendicular?
Moves straight through onto the retina
What is accommodation by the lens?
Light rays from distant objects are almost parallel by the time they reach the eye
But light rays from nearer objects are not parallel
- needs additional focusing power to focus them on the retina
The lens provide greater refractive power to focus these closer (<9m away) objects
What happens at the far point?
Ciliary muscles relax, zonal fibers stretch, lens flattensq
What happens at the near point?
Ciliary muscles contract, less tension in zonal fibers, lens becomes more spherical
Since the lens is naturally elastic…
If not stretched, it will become more spherical
What happens when the lens becomes rounder?
Increases the refractive power
What is hyperopia? (Far sightedness)
Eye is too short
Near objects are focused behind the retina
Not enough refractive power
Correction: Convex lens straighten out light
What is myopia? (near sightedness)
Eye is too long
Far objects are focused after the retina
Too much refractive power
Correction: concave lens makes light more divergent
What does the Pupillary light reflex allow?
Iris regulates light levels
- pupillary constrictor (smooth circular muscle)
- regulated by parasympathetic system
- pupillary dilator (smooth radial muscle)
- regulated by sympathetic system
Why is the pupillary light reflex useful?
Enables us to adjust to changes in light intensity
- increase in focus (clearer image)
What is the consensual light reflex? (occurs in both eyes even if only one is stimulated)
Shining light in each eye
- to see if both pupils will constrict
- if only one constricts, no proper function in brain
- sensory (afferent) neuron
- optic nerve (cranial nerve II)
- both Edinger-Westphal nuclei are innervated by sensory input from one eye
- Effector (efferent) neuron
- oculomotor nerve (cranial nerve III)
- sympathetic neurons via the superior cervical ganglion
