L22 - The Visual System: The Eye And Retina Flashcards
What does vision allow us to do?
Detect prey
(in modern world = source food)
Detect predators
(in modern world = danger)
Detect mates
Communicate
What 3 things does light have?
- A wavelength - distance between peaks or troughs
- A frequency - number of waves per second
- An amplitude - difference between wave peak and trough
Electromagnetic lights travels in straight line until it interacts with atoms and molecules. What 3 interactions can occur?
Reflection
Absorption
Refraction
What are the 6 main parts of the eye?
- Pupil
- Iris
- Cornea
- Sclera
- Extraocular muscles
- Optic nerve (cranial nerve II)
What is the role of the pupil?
Pupil lets light inside the eye.
What is the role of the iris?
They contain muscle which control the amount of light entering the eye.
What is the role of the cornea?
It is a glassy, transparent covering of the pupil and iris that refracts light.
What is the role of the Sclera?
It is continuous with cornea, forms the tough, protective wall of the eyeball to give its shape.
What is the role of the extraocular muscles ?
They move the eyeball, controlled by oculomotor nerve (cranial nerve 3).
What is the role of the optic nerve (cranial nerve II)?
It carries axons from retina to brain
What is the optic disk?
Origin of blood vessels and optic nerve, cannot sense light (blind spot)
What is the Macula?
It is a region of retina for central vision, devoid of large blood vessels to improve quality.
What is the Fovea?
Retina is the thinnest here and is the area of highest visual acuity. The fovea gives us the highest resolution image possible
What does the retina contain?
The retina contains the sensory receptor cells and afferent neurons.
How is the lens positioned in the eye?
The lens is suspended by zonal fibres (suspensory ligaments) which are attached to the ciliary muscle, enabling stretching of the lens.
Where in the eye does refraction occur?
Refrcation occurs 80% at the cornea and 20% at the lens.
What is the degree of refraction determined by?
Determined by:
* Difference in refractive indices between the two media
* The angle at which light hits the interface between these two media
What is refractive index?
Refractive index is essentially a measure of speed of light within it – so light moves quicker through air (1.0003) than the cornea (1.376) due to the increased density of the cornea.
Explain the process of refraction by the cornea
- Light is recieved at the cornea through air but the cornea is mainly water.
- Light travels more slowly through water that air due to higher density so refraction occurs.
What is focal distance?
Distance from refractive surface to convergence of parallel light rays
What structural difference takes place when lens focuses on distant and closer object?
- Rounding of the lens
- Contracted ciliary muscles
- Slackened (looser) suspensory ligaments
increases the refractive power to focus closer objects on the fovea.
How does the lens adjust according to the distance of the object?
- Distant objects have almost parallel light rays so the cornea provides sufficient refraction to focus them on the retina
- Closer objects (<7m away) have non parallel light rays which requires additional refraction to focus them on the retina which is provided by the fattening of the lens
What are the problems with focussing?
- Farsightedness (Hyperopia)
- Near (short) sightedness (Myopia).
What happens in farsightedness?
- eye is too short
- near objects are focussed behind the retina
- not enough refraction
How does reading glasses help those with hyperopia (farsightedness)?
- make light rays more parallel
- requires less refraction
Solution for farsightedenss.
What happens with myopia?
- eye is too long
- distant objects are focussed before retina
- too much refraction
How does glasses help those with myopia?
- make light rays diverge
- requires more refraction
What must light pass through before it reaches the photo receptors?
Light must pass through the ganglion cells and bipolar cells before it recahed the photoreceptors.
What happens to some of the light that doesn’t get absorbed by the photoreceptors?
The pigmented epithelium absorbs the excess light
What does emmetropic mean?
“emmetropic” describes a state of having normal, clear vision.
What are the different layers of cell of the retina?
- Ganglion cells
- Amacrine cells
- Bipolar cells
- Horizontal cells.
What do ganglion cells do?
They output information from retina.
What do Amacrine cells do?
They modulate information transfer between ganglion cells and bipolar cells.
What do bipolar cells do?
They connect photoreceptors to ganglion cells.
What do horizontal cells do?
They modulate information transfer between photoreceptors and bipolar cells.
What are photoreceptors?
They are sensory transducers, both rods and cones.
What is duplicity theory?
We can’t have high sensitivity and high resolution in single receptor.
Where would you find the photopigments?
They’re found within the membranous disks that contain light-sensitive photopigments that absorb light
How many rods cells are estimated to be in the human retina?
Approx. 92 million rods in each human retina.
What does resolution (visual acuity) and sensitivity mean?
- Sensitivity - ability of the retina to detect very dim light
- Visual acuity - ability of the retina to distinguish fine details and perceive sharp images
Why do rod photoreceptors have low visual acuity/resolution?
- Greater number of disks
- Higher photopigment concentration
- 1000 times more sensitive to light than cones.
- Enable vision in low light (scotopic) conditions that is at night time.
Why do cone photoreceptors have high visual acuity/ resolution?
- Fewer disks
- Used during daylight (photopic) conditions.
- Enable colour vision
- Lower sensitivity.
How many cones/rod cells does the fovea contain?
Fovea contains 5 million cones and no rods.
Describe the convergence, sensitivity and resolution in the central/peripheral retina
Central retina has low convergence, low sensitivity, high resolution.
Peripheral retina has high convergence, high sensitivity and low resolution.
