Special Senses Flashcards
What is the stimulus for vision?
Light
What is special about light that results in vision?
The only thing special about light is that we have special receptors that can interact with light.
What is the units for visual wavelengths?
Nanometers
nm
What wavelengths are our photoreceptors receptive to?
Approximately 400-750nm.
What do we perceive variations in wavelength as?
Changes in colour
What do we perceive variations in amplitude of wavelengths as?
Changes in brightness/intensity of light
What is the job of the visual system?
To detect visible radiation and use it as a basis for formation of visual perceptions of the world to guide behaviour
What is the special structure for light?
The Eye
What makes special senses special?
They are all a result of a unique structure that house the particular receptor - whereas other systems have the receptor all over the body.
What are the two basic components of the eye and their job?
An optical component that has the main job of collecting and focusing light into the plane of the retina.
A neural component that converts energy of light into patterned change of membrane potential that OTHER parts of the brain can decode to generate visual perceptions.
What is the retina part of?
The Brain / part of the CNS.
Why do we have a blind spot?
Because there are no photoreceptors on top of the optic disc where the optic nerve opening is (retina is not continuous).
Where is the optic nerve located?
Back of the eye on the side closest to the nose.
What is the sclera?
The whites of the eye.
it is a fibrous capsule.
What is the pigmented disc of the eye?
The iris - what gives eye its colour.
What is the black spot in iris?
Pupil
Pupil is perceive as a black spot but it is actually a hole in the iris. What we are seeing is black pigment at the back of the eye.
What prevents internal reflections?
The black pigment at the back of the eye (what we see are the pupil).
Black absorbs all wavelengths and reflects nothing therefore no internal reflections. Because there is no light reflecting off the black pigment it means we can localise where light is coming from = locality.
What is the Cornea?
The transparent structure in front of the iris.
Transparent because it is avascular (meaning it does not have any blood vessel and gets nutrients via transfusion from surrounding vessels).
Draw the major structures of the eye.
Refer to slide image.
Should include:
Cornea
Aqueous humor
Anterior chamber
Iris
Lens
Posterior Chamber
Zonule Fibers
Cilary body
Sclera
Viteous humor
Retina
Fovea
Optic disc
Optic nerve
What is the anterior chamber?
Structure that is filled with aqueous humor fluid.
It keeps pressure in the eye.
What continuously produces aqueous humor?
Cilary body.
As aqueous humor is continuously produced it is also continuously drained away.
What are the characteristics of the lens?
A fibrous capsule filled with crystallised cells.
It is avascular.
It is slightly elastic and flexible meaning the shape of the lens can change.
What are ligaments attaching the lens to the ciliary body called?
Zonule Fibres.
What is composition of the ciliary body?
Smooth muscle.
What is the vitreous humor?
The majority of the eye.
Mostly a solution of transparent protein - no tissues of cells in this area.
What is the optic disc?
Head of the optic nerve - where axons leave the back of the eye and go to visual centres in the brain.
What is the fovea?
A area of the retina.
It has the highest 2 point discrimination - bit of the brain/retina we use when we focus on something.
What is refraction?
Bending of light
What does refraction result in?
Upside down and back to front.
What is refraction related to?
The difference between the refractive indices of the two media, and to the curvature of the refractive surface.
What is the refractive index of water and why is this important?
1.333
Light coming from the air first hits water (tear drops that surround our eye) - the refractive index of air is 1.000. This change means that majority of refraction occurs when light goes from air to water because it is the biggest change in refractive index - most other areas of the eye have about the same refractive index as water.
What is refractive power?
Refractive power is the reciprocal of focal length in meters.
What is refractive power measured in?
Diopters
What is the focal point of a “relaxed” eye?
About 1.7cm (1/60).
Because the relaxed eye has a total refractive power of about 60 diopters.
What would the effect of a lens having more diopters be?
The more diopters a lens has, the closer it brings the light to a focus.
What are the three process of the “near response” that occur as you shift your gaze from distant to near objects?
(1) Accomodation
(2) Constriction of pupil
(3) Convergence of eyes
What is accomodation?
Contraction/relaxation of ciliary muscle to alter lens shape and change refractive power
What is happening during convergence of eyes?
(E.g., cross eyed).
Convergence of the eyes refers to the coordinated movement of both eyes towards each other to maintain focus on an object as it moves closer to the observer.
Object remain in register on corresponding parts of the two retinae.
What is the autofocus mechanism called?
The near response (it is the way we change refractive power of optical components).
What happens to refractive power when the lens becomes more round?
refractive power increases
If we want to bring the image forward what do we need to do to refractive power?
Adding refractive power brings the image forward.
When you increase the refractive power of the lens, you’re essentially increasing its ability to bend light rays. This means the light rays converge more sharply, bringing the focal point closer to the lens.
What shape is the lens when the ciliary muscle is relaxed?
Flat = shallow curvature
Is the ciliary muscle relaxed when looking at something close up or looking at something far away?
Far away
What is happening to the zonular fibres when the ciliary muscle is relaxed?
They are taut
When gaze shifts to close up objects what happens to the parasympathetic activity and beyond that allows you to see close up object?
It increases the parasympathetic activity of the ciliary muscle causing it to contract.
Tension is removed from zonular fibers.
Lens becomes more spherical due to natural elasticity.
Eyes total refractive power increase from about 60 up to about 75 diopters.
What happens to the size of the pupil when you look at something close up and why?
Decreases in size because ciliary muscle contracts.
What are 5 optical defects?
Myopia
Hypermetropia
Astigmatism
Presbyopia
Cataract
What is emmetropia?
Normal vision - 20/20
What is myopia?
Short sighted.
Myopia occurs when the eyeball is too long or the cornea is too steep, causing light rays to focus in front of the retina rather than directly on it. As a result, distant objects appear blurry, while close objects can be seen more clearly.
To correct myopia, concave (diverging) lenses are used. These lenses help to diverge light rays before they enter the eye, allowing them to focus correctly onto the retina. This effectively shifts the focus point back onto the retina, improving distance vision.
What is hypermetropia?
Far sighted.
Hypermetropia occurs when the eyeball is too short or the cornea is too flat, causing light rays to focus behind the retina rather than directly on it. This results in blurry vision, particularly when looking at close objects, while distance vision may remain relatively clear.
To correct hypermetropia, convex (converging) lenses are used. These lenses converge incoming light rays before they enter the eye, allowing them to focus correctly onto the retina. This brings the focus point forward onto the retina, improving both distance and near vision.
What are optical defects?
Abnormalities in the shape and size of the eyeball.
What is astigmatism?
Where the eye ball is aspherical (more of a rugby ball than a soccer ball).
The result of this is that there are different amounts of refraction in different planes (different refraction dependant on where the light comes from).
What corrects astigmatism?
Cylindrical lens
What is presbyopia?
Age related loss of accommodation = caused by lens losing elasticity.
What does the elasticity of the lens fall from children –> 45/50 year –> 70 year and the consequence on near point?
Therefore accommodation falls from about 15 diopters in children to 2 diopters at around 45-50 years and essentially 0 at 70.
recedes from 10cm at 20 years to 80cm at 60 years.
What lens is required to correct presbyopia?
Convex - restore near vision
What is a cataract?
Where the lens becomes opaque due to the lens being broken up inside lens capsule.
Occurs predominately with age but some children (Alice) are born with it.
How is a cataract fixed?
Plastic lens - but plastic is not flexible so therefore no accommodation so require convex lens.
Where is the neural circuit of the eye?
Retina
Where in the neural circuit are photoreceptors (light sensitive cells) located?
Furtherest from the light.
What are the three interneurons in the eye neural circuit?
Bipolar cells
Amacrine cells
Horizontal cells
What are the retinal ganglion cells?
The only cells within eye neural circuit with axons. They send information from retina to other visual centres in the brain.
What cells are pushed out in the Fovea Pitt and what is the effect of this?
Ganglion and interneuron cell bodies pushed out to side.
Means that photons have a more direct path to reaching photoreceptors as less intervening substances that may reflect light.
What are the proportion of Rods and Cones in eye?
More Rods than Cones in retina.
But less rods than cones in fovea.
What are the two types of photoreceptors?
Rods
Cones
Are rod or cones used for night vision?
Rods function in low light
Do rods or cones detect colour?
Cones - require relatively hight light levels - day and twilight vision.
What are the three colours types that Cone photoreceptors detect?
Red, Green and Blue light
Why do rods have bigger surface areas than cones?
Rod are made up of individual free floating discs whereas cones are made up of folding membrane.
What are photopigments?
Proteins that are sensitive to different wavelengths of light - I is what gives photoreceptors the capacity to response to light.
What are the two components that photopigments consist of?
Opsin (membrane spacing protein)
Retinal (chromophore)
What type of opsin do rods have?
Rhodopsin
What type of opsin do cones have?
Photopsin
Either Short (Blue), Medium (Green) or Long (Red)
What is retinal a derivative of?
Vitamin A
Describe phototransduction in the dark:
Phototransduction in the dark is essentially the excitatory process.
The inactive retinal is in its 11-CIS isoform.
We have enzymes in the eye making lots of cyclic GMP.
cGMP gated channels open and there is a influx of cations including Na (sodium).
Positive charge coming in means that photoreceptors are a little depolarised due to the cGMP.
Depolarisation results in releasing lots of neurotransmitter glutamate onto the interneuron terminals.
Describe phototransduction in the light:
Phototransduction in the light is essentially the inhibitory process.
Light energy is absorbed and used to convert retinal from CIS to TRANS isomer. TRANS is the active isoform.
cGMP is broken down - this results in less cGMP and gated channels close. Therefore less Na influx, photoreceptors hyperpolarised and less glutamate released.
What is the one thing that light does?
Converts retinal from CIS to TRANS form.
Everything else in the eye is all a consequence of this and not because of light itself.
What are the only cells in the retina that fire AP?
Retinal ganglia cells.
What is the perception of colour created by?
Relative activation of the three cone types of photopigments (S, M, L).
How can you become colourblind?
Opsin is a protein which is a product of a gene - therefore colour blindness can be inherited. Can also be acquired due to disease.
Why are males more likely to be colour blind than females?
Colourblindness affects 8% of males and 0.5% of females.
This is because the gene encoding production of M and L photopsin are on the X chromosome. Therefore males only get one of them so if have it, it will be expressed.
What do electrical responses of ganglion cells depend on for retinal processing?
The spatial and temporal pattern of light stimulation on the retina.
What role does receptive fields have in retinal processing?
The input to each ganglion cell arises from neighbouring photoreceptors in a circumscribed area of the retina = the cells receptive fields.
RF are roughly circular and divided into two parts = central and the surround.
Ganglion cells respond optimally to the differential illumination of the centre and the surround.
What is light illumination?
Contrast in the light - enables us to see shapes.
What are the two types of ganglion cell receptive fields?
On-centre and off-centre.
If you have an on-centre RF and light falls on the surround what happens?
If more light on the surround you inhibit AP.
If more light in centre then it is excited.
The opposite is true for off-centre RF.
What is the effect of large receptive fields on acuity?
Low acuity (Acuity meaning 2 point discrimination).
What is the pathway of information from the eye to the primary visual cortex?
Retina
Optic Chiasm
Optic Tract
Material Geniculate Nucleus
Primary Visual Cortex
If someone is in your left visual space where do axons from each eye go?
If in left visual space light reflects to the nasal of left eye and temporal of right eye.
Information is brought together on the same side of the brain due to different crossing pathways at chiasm - ipsilateral (same) projection of axons from right eye and contralateral (opposite) projection from left eye.
Means that information concerning images from visual space falling on equivalent parts of the two retinas are brought together for central processing.
Axons got to the primary visual cortex at the dorsal end of the brain for first visual processing.
What are the four subcortical visual nucleus that ganglion cell axons project to?
Superior Colliculus
Lateral Geniculate Nucleus
Pretectum
Suprachiasmatic Nucleus
What is the Superior Colliculus?
The subcortical visual nucleus that sends information of head movement to move eyes along with it to give stable vision.
Concerned with eye movements and orientation to visual stimuli.
What is the Lateral Geniculate Nucleus?
The subcortical visual nucleus concerned with the sensation of vision.
What is the Pretectum?
The subcortical visual nucleus that is concerned with the control of pupils - for focus.
What is the Suprachiasmatic nucleus?
The subcortical visual nucleus that controls diurnal rhythms (senses daylight).
What is sound?
Pressure waves in the air