Vision Flashcards
What are 3 functions of the eye?
- Provide information about the shape and colour of objects to allow recognition of what the object is
- Provide information about where the object is and how it is moving in 3D space
- Provide information about changes in absolute levels of illumination, which enables the animals to synchronise their circadian and circannual rhythms with changes in the environment.
What features of the cornea and lens are essential to vision?
For the eye to function effectively, the cornea and lens have to be transparent and devoid of blood vessels that would scatter the light passing through them and degrade the quality of the retinal image.
If the lens and cornea are devoid of blood vessels, how are they supplied?
- Composed of living cell that require oxygen and nourishment and removal of waste products.
- Provided for by continual circulation of aqueous humor, that fills that anterior eye chamber.
- Produced by filtration of blood through the walls of the blood capillaries in the pupil to form the aqueous humor.
How is aqueous humor reabsorbed?
Reabsorption of fluid occurs into a venous plexus around the edge of the anterior chamber.
Describe the balance of fluid filtration and absorption of aqueous humor.
Balance is vital for eye health.
A slight positive pressure is important for maintaining the shape of the eyeball, but too high an intraocular pressure can impair vascular perfusion of the retina, leading to ischaemic damage and physical damage, increasing the risk of a detached retina.
Describe the pupil in dim light.
Sympathetic efferent activity causes the radial muscle fibres of the pupil to contract increasing its diameter. This is mydriasis and allows more light into the eye in dim lighting conditions to maximise sensitivity of the visual system.
Describe the pupil in bright light.
Parasympathetic efferent activity contracts the circular muscle constricting the size of the pupil and reducing the amount of light entering the eye. This is known as miosis and is important to prevent the temporary saturation of the photoreceptors, for example by dazzling bright light when emerging from a burrow or moving from deep shade to sunlight.
What happens to ganglion cells as light level increases?
- As light levels increase, the activity of melanopsin containing retinal ganglion cells that project to the pretectal nucleus of the midbrain increase.
- Neurones in the pretectal nucleus then project to the parasympathetic motor nucleus of the oculomotor nerve, which is part of the Erdinger Westphal nucleus.
- This conveys excitation via parasympathetic preganglionic fibre to the ciliary ganglion and then via postganglionic parasympathetic fibres to constrict the iris.
Describe the pupillary light reflex.
Bilateral. Direct response in the eye that receiving light stimulation is more prominent than the consensual response in the contralateral eye, due to decussation at the optic chiasm and then contralateral projections from the pretectal nucleus to the parasympathetic nucleus of the oculomotor nerve. Important diagnostic test that can distinguish dysfunction of afferent or efferent components of the pathway.
What is indicated in the presence of consensual response but absence of direct response?
Indicates that the sensory component and pretectal nucleus on the stimulated side is intact, but that there is a dysfunction in the efferent component from the parasympathetic nucleus of the oculomotor nerve to the pupil.
What is indicated in the absence of both consensual and direct responses?
When light stimulates 1 eye but a normal response with stimulation of the other indicates the efferent pathway to the unresponsive eye is intact and that the problem is in the sensory pathway.
Describe the focusing power of the eye.
Most of the focussing power of the eye is due to refraction at the cornea due to the large difference in the refractive indices of air and water. But the power of the cornea is fixed.
How does the eye focus on distant objects?
Change in the shape of the lens:
- The lens is suspended by zonular fibres from the ciliary body, which contains the ring of the ciliary muscle.
- Relaxation of the muscle increases the diameter of the ciliary body exerting tension on the zonular fires.
- This stretches and flattens the lens, allowing it to focus on distant objects.
How does the eye focus on near objects?
Change in the shape of the lens:
- The ciliary muscle is contracted via parasympathetic control, which reduces the tension on the zonular fibres and allows the elasticity of the lens to accommodate to a more rounded and thicker profile.
- This focuses the more divergent light rays from the closer point in the visual field.
What is the other roles of the pupil?
Pupil constriction increases the depth of field of the image on the retina.
Pupil constriction improves the quality of the visual image by reducing the aberration of the optical image caused by rays of light passing through the periphery of the lens.
Distinguish chromatic and spherical aberration.
Chromatic aberration results from different wavelengths pf light being refracted to different extents and therefore brought into focus at different distances.
Spherical aberration results from the shape of the lens and again means that light passing through the peripheral lens is focussed at different distances to light passing through the central region, which degrades image quality.
Compare the fields of vision of predators and prey species.
- Primates ad predators have forward facing eyes with a high degree of binocular vision where the visual fields of the left and right eyes overlap.
- Comparison of the visual images from the 2 eyes enable the perception of depth for objects in the near distance.
- Important for predators that need to catch prey or primates who live in arboreal environments and need to judge distances accurately.
- Depth perception also requires the coordinated movement of the eyes, whereas animals with little binocular overlap have more ability to move their eyes independently.
What are 3 visual cues used in binocular vision?
Relative size of objects learned from experience.
Overlap of images.
Haze - caused by light scattering and a different spectral prolife due to absorption of certain wavelengths by the intervening atmosphere.
What are the photoreceptors present in different species types?
Predominantly rods in nocturnal species and rod and cones in diurnal species.
Describe the density of photoreceptors across the eye.
- The density of photoreceptors is lower in the peripheral retina and increases towards a maximum in a central area called the area centralis.
- The higher density of photoreceptors is mirrored by a higher density of retinal ganglion cells in the area centralis that transmit the information to the brain.
- This higher density of receptors is associated with higher visual acuity – a better ability to distinguish fine spatial detail.
What is the area centralis?
In diurnal species, the area centralis is packed with cones and provide the most acute vision in good light levels.
In most mammalian species, possibly excepting cats, the area centralis cannot be directly visualised using an ophthalmoscope.
What is a fovea?
Species with particularly high visual acuity, like humans, have a fovea. This appears as a depression in the retina, where the cell layers overlying the photoreceptors are displaced to the side to decrease light scattering.
What is the ‘blind spot’?
There are no photoreceptors in the optic disc, which is where the optic nerve leaves the eye.
What is the visual streak?
While predators have area centralis, many herbivores, such as cattle and horses, instead have an elongated strip of higher photoreceptor density called the visual streak. The orientation of the streak is horizontal and enables a high acuity image for horizon scanning to detect predators. Even when horses are grazing, their eye rotates so that the visual streak remains aligned to the horizon.
Define vergence eye movements.
Species with a high degree of binocular overlap rely on vergence movements of the eyes to move the area centralis of each eye to investigate objects at different distances.
Distinguish convergence and divergence eye movements.
Shifting the point of fixation to a closer object requires a convergence movement to swivel the eyes medially in order to maintain the image on the area centralis in each eye.
Conversely, shifting the point of fixation to a more distant image requires a divergence movement in which the eyes move laterally.
Describe velocity and purpose of convergence eye movements.
- 25˚ per second, relatively slow.
- This is slow enough for ongoing control of the eye movements in response to sensory feedback.
- The velocity of movement is initially rapid, but then slows down as the eyes get closer to the new fixation point.
- Convergence of the eyes onto a closer fixation point is associated with accommodation to change the focus of the lens and pupil constriction to increase the depth of field of the retinal image, so that the focus does not have to be precise.
What is saccadic eye movement and which species does it occur in?
- Species with an area centralis tend to make greater use of eye movements than species with a visual streak.
- Saccadic eye movements are used to move the area centralis from 1 part of the visual field to another.
- In these species, the eyes are continually and subconsciously moving around the visual image in what are known as microsaccades.
Compare avian visual field with other animals.
There is a motor loop via the basal ganglia and frontal eye fields that enables to animals to consciously move their point of fixation to explore the visual field. But there is great species variation and head movements are equally important in many species for moving their area of highest acuity around the visual field. Most birds have a relatively little eye movement but a great range of head movement, more than 180˚.
Describe the speed of saccadic eye movements.
- Saccadic eye movements are very rapid, around 500 ˚ per second, far too fast for any ongoing feedback control.
- Instead these are ballistic movements.
- Rapid movements completed very rapidly using motor neurone activity.
Describe the sensory feedback involved in saccadic eye movements.
- If movement is inaccurate, sensory feedback from the eye will drive a correcting microsaccade, after delay while feedback is processed.
- Sensory feedback will also provide parametric feedback to adapt the motor model to ensure the saccade is performed more accurately in the future.
- System works well for saccadic eye movements, as load on the eye does not normally vary, so the outcomes of the movement is highly predictable.
What is the optokinetic reflex of the eye?
Slow steady movements, such as those that track objects in the visual field.
- Tracking movement is followed by a rapid saccade in the opposite direction to rest the position of the eye.
- For objects that leave the visual field, there is no head movement then tracking stops after the saccade.
- If a larger scale movement, sawtooth pattern of the eyes known as nystagmus. The direction of the nystagmus is defined by the direction of the rapid saccades.
Describe the vestibulocular reflex as another form of tracking eye movement.
- Sensory input from the vestibular system moves the eyes in the opposite direction to the head to stabilise the retinal image during head movement,
- If the head keeps rotating, then nystagmus is induced with the same direction as the head rotation.
- This response reduces and eventually disappears if the head continues, due to adaptation of the vestibular system.
- But if movement is stopped, this will again stimulate the vestibular system and induce nystagmus in the opposite direction to the original head movement.
How can optokinetic and vestibulocular reflexes be tested?
Can sit a large dog in an office chair and rotate it to test the vestibulocular reflex.
It is easier for a handler to hold smaller dogs whilst spinning around with them and then get an observer to look for the physiological nystagmus when the movement stops.
Define pathological nystagmus.
Pathological nystagmus, which can either be spontaneous or positional (elicited when the head is held in a certain position). Pathological nystagmus is a symptom of a dysfunction in the neural pathways underlying these tracking responses.
Compare the size and shape of avian eyes.
Avian eyes are typically larger than a similarly sized mammal.
Variety of eyeball shape exists in birds. Eagles and other birds of prey have tubular shapes, the eyes can be moved and accommodated independently in many birds.
Describe the position of the avian eyes.
Predators typically have forward facing eyes, enforcing binocular vision. But eye movements are not so important in birds as they are in mammals, as the avian head is so much more mobile.
Describe the avian nictating membrane.
Birds possess a nictating membrane as do many mammals, but in some migrating birds, it is transparent and protects the eyeball from drying during long migrations.
Describe the avian area centralis and fovea.
Eagles and birds of prey have 5 times the cone density in the area centralis than the human eye, providing extraordinarily high visual acuity.
Many species of bird have 2 foveas, 1 for lateral monocular vision and the other for frontal binocular vision.
What allows birds to hunt from great heights and maintain a focused retinal image of prey in a steep dive?
Contain striated ciliary muscles and the lens is directly suspended from the muscle fibres, which makes the lens shape changed during accommodation far more rapid than in mammals.
What is pecten in the avian eye?
Projects into the vitreous humor from close to the optic disc. This is to supply oxygen to tissues of the retina, as the retina is poorly supplied with blood vessels that would degrade the optical quality of the retinal image.
Name the 5 classes of neurones in the retinal structure.
Photoreceptors
Horizontal cells
Bipolar cells
Amacrine cells
Retinal ganglion cells
Describe the photoreceptor cells of the retina.
The sensory neurones that transduce the light energy in photons into an electrical response that can be processed by the nervous system. Rods and cones respond to light over different ranges of intensity and wavelength.
Describe the horizontal cells of the retina.
Photoreceptors are linked by synapses to horizontal cells, which mediate lateral inhibitory interactions between neighbouring photoreceptors. This increases the contrast between those that are more or less stimulated, which is the first stage in the high acuity processing that enables the recognition of features in the retinal image.
Describe the bipolar cells of the retina.
Convey information from photoreceptors to the ganglion cells, and some classes respond to light stimulation, either by depolarisation or hyperpolarisation. This enables the visual system to be equally sensitive to darker stimuli on lighter backgrounds as it is to lighter stimuli on darker backgrounds.
Describe the amacrine cell of the retina.
Come in a number of different classes with a variety of functions, including motion detection in some species.
Describe the retinal ganglion cells of the retina.
Receive inputs from bipolar cells directly or indirectly from amacrine cells. Different classes process different information from the visual image, some of which convey information about the detailed pattern of light stimulation and from the basis of processing streams for recognition. Larger retinal ganglion cells show more rapid changes in light intensity.
Describe the ion channels in the photoreceptor membranes in the dark.
- Open and allow the influx of sodium ions that depolarise the photoreceptors membrane.
- The current circuit is completed by the efflux of sodium ions form the inner segment via the sodium potassium ATPase.
- Membrane of photoreceptors depolarised at -35 to -40mV in the dark and continually releases glutamate neurotransmitter at its synapse with retinal interneurons.
Describe the ion channels in the photoreceptor membranes upon light stimulation.
- Close, reducing current influx and causing membrane hyperpolarisation.
- This differs from most other sensory transduction mechanisms that have a depolarising response to a stimulus.
- But does share the basic principle of being graded change in membrane potential.
- As intensity of light stimulus is increased, the size of the hyperpolarising response increases, before saturating around -65mV.
Distinguish opsin and rhodopsin.
Opsin proteins in cones or rhodopsin in rods are G-protein coupled receptors, as are beta-androgenic receptors and olfactory receptors.
Describe the action of opsin in light transduction.
- Cis-retinal ligand bound to opsin in the dark, activating the receptor.
- Absorption of light converts cis-retinal to trans-retinal which no longer fits into the binding site and dissociates from the opsin.
- Activates the receptor, causing the G protein, transducing, to dissociate into the alpha and beta-gamma subunits.
- Alpha subunit of transducing activates phosphodiesterase, which hydrolyses cGMP.
- cGMP keeps the cyclic nucleotide gated ion channels in the rod outer segment open in the dark.
- The light induced fall in cGMP causes the cyclic nucleotide gated ion channels to close, reducing the influx of sodium ion and hyperpolarising the membrane.
- Reduces the release of glutamate form the synaptic terminal.
How can trans-retinal be converted back to cis-retinal?
Trans-retinal can be converted back to cis-retinal. Takes several enzymatic steps and occurs in the pigment epithelial cells and not photoreceptors. This is why photoreceptor outer segments are closely associated with the pigment epithelial cells and detachment of the retina from the pigment epithelium in blindness.
What is scotopic vision?
Rod-mediated, low light vision is known as the scotopic range.
How are rod cells adapted to scotopic vision?
- Their photopigment rhodopsin is packed into the stacked membrane discs of the rod outer segment.
- Efficient array for capturing low number of photons in the lowest light levels.
- Rods are highly sensitive to these lowest light levels and can produce an electrical response to a single photon of light.
- Their sensitivity is far higher than cones and so provide input into the visual system at the lowest light levels, which are below the threshold for activation of cones
What must be overcome with highly sensitive rods?
Basal level of thermal isomerisation of cis to trans retinal even in total darkness. So, a dim light stimulus needs to overcome this thermal noise to be detected.
What is the effect convergence on scotopic vision?
- Greatly enhances by the convergence of input from hundreds of rods via rod bipolar cells and amacrine cells onto the retinal ganglion cells.
- Convergence of information in the rod bipolar pathway greatest in the peripheral retina.
- It increases the sensitivity of scotopic vision but decreases its spatial acuity.
Distinguish the choroid layer of the eye of diurnal and nocturnal animals.
Diurnal: choroid layers beneath the retina is pigmented and appears black. Helps to absorb stray photons that have escaped capture by the photoreceptors, preventing them from being scattered, which would reduce visual acuity at high light levels.
But scotopic night time vision is to maximise photo capture by the photoreceptors. So most nocturnal animals (as well as some both active in day and night, such as cats) have a choroid containing a highly reflective layer called the tapetum lucidum.
What are the species differences in the tapetum lucidum?
In ungulates, it is collagen fibres, in carnivores, it is crystals of guanine.
A cat has far better sensitivity than humans at night, but humans in daylight can see at a certain distance that would need to be 6 times closer by a cat.
What is the consequence of the tapetum lucidum?
The cost of this higher night time sensitivity is lower spatial acuity in daylight.
What is photopic vision?
Cones respond to higher light levels than rods and are the sole source of visual information when rods responses have becomes saturated in what is known as the photopic range.
What characteristics of cones allow for photopic vision?
- Less sensitive than rods but much less convergence.
- This particularly around the area centralis and fovea in species with the highest visual acuity, such as primates, in which there can be a 1:1 relationship between a single cone and a retinal ganglion cell, so no convergence.
- The response of cones to a brief flash of light is also a lot faster than rods, so greater spatial acuity and temporal acuity.
What are the 3 opsins in humans involved in colour vision?
Absorb different wavelengths of light based on their protein structures. A short wavelength opsin (S) absorbs light in blue and violet range, a mid (M) wavelength opsin absorbs green and yellow range and long wavelength opsin (L) absorbs orange and red range.
How is information about colour obstained?
Each cone has only 1 types of opsin, so each cone on its own is unable to signal information about colour. It is only be analysing the pattern of activity across different cone types that information about colour can be obtained.
Distinguish dichromats and trichromats.
Dichromats see a more limited colour range than trichromats and there is less colour contrast, which means that colours that look very different to a trichromat, such as red and yellow may look very similar to a dichromat.
What is tetrachromatic vision?
- Most non-mammalian vertebrates, including fish, amphibians, reptiles and birds
- 4th type of cone opsin that can absorb in the UV region.
- Many species of birds also have coloured oil droplets in their cones, which act as filters to reduce overlap between the wavelength ranges of the different cone photopigments.
- Only a few rodent species have evolved the ability to see in the UV range, although what selective advantage that may provide is unclear.
What is adaptation of the visual system?
At the lowest light levels, the visual system is at its most sensitive, but as light levels increase at dawn, the visual system adapts to the increasing light levels but decreasing the sensitivity of the receptors to match the overall light intensity in the environment.
Describe the process behind light adaptation of the eye.
- A reduction in the sensitivity of the photoreceptor to maintained stimulation.
- Because in the dark, calcium ions as well as sodium ions flow through the cGMP gated ion channels in the outer segment.
- Reduced cGMP and closure of channels in outer segment due to hyperpolarisation of the photoreceptor and reduced influx of calcium ions.
- Relieves some inhibition of guanyl cyclase, which increases its rate of cGMP production to oppose the effect of the phosphodiesterase.
- The increasing cGMP levels allow more channels to open and reduce the hyperpolarising response.
- Sensitivity of the photoreceptor is being controlled by the level of guanyl cyclase, which is in turn being controlled by absolute light level.
- Light adaptation shifts the sensitivity of the photoreceptors as light level change during the day quickly, along with the pupillary reflex to prevent saturation of the photoreceptor response as an animal moves from a dark burrow or deep shade to bright daylight.
Why is the sensitivity of the visual system not solely determined by the light adaptation mechanism?
- If so, you would expect the increase in sensitivity going form light to darkness to be as paid as the decrease in sensitivity going from darkness to light. But this is not the case.
- It takes 20-30 minutes for the sensitivity of the visual system to fully recover after moving from bright daylight to total darkness.
Describe the process of dark adaptation of the eye.
- Increase in sensitivity is initially determined by recovery of the cones.
- Break at the threshold for the rods, and overall threshold follows the rods threshold.
- Because bright light isomerises a certain fraction of the cis-retinal to trans-retinal.
- Trans-retinal dissociates from the opsin, which is no longer sensitive to light.
- Takes time for the trans-retinal to be metabolised back into cis-retinal and to be available to bind the opsin again.
- So the dark adaptation threshold curves for rods and cones reflect the recovery in the proportion of photopigment with cis-retinal, which is available to detect light.
What happens in the retina when information must be transmitted across short and long distances?
Up to the ganglion cells, the distances in eth retina are short enough for the information to be conveyed by graded changes in membrane potentials without action potentials.
It is only when information needs to be transmitted the long distances to the brain that the retinal ganglion cells convert the graded potentials to action potentials.
Describe the mechanism of retinal processing for longer transmission distances.
- In the dark, photoreceptors depolarise and continually release glutamate neurotransmitter onto bipolar cells.
- But the postsynaptic effect of glutamate depends on the receptor type expressed by the bipolar cells:
- 1 popular cells known as off-cone bipolar cells is excited by glutamate and therefore responses to light by hyperpolarising.
- On-cone bipolar cells is inhibited by glutamate and therefore responds to light by depolarising. - These in turn feed their signals onto separate populations of on and off ganglion cells.
- Makes the visual system as sensitive to small spots of light o a dark background as it is to small dark spots on a light background.
How are horizontal and amacrine cells involved in retinal processing?
- Another form of processing is mediated by the horizontal cells, which connect a photoreceptor with neighbouring photoreceptors.
- Horizontal cells are inhibitory and when a photoreceptor is stimulated by light it will inhibit surrounding photoreceptors.
- The amacrine cells can also mediate lateral interactions in the retina.
What is an electroretinogram used for?
An ERG records the summed electrical activity in the retina. It is used to check retinal function, as it will not show activity if the rod and cone photoreceptors are non-functional.
What is an electroretinogram used for?
An ERG records the summed electrical activity in the retina. It is used to check retinal function, as it will not show activity if the rod and cone photoreceptors are non-functional.
What is lateral inhibition?
Common to sensory system, except olfactory. In the retina, helps to sharpen up the spatial differences in the retinal image:
- Sensory neurones directly excite second order neurones and indirectly inhibiting neighbouring second order neurones via inhibitory interneurons.
- Lateral inhibition increases the contrast at the boundaries between darker and lighter areas of the image.
- This concentrates the information in the retinal output by highlighting edges in the visual image.
Describe the structure of the receptive fields of retinal ganglion cells.
- Region in the centre of the receptive field where a light stimulus evokes either an increase or decrease in firing rate, depending on whether it is an on or off retinal ganglion cell.
- But this is surrounded by a peripheral region in which the light stimulus elicits the opposite response due to the lateral inhibitory action of the horizontal cells.
- The size of the retinal ganglion cell receptive fields varies across the retina.
Distinguish scotopic and photopic vision.
Scotopic: convergence is greatest in scotopic vision where the signal from the rods is transmitted to rod bipolar cells and then via all amacrine cells to the retinal ganglion cells. No lateral inhibition in scotopic vision which would reduce sensitivity.
Photopic: cones signalling via the cone bipolar cells with lateral inhibition between cones being mediated by the horizontal cells.
Name and describe the intermediate range between scotopic and photopic visions.
Mesopic range, between cone threshold and rod saturation, in which both rods and cones are functioning.
- Has rod signal fed into the horizontal cell network and also mediates lateral inhibition.
- Can combine the high sensitivity of the rod photoreceptors with lateral inhibition to enhance contrast and enhance spatial acuity.
Why is there little change in firing rate of ganglion cell receptive fields?
The responses are opposite for the off centre ganglion: a light stimulus in the central region inhibits the cell, whereas a light stimulus in the peripheral region is excitatory.
Opposing effects are balanced, so for both off and on-centre retinal ganglion cells, even illumination across the whole receptive field results in little if any change in firing rate.
What are the species differences in conscious visual perception?
70% in cats, 85% in horses and 100% in birds (they have less binocular vision) of the retinal ganglion cell axons decussate at the optic chiasm.
What is the retina surrounded by and why?
Retina is embryologically an outgrowth of the brain so it is surrounded by meninges like the brain and it is myelinated by oligodendrocytes rather than the Schwann cells found in peripheral nerves.
Describe how the conscious visual perception is represented contralaterally.
- Retinal ganglion cells project via the optic nerve to the optic chiasm.
- The results is that the visual field is represented contralaterally in the pathway for conscious visual perception via the lateral geniculate nucleus in the thalamus to the primary visual cortex in the occipital lobe.
What is cortical feature extraction and where does it take place in the brain?
The separate pathways form the retinal ganglion cells for detailed form, recognition, position and movement are maintained as separate projections via the lateral geniculate nucleus to the visual cortex. From the primary visual cortex, the information is processed in a variety of higher visual association areas of cerebral cortex. The ‘what’ pathway is associated with the temporal cortex memory systems and the ‘where’ pathway is associated with the sensory representation of the world in the posterior parietal cortex.
Describe tuning of receptive field responses.
- At higher levels of visual processing in the ‘what’ pathway, neurones can respond selectively complex features in the visual image.
- Neurones in temporal cortex are tuned to respond to combinations of features that allow recognition of objects.
As well as the visual cortex, name the 3 subcortical targets that ganglion cell afferents project to.
Rostral colliculus of the midbrain
Pretectal nucleus
Suprachiasmatic nucleus of the hypothalamus
Why do ganglion cell afferents project to the rostral colliculus of the midbrain?
This has a sensory map of the visual field, which is aligned with a map of the auditory space. The function of the tectal projection in mammals is to respond to a sudden stimulus in the environment, driving saccadic movements of the eyes to fix on the stimulus, in coordination with reflex head movement via the tectospinal tract.
Why do ganglion cell afferents project to the pretectal nucleus?
Pretectal nucleus, which forms the afferent pathway for the pupillary reflex. Arises from a specialised subpopulation of retinal ganglion cells. These intrinsically photosensitive retinal ganglion cells contain the photopigment melanopsin. They receive indirect input form rods but can also respond directly to light.
Why do ganglion cell afferents project to the suprachiasmatic nucleus of the hypothalamus?
Even if an animal lacks both rods and cones and are functionally blind still have a pupillary light reflex and can entrain their circadian rhythm to environmental light levels.
What is the purpose of the aqueous and vitreous humour?
Maintain pressure and shape of eyeball
Describe the aqueous humour.
- Nutrition for anterior chamber and cornea
- Blood filtrate of ciliary process
- Returns via venous plexus or canal of Schlemm
- Glaucoma
Describe the vitreous humour.
- Slower turnover
- Holds lens in place
What is the role of extrinsic muscles of the eye?
- Convergence
- Tracking (also neck movements)
- Saccades
- Holding eyes horizontal
What is the role of the intrinsic muscles of the eye?
- Pupillary muscles – dilator (radial) and constrictor (sphincter, circular)
- Ciliary body – accommodation (changing the shape of the lens
- Dim light or fear, sympathetic
- Strong light, parasympathetic
Name the layers of the sensory retina.
1) Pigment epithelium
2) Photoreceptor layer
3) Outer limiting membrane
4) Outer nuclear layer
5) Outer plexiform layer
6) Inner nuclear layer
7) Inner plexiform layer
8) Ganglion cell layer
9) Nerve cell layer
10) Inner limiting membrane
What are the histological components of the sensory retina?
Choroid
Pigment cells
Cones
Rods
Horizontal cells
Amacrine cells
Bipolar cells
Muller cells - supports glial cells
Ganglion cells
Axons at surface passing via optic nerve, chiasm and tract to lateral geniculate body
What is the mechanical advantage of a lamina cribrosa over the alterative of a single large hole for the emergence of the optic nerve?
It is effectively like a net, one large hole would be much weaker than multiple small holes.
Why is the optic disc functionally the blind spot?
It is where the axons leave the eye therefore there is no sensory retina in this area.
In what ways is the retina specialised at the area centralis, the fovea of primates?
It is thinner and composed of cones. It is the region of highest visual acuity.
What is the importance of compartmentalisation within the cochlea duct?
Maintenance of separate fluids. The scala media contains endolymph which is high in potassium. It has a high potential compared to the resting potential of the hair cell which is the driving force for the flow of potassium ions into the cell and therefore the transduction mechanism.
What is the name of the structure which houses the spiral ganglion nerve cells bodies and their central projections?
Modiolus
What 3 events combine to permit an object to remain in focus when it is moved closer?
Lens accommodation – increasing its optical power
Pupillary constriction – increasing the depth of field
Inward movement of the visual axes of both eyes – adduction
