PB - special senses: vision Flashcards
(35 cards)
How is the aqueous humour removed from the eye?
By the venous plexus in the anterior chamber
What pressure is required in the eyeball to maintain eyeball shape?
Slightly +ve
What is the consequence of a high intraocular pressure?
Impairs vascular perfusion.
Ischaemic damage
Physical damage
Risk of detached retina.
What does the term mydriasis mean? Which nerve is stimulated and what muscle is activated?
Dilation of the pupil in dim light. Parasympathetic efferents causes radial muscles (iris dilator) of the pupil to contract.
What does the term miosis mean? Which nerve is stimulated and what muscle is activated?
Constriction of the pupil in bright light. Sympathetic efferents causes circular muscles (iris sphincter) to contract and constrict the pupil
How is a dilated pupil maintained?
Maintained by a lack of PS activity and a basal level of S activity. This maximises the sensitivity to light.
What does melanopsin contain, which is stimulated by light?
Retinal ganglion cells.
What is the pathway, which causes pupillary constriction in response to light?
Retinal ganglion cells Pretectal nucleus of the midbrain PS motor nucleus of oculomotor nerve Ciliary ganglion Circular muscle Constriction
What does the pupillary light reflex test show?
This is a bilateral response. So this test shows whether the efferent pathway of each eye is intact.
Why does a close image require a thicker lens to focus?
The light focusses at a more distant point behind the lens.
How is a thinner lens achieved?
Relaxation of the ciliary bodies increases their diameter.
This exerts tension on the zonular fibres, which causes the lens to become thinner.
How is a thicker lens achieved?
Contraction of the ciliary bodies is stimulated by PS control.
This decreases the tension of the zonular fibres.
Thicker lens results due to the elasticity of the lens.
Do the ciliary bodies need to be relaxed or contracted for more distant vision?
Relaxed
What is the effect of pupil constriction of the depth of field? How does this happen?
Increases the depth of field because the rays of light are brought closer to the visual axis.
What is aberration caused by?
Rays of light passing through the periphery of the lens.
What is chromatic aberration?
Different wavelengths are refracted to different extents. So they are focussed at different points.
What is spherical aberration?
This is caused by the shape of the lens.
Light passes through the periphery of the lens, which reduces image quality.
What is the primary function of binocular vision? Which type of animals benefit from this?
Comparison of images allows for the perception of depth.
Primates and predators.
What is the primary function of monocular vision? Which type of animals benefit from this?
Does not provide depth for distant images because each image is arriving at the same time.
Uses size, haze and overlap to judge distance.
Prey animals.
How does the receptor density vary across the retina?
Low receptor density in the periphery.
High receptor density in the area centralis.
Directly correlates with the number of retinal ganglion cells, which is related to visual acuity.
Why is the optic disc a blindspot?
This is where the optic nerve leaves the eye, so there are no photoreceptors in this region.
Do carnivores or herbivores have a circular area centralis? What is the function of this?
Carnivores.
Allows for a central area of sharp detailed vision, which is useful for recognising prey.
Do carnivores or herbivores have a ‘visual streak’ area centralis? What is the function of this?
Herbivores.
Elongated strip has a high photoreceptor density, which allows for high visual acuity for horizon scanning.
What determines the position of a species blindspot?
Eye position
