The Visual System Flashcards
What are the layers of the layer?
Fibrous layer
Vascular layer
Neural layer
What is the fibrous layer of the eye?
Contains the cornea and sclera, formed of collagen fibres.
What is the cornea?
Fibrous layer overlying the iris, responsible for focussing the light rays to the back of the eye in the retina, innervated by the trigeminal nerve.
What is the sclera?
Tough fibrous layer over the majority of the eye, innervated by the ophthalmic nerve which protects the eye. This is where the extra-ocular muscles attach.
What is the uvea?
Vascular layer of the eye which contains the
Iris
Ciliary body
Choroid plexus, a layer of connective tissue with the blood vessels of the eye from branches of the ophthalmic artery, ciliary muscles and suspensory ligaments.
What is the ciliary body?
Structure in the urea layer that is an extension of the iris. It produces aqueous humour for the anterior eye cavity, to nourish the lens and the cornea and keep the eye round and rigid.
It also has the ciliary muscle that have processes to connect to the pupils via suspensory ligaments for controlling the size of the pupil for light entry to the retina.
Where does the aqueous humour drain?
Aqueous humour travels through the trabecular mesh work and leaves via the canal of Schlemm, through the anterior ciliary veins.
What is the neural layer of the eye?
Contains a light sensitive layer of tissue called the retina which is connected to the optic nerve and contains the optic disc.
What is the blood supply to the retina?
Outer layer and retinal epithelium photoreceptors is supplied by the innermost layer of the choroid via choriocapillaris..
Inner layer is supplied by central retinal artery.
What is the optic disc?
Round spot on the retina where the axons of the retinal ganglion pass through to the optic nerve.
What is the iris?
Coloured part of the eye which contains:
Circular muscle formed by the sphincter pupillae under parasympathetic control.
Radial muscle formed by the dilator pupillae under sympathetic control.
What is the pupil?
Opening in the centre of the iris where light enters the eye. The diameter of this is controlled by the suspensory ligaments, ciliary muscles and muscles of the iris.
What is the fovea?
Pit in the retina which contains only cone photoreceptors, and has the highest visual acuity and resolution. This is located in the macula.
What is the lens?
Responsible for focussing light towards the retina, which is able to change shape via the accommodation reflex for focussing on close and distant objects.
What is the anterior cavity of the eye?
Located between the cornea and iris, filled with aqueous nutrient rich fluid produced by the ciliary body.This fluid nourishes the lens and cornea which maintains ocular pressure and drains into the Schlemm canal, held open by the trabecular mesh work.
What is the posterior cavity of the eye?
Located between the iris and the lens, filled with vitrous fluid to maintain eye shape and attachment to the retina and vision clarity.
What is the optic chiasm?
Crossing over of the optic nerves from each eye in the base of the hypothalamus, by the nasal field of vision. This is why the vision in the left eye is partially controlled by the right hemisphere.
Which muscles are present in the iris?
Dilator pupillae which causes dilation of the iris, under sympathetic control and forms radial muscle.
Sphincter pupillae which causes constriction of the iris, under parasympathetic control and forms circular muscle.
Which muscles are involved in pupillary constriction?
Sphincter papillae and ciliary muscle which contract due to parasympathetic innnervation via the Edinger-Westphal nucleus.
Which muscles are involved in pupillary dilation?
Dilator pupillae due to sympathetic innervation.
What is the accommodation?
To focus on a near or distant object, the lens will change shape.
How does the eye focus on distant objects?
Light enters in a parallel angle so lens should be unfocused. Ciliary muscle is relaxed, suspensory ligaments are now tight and cause constriction of the pupil, so less light enters. Each eye must be divergent and turn outwards.
How does the light focus on close objects?
Light rays enter at a divergent angle, so lens needs to increase focusing power by becoming short and curved. Ciliary muscle will contract to change lens shape as suspensory ligaments loosen and eyes converge and turn inwards.
What is the accommodation reflex?
Co-ordinated change in the eye from far away to focus on a near object consisting of:
Convergence via medial rectus muscle
Constriction/miosis through Edinger-Westphalia nucleus parasympathetic innervation
Contraction of the lens torn fatter and more Convex
What is the accommodation reaction?
Contraction of the ciliary muscle to increase the size of the lens and make it round and more convex.
What is miosis?
Decreasing diameter of pupil which occurs when
Objects are close to the eye to avoid it being “zoomed in:
In a bright room
This is innervated by the parasympathetic system.
What is mydriasis?
Increasing diameter of pupil which occurs when:
Objects are far away from the eye to increase light entering
In a dark room
This is innervated by the sympathetic nervous system
What is the innervation for pupil constriction?
Miosis-Parasympathetic fibres, via the Edinger-Westphal oculomotor nucleus in the midbrain which synapses on the ciliary ganglion and short ciliary nerve for
—> relaxation of ciliary muscle
—> contraction of sphincter papillae muscle
This decreases the pupil diameter and less light enters.
What is the innervation for pupil dilation?
Mydriasis- Sympathetic fibres of the superior cervical travel along the carotid plexus and give an ophthalmic nerve and long ciliary nerve for:
-> Contraction of ciliary muscle
-> Relaxation of suspensory ligaments and pupil sphincter muscles
-> Contraction of dilator sphincter
This causes the eye to dilate and allow more light to enter.
Where do the extra-ocular muscles attach?
Sclera.
How does the eyelid close?
Orbicularis iris, innervated by the facial nerve.
How does the eyelid open?
Lavatory palpabrae superioris and the superior tarsal muscle.
What is the innervation to the levator palpabrae superioris?
Oculomotor nerve.
What is the innervation to the superior tarsal muscle?
T1 sympathetic nerve.
What is the role of the lacrimal gland?
Produces tears to cleanse and moisten the surface of the eye, innervated by the extraperoneal nerve, a branch of the facial nerve.
Which muscle is responsible for eye abduction?
Lateral rectus muscle, innervated by the abducens nerve.
Which muscle is responsible for eye adduction?
Medial rectus muscle, innervated by the oculomotor nerve.
What is the role of the superior rectus muscle?
Moves the eye upwards and inwards. It is innnervated by the oculomotor nerve.
What is the role of the inferior oblique muscle?
Moves the eye upwards and outwards. It is innnervated by the oculomotor nerve.
What is the role of the inferior rectus muscle?
Moves the eye downwards and inwards. It is innnervated by the oculomotor nerve.
What is the role of the superior oblique muscle?
Moves the eye downwards and outwards. It is innervated by the trochlear nerve.
How can the superior rectus muscle be tested in isolation?
Abduction of the eye + look up.
How can the inferior rectus muscle be tested in isolation?
Abduction of the eye + look down
How can the superior oblique be tested?
Adduction of the eye + look down.
How can the inferior oblique be tested?
Adduction of the eye + look down.
What is binocular disparity?
Difference in vision between the two eyes, which is greater the closer an object is.
What cells do the retina contains?
Photoreceptors, horizontal, bipolar, amocrine and retinal ganglion.
How does light enter the eye?
Passes through the cornea -> lens. Once it reaches the retina, it passes through all the layers to reach the photoreceptors where the image is reversed and transformed upside down in the eye.
How does information from the eye travel to the brain?
Photoreceptors in the retina detect light which travels along the retinal ganglion and leaves the retina in the optic disc.
The nerves from each eye cross over at the optic chiasm of the hypothalamus to the lateral geniculate nucleus of the thalamus.
It reaches the primary visual cortex in the occipital lobe-> secondary visual cortex -> visual association centre.
What are the types of photoreceptors?
Rods and Cones.
What is the role of rods?
Ideal for dim scototopic vision for high light sensitivity via their connections to many ganglions. However, they have a low acuity and are concentrated peripherally around the fovea.
What is the role of cones?
Ideal for phototopic bright vision, for low light sensitivity but high visual acuity and for colour vision. These are the only type of photoreceptors present in the fovea.
What are the ganglion cells?
Most superficial layer of the retina which sends signals to the optic nerve.
What are the cells in the intermediate layer of the retina?
Horizontal and amacrine cells for integration of information across the layers.
Which type of photoreceptors for green + blue light?
Rod photoreceptors.
Which type of photoreceptors for yellow + red light?
Cone photoreceptors.
What is disconjugate eye movements?
Eyes move in different directions.
What is conjugate eye movement?
Eyes move in the same direction.
What are examples of disconjugate eye movements?
Vergence eye movement, which adjust the vision of each eye to determine depth.
What are examples of conjugate eye movement?
Vestibulo-ocular, optokinetic and saccade.
What is vestibule-ocular movement?
Initiated in rapid head movements where the gaze shifts to the opposite direction to maintain balance and is a gaze-stabilising movement.
What is optokinetic movement?
Gaze shifts to the opposite direction in slow head movement, to maintain balance and is a gaze-stabilising.
What is a saccadic eye movements?
Gaze shifts from different distances, such as from reading a book to looking across the room.
What is intortion?
Rotation of the cornea towards the nose.
What is extortion?
Rotation of the cornea away from the nose.
Which muscles are responsible for intortion?
Superior oblique and superior rectus.
Which muscles are responsible for extortion?
Inferior oblique and inferior rectus.
Which extraocular muscles are innervated by the oculomotor nerve?
Superior rectus, medial rectus, inferior oblique.
Which extraocular muscles are innervated by the trochlear nerve?
Superior oblique.
Which extraocular muscles are innervated by the abducens nerve?
Lateral rectus muscle
What are the layers of the lateral geniculate nucleus?
Parvocellular layer and magno-cellular layer.
What is the parvocellular layer?
Makes up majority of the lateral geniculate nucleus of neurons that have small cell bodies which receive input from B ganglions. Processes colour, fine detail and still or slow moving objects.
What is the magnocellular layer?
Neurons with larger cellular bodies which receive input from A ganglions for processing vision for objects in motion.
Where does the lateral geniculate nucleus project to.
Calcarine sulcus of the Primary visual cortex, located in the occipital lobe. This sends information to the secondary visual cortex and visual association areas.
What is the primary visual cortex?
Located in the occipital lobe which is responsible for receiving visual information and sending it to other areas of the brain to be analysed. It is highly myelinated.
What is the voluntary eye control in the brain?
Frontal cortex contains the primary and Supplementary eye fields
It travels down to the brain stem to project to the eye,
What is the role of the posterior parietal cortex in central eye control?
For attention and visuospatial integration in navigating and depth.
What is the role of the superior colliculi in central eye control?
Multisensory integration
Target selection
What is the of the frontal eye field cortex in central eye control?
Execution of eye movement.
What is the role of the reticular formation in central eye control?
Co-ordination of eye movement.
What is the role of the caudate nucleus in central eye control?
Modify motor commands from the frontal eye fields.
Where does the parasympathetic nerve supply to the eye run?
Runs outside the optic nerve
What is the course of the sympathetic nerve supply?
Runs alongside the blood vessels.
What is the leading cause of blindness worldwide?
Cataracts
What is the leading cause of blindness in the UK?
1) Age related macula degeneration. This commonly occurs because the macula becomes thinner with age and protein clumps form OR abnormal blood vessels form in the retina. There is a higher risk with a high cholesterol diet.
2) Glaucoma
What is the macula?
Located in the retina and is responsible for central vision and colour vision and fine detail. The centre of the macula is the fovea.
What is the nasal field of vision?
Projects onto the contralateral cerebral hemisphere.
What is the temporal field of vision?
Projects onto the ipsilateral cerebral hemisphere.
What is the Edinger-Westphal nucleus?
Accessory oculomotor nucleus which provides parasymapthetic innervation to the sphincter pupillae and ciliary muscle. It is located in the midbrain at the level of the colliculi.
What is the pathway for the oculomotor nuclei in response to bright light?
Fibres from the temporal and nasal field of vision majorly project onto the visual cortex.
There is projection onto the pre-tectal nuclei in the midbrain which transmits signals to the Edinger-Westphalia nuclei at the level of the colliculi to the oculomotor nuclei in the ciliary ganglion
Where does the optic nerve enter the eye?
Optic disc, which is where retinal ganglion cells pass through. It contains no photoreceptors and is known as the blind spot of the eye.
