Case 21- Eye and questions Flashcards
Projections of the retinal ganglion cells (RGC)
The retinal ganglion cell (RGC) axons project via the optic nerve, chiasm and tract to terminate in the lateral geniculate nucleus of the thalamus. They are functionally classed as either parvocellular or magnocellular. The parvocellular RGC’s give information regarding the specific detail of vision e.g. colour. The magnocellular RGC’s have information about shape and movement.
Parvocellular or Magnocellular pathway
This is important when the information reaches the primary visual cortex which is located in the occipital lobe. It sits on either side of the calcarine sulcus and is known as V1. Within the cortex, there are cortical columns that receive information from either the parvocellular or magnocellular pathways.
The where pathway
Neurones from V1 project to V2, V3 and V5.
Dorsal pathway, to the parietal region
This is an extension of the magnocellular pathway processing shape, movement and motion.
The what pathway
Neurones from V1 project to V2 and V4.
The ventral stream, to the temporal region
This is an extension of the parvocellular pathway for the analysis of details.
Akinetopsia
Caused by a lesion of the V5 area
Akinetopsia causes a patient to experience an impaired perception of motion. An object may jump from one position to another.
This could cause many difficulties, one example being that it may be hard to know when to cross the road as a car may appear to be a long way in the distance and then jump to its actual position close by.
Prosopagnosia
The inability to recognise faces.
The medial temporal lobes have a record of faces (it is right-side dominant).
Damage to the temporal lobe or ventral stream from the visual cortex can cause this condition.
Blindsight
This is an extremely rare phenomenon.
It can occur after damage to the occipital lobe e.g via a stroke.
A patient becomes cortically blind- i.e. there is no conscious awareness of sight as the primary visual cortex is damaged.
The rest of the visual pathway is still intact and the information processing can be taken over by other areas of the brain.
This means patients with blindsight cannot see but can walk down a corridor avoiding objects.
Lesions in the L optic nerve
Monocular visual loss
Lesions in the optic chiasm
Bitemporal hemianopia- no sight on the temporal side of each eye
Lesions in the left optic tract
Homonymous hemianopia- no sight on the right side of both eyes
Lesions in the left optic radiation- lateral fibres
Superior quadrantopia- no sight in the upper right
Lesions in the left optic radiation- medial fibres
Inferior quadrantopia- no sight in the lower right
Occipital visual cortex lesions
Homonymous hemianopia with or without macula sparing.
Pupil- afferent and efferent connections
Afferent= Light entering the eye-> pre-tectal area
Efferent=Pre-tectal area->sphincter pupillae
Sphincter pupillae
Circular muscle
Parasympathetic
Constriction/miosis
Dilator pupillae
Radial muscle
Sympathetic
Dilation/mydriasis
A patient has a lesion of CN II on the right side- what is the response of both pupils when light is shone into the right eye?
Right eye- no response
Left eye- no response
A patient has a lesion of CN II on the right side- what is the response of both pupils when light is shone into the left eye?
Right eye- constriction
Left eye- constriction
A patient has a lesion of CN III on the left side- what is the response of both pupils when light is shone into the left eye?
Right eye- constriction
Left eye- no response
A patient has a lesion of CN III on the left side- what is the response of both pupils when light is shone into the right eye?
Right eye- constriction
Left eye- no response
What other symptoms would you see with a CN III lesions
Dilated pupil- loss of parasympathetic supply to sphincter pupillae.
Full ptosis- loss of levator palpebrae superioris function.
Eye looks down and out- only lateral rectus and superior oblique can function.
Failure to accommodate- loss of function of sphincter pupillae, ciliary muscle and medial rectus
Range our visual system is sensitive to
The visible light range of the electromagnetic wave spectrum ranges from 380nm (purple) to 740nm (red). Most electromagnetic energy emitted by the sun is within the visible range. We have blue, red and green cones which then combine to form other colors. Our perception of the world is filtered
Importance of the fovea
Contains most of the cones (no rods)- where you have eye acuity and colour vision. The further out you go its most rods where you have night vision and movement. The photoreceptors have been pushed apart to create the foveal pit
Optic disc
Where the optic nerve and blood vessels enter the eye. Is a blindspot with no rods or cones. Closest to the nose
Macula
Surrounds the fovea. Has some rods but the majority is cones, important in color vision. Surrounded but the superior and inferior branch of the retinal vein or the radius from the fovea to the optic nerve. Location of central vision. The macula layer is composed of two or more ganglion cells on top of each other
Organisation of the retina
Has pigmented epithelium at the back, reduces the light scatter and feeds the photoreceptors
Phtoreceptors- cell bodies are in the outer nuclear layer. The outer nuclear layer is towards the back of the eye and the inner nuclear layer is near the front. In the outer plexiform layer the photoreceptors synapse onto the bipolar cells. The photoreceptor and bipolar cell are in parallel to each other. Horizontal cells are between them in outer plexiform layer
The cell bodies of the bipolar cells are in the inner nuclear layer
The bipolar cells project to the inner plexiform layer and synapse with the inner plexiform layer. The axons from the ganglion cells become the optic nerve
Layers of the retina
1) Nerve fibre layer
2) Ganglion cell layer
3) Inner plexiform layer
4) Inner nuclear layer
5) Outer plexiform layer
6) Outer nuclear layer
7) Photoreceptors outer segments
8) Pigmented epithelium
Hormone photoreceptors produce
Costantly produces Glutamate which goes down to the bipolar cells in the dark which the brain interprets as there being no signal. When light hits the photoreceptors the Na+ channels close down stopping the Glutamate from being released. In low light levels some Glutamate, in high levels less. GABA, Glycine, Acetylcholine and Dopamine can be released from amacrine cells and inhibit Glutamate production. Ganglion cells fire action potentials based on how much glutamate is produced
Columns in the retina
Photoreceptors, Ganglion, Muller and Bipolar cells form columns
The Horizontal and Amacrine cells form layers
Ganglion cells go along in layers
Muller cells form the innermost layer, the feet of glial cells are support cells
The synaptic
Effects of raised intra ocular pressure on the optic disc
There will be a cupped optic disc with a high cup to disc ratio, cup will appear bigger
Can be due to Glaucoma
Visual acuity and driving
The minimum eyesight standard for driving is 6/12
You must- have an adequate field of vision and be able to read a car number plate made after 1/09/2001 from 20m
Visual acuity and occupation
Lorry and bus drivers- must have a visual acuity at least 6/7.5 in their best eye and at least 6/60 in the other eye
Other restriction- Armed forces, Air traffic control and Electrical engineers
What nerve supplies the lateral rectus muscle
Abducens
Result of right CN IV (trochlear) palsy
Superior oblique-normally helps pull eye down and out.
On looking forwards: elevation and extorsion of the eye.
On looking left: the eye elevates as it moves medially
Oculomotor CNIII palsy
- ‘Down and out’ palsy
- Ptosis (innervates levator palpebrae superioris)
- Mydriasis (large pupil)– carries parasympathetic fibres
What are the afferent and efferent branches of the corneal reflex
Afferent- Trigeminal (V1)
Efferent- facial nerves
What does the greater petrosal nerve supply
The greater petrosal nerve (a branch of the facial nerve – CNVII) gives parasympathetic supply to the lacrimal gland. If the cornea is dry then the greater petrosal nerve may be injured
Where does the nerves for the lacrimal gland have their cell bodies
Superior salivatory nucleus
Symptoms of cataracts
1) Change in refractive error
2) Blurred vision
3) Glare
Most common complication after a cataract operation
Endophthalamitis
Why does Presbyopia lead to reduced accomodation
There is reduced lens flexibility with age
What structure in the eye is responsible for the most refractive power
The cornea
What are the visual acuity standards for lorry and bus drivers
visual acuity at least 0.8 (6/7.5) in your best eye
and at least 0.1 (6/60) in the other eye
glasses can’t be more than +8 (legally blind)
uninterrupted horizontal visual field of at least 160 degrees
- with an extension of at least 70 degrees left and right
- and 30 degrees up and down
What neurotransmitter is the cause of excitotoxicity
Glutamate
What neurotransmitter is the cause of excitotoxicity
Glutamate
Description of phototransduction
Light hitting photoreceptors hyperpolarise the photoreceptors by closing Na+/Ca+2 channels thus membrane potential hyperpolarises due to K+ effluc
What neurotransmitter is recycled by monoamine oxidase (MAO) only
Serotonin
Nerve supply to the cornea
It has a dense nerve supply derived from the long ciliary branches of the ophthalmic division of the trigeminal
What structure of the temporal lobe is indicated in schizophrenia and epilepsy
The Hippocampus
Where is a flame haemorrhage located
The Nerve fibre layer
When do you get tardive dyskinesia
Following chronic use of antipsychotic drugs. There is proliferation of dopamine receptors and hypersensitivity of the nigrostriatal pathway. The effects are often irreversible
What part of the eye is affected by cataracts
The Lens
Occupations where a higher standard of visual acuity is required in the Uk
Bus and lorry driver
The Pharmacokinetic property of typical antipsychotics
High plasma protein bound
Cause of positive symptoms in schizophrenia
Dysregulation of dopaminergic pathways to the subcortical and limbic systems through hyperactivity acting on dopamine D2 receptors.
The pathways to the prefrontal cortex are hypoactivated, are D1 receptor mediated and lead to negative symptoms of schizophrenia.
Cause of positive symptoms in schizophrenia
Dysregulation of dopaminergic pathways to the subcortical and limbic systems through hyperactivity acting on dopamine D2 receptors.
The pathways to the prefrontal cortex are hypoactivated, are D1 receptor mediated and lead to negative symptoms of schizophrenia.
