4a.) The Retina & Central Vision Pathways Flashcards
Describe the 3 layers of the eye (outer to inner)
- Fibrous outer layer: tough and continous posteriorly with dural sheath of optic nerve and continues anteriorly as the conrea
- Vascular middle layer: choroid which continues anteriorly as ciliary body (ciliary body has two parts: ciliary muscle and ciliary process). Ciliary body connects choroid with iris
- Inner neural layer: retinal layer. Has outer pigmented epithelial layer and inner neurosensory layer
State what cell types are found in the neural layer of the retina and state what each does
- Photoreceptor cells: convert light enery into form of energy CNS can understand “signal transduction”
- Bipolar cells: first order neurones receiving input from photoreceptors
- Ganglion cell layer: receives input from bipolar cells and axons of the ganglion cells form the nerve fibre of optic nerve
- Horizontal cells: connect bipolar cells together and help to enhance contrast through a process called lateral inhibition; send out inhibitory signals to photoreceptors either side of the photoreceptor which is central to prevent too many signals coming in
What do we mean when we say our retina is the ‘wrong way round’?
Light has to pass through the nerve fibre layer and other layers before getting to the photoreceptors
Fundoscopy is useful in detect signs of many diseases; state some disease which would have clinical sings visible on fundoscopy
- Hypertension- retinopathies
- Diabetes- retinopathies
- Macula degeneration
- Raised ICP- papilloedema
- Vascular occlusions
What is amaurosis fugax?
Temporary loss of vision in one or both eyes due to lack of blood flow to the eye
Remind yourself of how you can determine whether you are looking at the left or righ eye
Macula is lateral to the optic disc
The medial retina is referred to as the ________ retina
The lateral retina is referred to as the ______ retina
- Medial retina= nasal retina
- Lateral retina= temporal retina
Remind yourself of the pupillary light reflex
- Light stimulates the afferent nerve= optic nerve (CNII)
- Impulse travels via optic nerve which decussates at optic chiasm
- Optic nerve synapses in pretectal nucleus in brainstem
- Connection with Edinger-Westphal nucleus (pretectal nucleus gives projections to EDW on same side and on opposite side- this is what allows consensual light reflex)
- Parasympathetic fibres from EDW leave brainstem
- Pass via ciliary ganglion
- Reach sphincter pupilae of iris
- Cause contraction
Remind yourself of the accomodation reflex
Accomodation used when focusing on nearby objects. There are 3 main components:
- Convergence of eyes: ensures both retinas are focusing on one object and keeps the image centred of the fovea (where resolution is highest). Medial rectus is responsible
- Autonomic contraction of pupils: ensures light passes through centre of lens
- Thickening of lens: ciliary muscle contracts causing suspensory ligament to become lax so lens can become rounder/more convex
Cerebral cortex must be involved because it is relating to image analysis. Hence, reflex follows visual pathway to visual cortex via lateral geniculate nucleus. Then get projections from visual cortex to EDW and occulomtor nuclei
Describe, the visual pathway (do not include the visual fields in this explanation)
- Medial retina= nasal retina, lateral retina= temporal retina
- Each half of retina is split into superior and inferior hence we have: superior nasal fibres, inferior nasal fibres, superior temporal fibres, inferior temoral fibres (for each eye)
- At the optic chiasm, nasal fibres (both superior and inferior) decussate. Temporal fibres reamin ipsilateral
- The optic tracts (which run from optic chiasm to lateral geniculate nucleus) contain temporal retinal fibres from ipsilateral side and nasal retinal fibres from contralateral side
- The optic radiations (from lateral geniculate nucleus to primary visual cortex) can have two different pathways:
- Superior quadrant fibres form the superior optic radiations (one for each side) which runs through the parietal lobe and also known as “Baums loop”
- Inferior quandrant fibres form the inferior optic radiations (one for each side) which runs through the temporal lobe and is also known as “Meyers loop”
The superior quandrant fibres that form the superior optic radiation and pass through parietal lobe are also known as….?
Baum’s loop
The inferior quadrant fibres that form the inferior optic radiations and travel through the temporal lobe are also know as….?
Meyer’s loop
Describe the visual fields of a human
- Each eye has it’s own set of visual fields: a nasal (medial) and temporal (lateral) visual field
- These visual fields overlap in middle to form our binocular vision which is good for depth perception
*NOTE: for simplexity image does not show overlap of vision
Which retinal fibres are responsible for each field of vision? Explain why
- Nasal retinal fibres are responsible for our temporal field of vision
- Superior nasal fibres responsible for inferior temporal field
- Inferior nasal fibres responsible for superior temporal field
- Temporal retinal fibres are responsible for our nasal field of vision
- Superior temporal retinal fibres responsible for inferior nasal field
- Inferior temporal retinal fibres responsible for superior nasal field
… because light travels in straight lines!
NOTE: colour coding has been done to allow them to match up. In retina, green= nasal, orange= temporal. In visual fields, green= temporal, orange= nasal. Diagram also doesn’t split sup and inf
Describe how we name visual field defects
Name them based on the area of visual loss rather than the site of the lesion e.g.:
- Monocular blindness
- Bitemporal hemianopia
- Homonomous hemianopia
If you had a lesion in left optic nerve (see red line on diagram), describe:
- The visual field losses
- Name for this type of visual field loss
Visual Field Losses
- Ipsilateral temporal retinal fibres are damaged meaning you would have ipsilateral nasal visual field loss
- Ipsilateral nasal retinal fibres are damaged meaning you would have ipsilateral temporal visual field loss
- Hence would lose all visual fields in left eye
Visual field loss= monocular blindness (lost all visual fields in one eye)
If you had a lesion at optic chiasm (see red line on diagram) describe:
- Visual field losses
- Name for this type of visual field loss
Visual Field Losses
- Nasal retinal fibres deccusate at optic chiasm, temporal fibres remain ipsilateral
- Nasal retinal fibres from both eyes affected causing bilateral temporal visual field loss
- Temporal retinal fibres from both eyes unaffected
Name for visual field loss: bitemporal hemianopia
Bitemporal hemianopia can be described as….?
Tunnel vision
State two posible causes of bitemporal hemianopia
- Pituitary adenoma
- Aneuryseum of anterior communicating artery
*Both could cause compression of optic chiasm (and hence the nasal retinal fibres)
If you had a lesion on the right optic tract (see red line on diagram) describe:
- Visual field losses
- Name for this type of visual field loss
Visual Field Losses
- Lesion on right optic tract will cause damage to ipsilateral temporal retinal fibres and contralateral nasal fibres
- Damage to ipsilateral temporal retinal fibres = loss of nasal visual field in right eye
- Damage to contralateral nasal retinal fibres= loss of temporal visual field in left eye
Name for visual field loss: homonomous hemianopia (this means same and half can’t see- idea that in this case left visual of each eye lost- see diagram to make more sense of this)
If you had a lesion in right inferior optic radiation (see red line on diagram), describe:
- Visual field losses
- Name for this type of visual field loss
Visual Field Losses
- Damage to ipsilateral inferior temporal fibres= loss of supeior nasal visual field on ispsilateral side (right)
- Damage to contralateral inferior nasal fibres= loss of superior temporal visual field on contralateral side (left)
Visual field defect: homonomous superior quadrantanopia (idea that it is same in each eye, so left sided visual field loss, in superior quadrant)
If you had a lesion which affected both superior and inferior radiations on same side- in this case the right side- are affected, e.g. in a stroke say of middle cerebral artery, describe:
- Visual field loss
- Name for this type of visual field loss
Visual Field Loss
- Superior & inferior temporal fibres on ipsilateral side loss- meaning loss of nasal visual field on ipsilateral side (right side)
- Superior & inferior nasal fibres from contralateral side loss- meaning loss of temporal visual field on contralteral side (left)
Name for visual field loss: homonomous hemianopia
*NOTE: we wouldn’t be able to tell, from visual field loss, whether this was damage to right optic tract or the superior and inferior radiations on right by just looking at visual field loss alone
Where is the primary visual cortex?
Occipital lobe
Describe what can happen, in terms of blood supply to the occipital lobe and hence visual field loss, if you have a stroke affecting the posterior cerebral artery
Occipital lobe has dual blood supply:
- Posterior cerebral artery (most of occipital lobe)
- Middle cerebral artery (occipital pole)
In a stroke affecting posterior cerebral artery, most of occipital lobe will be lost. However, middle cerebral artery supplies occipital pole and the occipital pole represents the macula hence macular function (central vision) will be spared “macular sparing”
What is optical coherence tomography (OCT)?
Optical coherence tomography (OCT) is a non-invasive imaging test. OCT uses light waves to take cross-section pictures of your retina. It allows you to see different layers of retina and measure their thickness- this can help with diagnosing patholog e.g age related macular degeneration, diabetic eye disease, treatment for glaucoma etc…
What is the role of the medial longitudinal fasciculus?
- Allows eyes to communicate and work together (so that we get coordinated eye movements)
- Allows eyes to respond to changes in head position
- Allows communication between eyes and trunk and neck so that neck and trunk can compensate for eye position
Describe what happens in medial longitudinal fasciculus
- Connections, which are crossing midline, between nuclei that control eye (CNIII, CNIV, CNVI) which allows coordinated eye movements
- CNVIII also connected with medial longitudinal fasciculus allowing eyes to compensate for position of head.
- All of nuclei mentioned send descending connections into spinal cord allowing us to make compensatory movements of neck or trunk
All of the above allows us to maintain a level horizon
If you had multiple sclerosis plaque in the medial longitudinal fasciculus
We wouldn’t be able to make communications between abducens and occulumotor nucleus hence we would lose conjugate eye movements leading to internuclear opthalmaplegia (paralysis of eyeballs caused by loss of connections between cranial nerve nuclei)
*REMEMBER: to look right we would have to contract lateral rectus of right eye and medial rectus of left eye
Dolls eye reflex can be used to assess brainstem function; describe the dolls eye reflex
Eyes move in opposite direction to head
