Vision Flashcards
What is contained within the outer coat of the eyeball and what is its function?
Cornea and sclera
Provides strength
What is contained within the middle coat of the eyeball and what is its function?
Uvea
Provides nutrition
What is contained within the inner coat of the eyeball and what is its function?
Retina
Responsible for vision
List the 3 optical factors affecting visual acuity
Pupil size (smaller = clearer) Clarity of optical media (compromised in cataracts, corneal opacities, etc.) Refractive errors (causing blur)
List 4 causes of refractive error
Myopia
Hypermetropia
Astigmatism
Presbyopia
What is the best visual acuity that can be achieved at photopic light levels, and where is it achieved? Why?
VA=6/6
At fovea due to high density of cones
What is the best visual acuity that can be achieved at scotopic light levels, and where is it achieved? Why?
VA=6/60
5-15° due to high density of rods
List the 6 types of neurons in the retina
Rods Cones Horizontal cells Bipolar cells Amacrine cells Ganglion cells
List the layers of the retina from inner to outer
Ganglion cells
Inner plexiform layer with amacrine cells
Inner nuclear layer with bipolar cells
Outer plexiform layer with horizontal cells
Outer nuclear layer with photoreceptors
What % of the photoreceptors are rods?
95% (100 million)
Are rods or cones more densely packed? Which are smaller?
Rods are smaller and more densely packed
How many different types of bipolar cells are there? What are the 2 classifications?
10 types (1x rod, 9x cone) Can be OFF or ON
Explain the “through” pathway in the retina in the dark
Photoreceptor is at rest; when at rest, the photoreceptor is depolarised and this causes release of glutamate
Glutamate stimulates the AMPA/Kainate (inotropic) receptors on OFF bipolar cells, resulting in depolarisation and release of glutamate onto OFF ganglion cells (also stimulates the mGluR6 metabotropic receptor on ON bipolar cells but this causes hyperpolarisation)
OFF ganglion cells increase their AP firing rate
This signals to the brain and the brain recognises it is dark
What kind of membrane potentials do photoreceptors have?
Graded
How are photoreceptors activated by light?
Light changes the conformation of retinal, to which various photopigments (opsins; either rhodopsin or 1 of 3 cone-opsins) are attached, causing a secondary change in their conformation
Conformational change in the opsin activates transducin, which then initiates a signalling cascade culminating in the breakdown of cGMP to GMP by PDE; cGMP ordinarily gates a Na+ channel, causing continuous influx of Na+ ions, so breakdown of cGMP results in closure of the channel and therefore hyperpolarisation
What causes depolarisation of the photoreceptor in the dark?
cGMP gates a Na+ channel, causing a continuous influx of Na+ ions
What is the function of horizontal cells and how do they work?
Responsible for “centre-surround”
They are depolarised by glutamate released from photoreceptors in the dark
This causes release of inhibitory NT GABA onto an adjacent photoreceptor, causing the photoreceptor to hyperpolarise (this causes depolarisation of ON bipolar cells or hyperpolarisation if an OFF bipolar cell)
On exposure to light, horizontal cells are hyperpolarised and do not release GABA to inhibit neighbouring photoreceptors (causing hyperpolarisation of ON bipolar cells and depolarisation if an OFF bipolar cells)
What are amacrine cells and what is their function? What NTs are released by amacrine cells?
Axonless cells responsible for lateral inhibition (also have a role in motion sensing)
Generally considered inhibitory and release EITHER glycine or GABA
What are M and P ganglion cells? What are their alternative names? What are the relative %s of each?
Magnocellular (big) aka parasol
Parvocellular (small) aka midget
Where do most ganglion cell axons terminate?
In the lateral geniculate nucleus (LGN) of the thalamus
Which layers are assigned to M vs. P cells? Which are allocated to the right eye and which to the left?
M cells: layers 1-2 (1 layer for each eye)
P cells: layers 3-6 (2 layers for each eye)
Left eye: layers 1, 4, 6
Right eye: layers 2, 3, 5
What is the corresponding Brodmann’s area for V1? What is its neuroanatomical location?
Area 17
Occipital lobe around the calcarine fissure
Where do LGN neurons project to?
V1
Describe retinopic organisation
Neighbouring cells in the retina project to neighbouring cells in the LGN and V1
What part of the field of vision do the areas below the calcarine fissure in V1 correspond to? Where is central vision located in V1?
Below the calcarine fissure = upper field of vision
Central vision is located most posteriorly
Why does macula sparing occur in some vision loss?
May be due to the differing blood supply of the occipital lobe of V1 (this is supplied by MCA; the rest of V1 is supplied by PCA)
What are ipGCs?
Intrinsically photosensitive ganglion cells are a small population of GCs which contain the photopigment melanopsin, which is activated by light and causes depolarisation of the ipGC
What are the 5 functions of ipGCs?
Circadian rhythm Sleep regulation Pupil responses General information about light levels Light allodynia (photophobia associated with migraine, ocular injury or infection)
What are the 2 muscles involved in pupil responses and what is their innervation?
Sphincter pupillae for constriction (innervated by PNS fibres from EW nucleus carried by CNIII)
Dilator pupillae for dilation (innervated by SNS)
Describe the neural pathway in the pupil responses
ipGCs project to the optical pretectal nucleus (OPN) in the rostral midbrain
OPN projects to EW nuclei on both sides, which project back towards the eye, synapsing in the ciliary ganglion, to produce direct and consensual constriction in response to light
Where is the OPN located?
In the rostral midbrain
Via which neural structure do ipGCs regulate the circadian rhythm?
The suprachiasmatic nucleus of the hypothalamus
What is the proposed cause of migraine pain? What nerve carries this pain and where is the signal carried?
Due to stimulation of nerves in the dura, carried by CNV to the posterior thalamic nucleus
Why is migraine pain often worse with light?
Due to the projection of ipGCs to the posterior thalamic region (where dural pain is carried by CNV)
What are the 2 components involved in control of gaze and what structures/systems are involved in each?
Oculomotor (movement of eyes in the orbit whilst head is still): involves extraocular muscles and the nerves responsible for their innervation
Head-movement (movement of eye sockets as a whole whilst head moves): involves vestibular and oculomotor system
What are the 5 different types of eye movements?
Saccades Smooth pursuits Vergence Vestibular ocular Optokinetic
What are saccades?
Rapid shifting of the fovea to a new visual target
What are smooth pursuits?
Eye movements designed to keep the image of a moving target on the fovea
What is vergence?
Movement of the eyes in opposite directions (may be divergence or convergence)
What is the role of vestibular ocular movements?
To hold an image still on the retina during brief head movements
What is the role of optokinetic movements?
To hold an image still during sustained head rotation or translation
What is the primary action of SR?
Elevation
What is the primary action of IR?
Depression
What is the primary action of MR?
Adduction
What is the primary action of LR?
Abduction
What is the primary action of SO?
Torsion inferiorly (looks like depression)
What is the primary action of IO?
Torsion superiorly (looks like elevation)
Where do SO and IO insert? What is the main difference in terms of their structure?
Insert behind the equator close to LR
SO runs through a fibrocartilage pulley or trochlea
How are movements from 1 eye coordinated with the other?
Via the medial longitudinal fasciculus (MLF) and reticular formation
Which parts of the reticular formation are responsible for horizontal and vertical movements respectively? Which nerves and muscles are involved?
Horizontal (MR, LR; CNIII, CNVI): pontine paramedian RF
Vertical (SO, SR; CNIV, CNIII): mesencephalic paramedian RF
What is the MLF?
White matter tract connecting the various cranial nerve nuclei involved in eye movements and gaze (CNIII, CNIV, CNVI)
What are the 2 types of neurons in the pontine paramedian RF?
Burst neurons
Omnipause neurons
What is the role of burst neurons and what are their different types? Describe their activity
Burst neurons fire at high frequency just before movement to signal to the cranial nerves responsible, thereby driving contraction of the extraocular muscles
Different types are excitatory for ipsilateral CNVI or inhibitory for contralateral CNVI
Describe the activity of omnipause neurons
Omnipause neurons fire continuously whenever movement is not occurring and project to the contralateral CNVI (where they release GABA for inhibition)
What is internuclear ophthalmoplegia and what causes the condition?
Internuclear ophthalmoplegia is a disorder of lateral gaze in which the affected eye is unable to adduct
Caused by damage to the MLF (results in an ipsilateral presentation)
List the 4 UMNs responsible for control of saccades
Frontal eye fields (FEFs)
Posterior parietal cortex
Superior colliculus
Basal ganglia
Does the FEF control saccades in the ipsilateral or contralateral direction?
Contralateral (so input from the left FEF causes saccades to the right)
Explain the neural signalling from the semicircular canals
As the head turns to the right, fluid movement in the horizontal duct of the right ear accelerates and deceleration is detected in the left ear
Right ear fluid acceleration causes increase in CNVIII firing, deceleration causes a decrease in CNVIII firing
