Lecture 10: Visual System Flashcards
What are the 3 layers of tissue in the eye?
- Retina
- Uveal tract
- Sclera
Describe the retina.
- Contains all neural cells of the eye (e.g. neurons, photoreceptors)
- innermost layer
- Only part of eye that contains neurons sensitive to light and capable of transmitting visual signals to central targets
Describe the uveal tract. What 3 structures is it made up of?
- 3 distinct but continuous structures
- Middle Layer
- Made of: choroid, ciliary body, and iris
Describe the choroid.
- Largest component of uveal tract
- Rich capillary bed that nourishes photoreceptors (contains light-absorbing melanin in ‘pigment epithelium’)
- Keeps photoreceptors healthy
Describe the ciliary body.
- Extends from choroid near front of eye
- Ring of tissue that circles the lens
- Muscular component (ciliary muscle) adjusts lens refractive power
- Vascular component (ciliary processes) produces aqueous humour
Describe the iris.
- Most anterior component
- Coloured
- Contains 2 sets of muscles with opposing actions to adjust size of pupil (opening in its centre) with neural control
Describe the sclera.
- Opaque ‘white of the eye’
- Continuous with transparent cornea at front of eye
- Outermost tissue layer
- Composed of tough, white fibrous tissue
What are the 2 separate fluid environments between the cornea and retina?
- Aqueous humour
- Vitreous humour
- Light must pass through cornea and these fluid environments before reaching retina
Describe the aqueous humour.
- In anterior chamber (behind cornea, in front of lens)
- Clear watery liquid
- Nourishes cornea and lens
- Produced by ciliary processes in posterior chamber and flows into anterior chamber through pupil
- Replaced about 12x a day
- Drained by meshwork of cells junction of iris/cornea
- Failure to drain leads to glaucoma
Describe the vitreous humour.
- In posterior chamber
- Between back of lens and surface of retina
- Clear thick gelatinous substance
- 80% of eye volume
- Holds shape of eye
- Phagocytic cells clear debris (e.g. blood, old retinal cells) that might interfere with light transmission
What is accommodation and what does it consist of?
- Change of lens shape that alters how light is refracted when it enters the eye
- Focusing on near objects: contraction of ciliary muscle, reduces tension in zonule fibers and allows elasticity of lens to increase its curvature
- Thin lens = distant object
- Thick lens = near object
What are the 2 opposing forces that determine the shape of the lens?
- Elasticity of lens which tends to keep it rounded up
- Tension exerted by zonule fibers which tends to flatten it
How does the eye accommodate to viewing distant objects?
- Force exerted by zonule fibers is greater (ciliary muscle is relaxed) than elasticity of lens
- Flatter shape for distance viewing
- Least refractive power
How does the eye accommodate to viewing close objects?
- Contraction of the ciliary muscle relaxes tension of zonule fibers
- Allows inherent elasticity of lens to increase its curvature
- Highest refractive power
What is the relationship between age and capacity for accommodation?
- As you age, capacity for accommodation is gradually diminished
- Lens weakens
What is the fundus?
- Surface of the retina
What is the optic disk/optic papilla?
- Site where retinal axons leave the eye
- Entry/exit point of blood supply
- Lies nasally
- Contains no photoreceptors
- ‘blind spot’ (scotoma, white matter pathway)
- Not dark, just can’t receive visual info here
- Travel through optic nerve to reach target structures in brain
What is the macula lutea?
- On fundus/inner retina
- Circular region with yellow pigment
- 3mm
- Near centre of retina
- High visual acuity
What is the fovea?
- Depression in retina located in centre of macula lutea
- 1.5mm
- Greatest visual acuity
- Dense concentration of small diameter cones
- 1:1 relationship b/n cones and bipolar ganglion cells
What is the purpose of the optic nerve?
- Transmits signals from retina to brain
What is the cornea?
- Specialized transparent tissue that permits light rays to enter the eye
What are the primary functions of the optical components of the eye?
- Efficiently transmitting light energy
- Achieve focussed image on surface of retina
What parts of the eye are primarily responsible for refraction/bending of light? What does this bending do?
- Cornea and lens
- Necessary for formation of focussed images on photoreceptors of retina
What tool is useful for visualizing inner surface of retina (or fundus) through the pupil?
- Ophthalmoscope
How are blood vessels organized on the inner surface of the retina?
- Fan out
- Arise from ophthalmic artery/vein
Where do the ophthalmic artery and vein enter the eye?
- Through whitish circular area called optic disk/papilla
Why must humans move their eyes and head around when looking at things?
- Direct fovea to objects of interest
- High acuity of vision is restricted to a very small portion of the retina
What are saccades?
- Rapid/orienting movements of the eyes
What do retinal bipolar cells consist of?
- Concentrated, few dendrites at one end
- Cell body
- Axon extending from other side of cell body
What do retinal ganglion cells consist of?
- Widespread dendrites
- Cell body
- Axon extending from other side
What do retinal amacrine cells consist of?
- Lots of dendrites interwoven
- Extend from cell body
- No axon
What is the simplest path that retinal cells take?
Photoreceptors -> Retinal bipolar cells -> Retinal ganglion cells
- Horizontal and amacrine cells provide lateral interactions and maintain visual system sensitivity to contrasting light changes
How many neuron classes can be seen in inner nuclear, outer nuclear and ganglion cell layers?
5
- Photoreceptors
- Retinal bipolar cells
- Retinal ganglion cells
- Horizontal
- Amacrine cells
Where are retinal processes and synapses located?
- Inner plexiform and outer plexiform layers
What kind of cells must light travel through in order to reach outer segments of photoreceptors?
- Non-light-sensitive cells
What is phototransduction?
- Light absorption initiates intracellular cascade which changes membrane potential of receptor
What are photoreceptors embedded in?
- Pigment epithelium
- Composed of disks to nourish photoreceptors
- Outermost layer of retina
What do epithelial processes do?
- Extend down b/n outer segments of photoreceptors
How long do the membranous disks in the pigment epithelium last?
- About 12 days
Where are new outer segment disks being formed? What disks are removed and how?
- Formed near base, then grow outward toward pigment epithelium
- Oldest disks (those closest to outer pigment epithelium) are removed/phagocytosed
How are the membranous disks phagocytosed?
- Disks curl
- Tip becomes spherical
- Tip separates from rod
- Tip is engulfed by pigment epithelium
- Broken down/recycled for parts
Does light stimulation lead to depolarization or hyperpolarization?
- Hyperpolarization
Do different levels of light stimulation lead to different effects on neurons? Why?
- Yes: more/less hyperpolarization
- Show a graded change, changes in light
- There is continuous ntx release because they have a higher resting MP
- More stimulus = less cell excitement
What happens on the molecular level in the dark?
- cGMP levels in outer segment membrane are high
- cGMP binds to Na+-permeable channels in membrane, keeping them open
- Allows Na+ and other cations to enter, depolarizing cell
What happens at the molecular level in the light?
- Absorption of photons leads to a decrease in cGMP levels
- Closes cation channels
- Results in receptor hyperpolarization
What is seen dependent on light absorbed?
- Stimulation-specific graded change in how much ntx is being released
Does the direction of K+ movement change with light absorption?
- NO
- Always an efflux
What are the two types of photoreceptors?
- Rods and cones
What does the outer segment of photoreceptors consist of?
- Membranous disks
- Contain light-sensitive photopigments
What does the inner segment of photoreceptors consist of?
- Cell nucleus
- Gives rise to synaptic terminals that contact bipolar and horizontal cells
What is phototransduction?
- Process of absorbing light and creating a response
- Change in amount of ntx released on target neurons
What is the photopigment in rods? What does it consist of?
- Rhodopsin
- 7 transmembrane domains of protein portion (opsin) traverse membrane bilayer
- Opsin forms pocket where light absorbing portion (retinal molecule) resides
How do rods and cones differ?
- Protein portion
What do the different forms of retinal do?
- Absorb different wavelengths of light
What does a photon do to retinal?
- Breaks its double bond, which leads to a cascade of events
What does transducin look like in its inactivated state?
- Bound to GDP
What happens when transducin is activated?
- Exchanges GDP for GTP
- Alpha subunit activates phosphodiesterase in disk membrane, which hydrolyzes cGMP thus lowering cGMP concentration in outer segment
What happens when the concentration of cGMP falls?
- Molecule no longer binds to and holds open ion channels on surface of outer segment membrane
What does the flow of cations in the dark look like?
- Ion channels are open
- Influx of Na+ and Ca2+ depolarizes cell
- Efflux of K+ opposes, as a hyperpolarizing influence
- Combined result = depolarization of photoreceptors
- Continuous release of NTX from synaptic terminals of photoreceptors
How does light affect ion flow?
- Light reduces cGMP concentration
- Na+ channels close
- K+ flows out faster than Na+ or Ca2+ can flow in
- Hyperpolorization
- Decreases NTX release
- Enormous signal amplification
- Net change of about 1 mV per rhodopsin molecule
What is responsible for limiting amplifying cascade and restoring molecules to inactivated state?
- Proteins in photoreceptor
When are molecules converted back to a form that can absorb light again?
- In the dark
What is the optic chiasm?
- 60% of fibres (nasal retina) cross to contralateral side
- 40% (temporal retina) continue ipsilaterally to thalamus and midbrain targets
- Anterior to stalk of pituitary gland
What is the optic tract?
- Contains ganglion cell fibres from both eyes (contralateral and ipsilateral nerves)
- Major target is dorsolateral geniculate nucleus of thalamus (primary relay nucleus for visual processing by cerebral cortex)
- Secondary target of pretectum
- Information from left visual field is carried in right optic tract and vice versa
What is optic radiation?
- Axons from lateral geniculate nucleus to the striate cortex (part of the primary visual cortex, or V1, in occipital cortex)
What is the pretectum?
- Responsible for pupillary light reflex via circuitry interacting with oculomotor nerve (CNIII)
- Managed by projections from optic nerve
- Collection of neurons b/n thalamus and midbrain
What are 2 other projections/targets of optic nerve?
- Suprachiasmatic nucleus of the hypothalamus
- Retinohypothalamic pathway
- Circadian rhythms (comes mostly from light sources)
- Superior colliculus
- Coordination b/n head and eye to visual targets
- Orienting toward/away from visual stimuli
- Includes ganglion cell types distinct from rod/cone light sensitivity
How are images formed on the retinal surface?
- Light rays pass through pupil
- Results in images that are inverted and left-right reversed
What is a visual field?
- Each eye sees a part of the visual space that defines its visual field
- Divisions of nasal v temporal and superior v inferior
- Portion of the world you can see without moving your head or eyes
What does monocular mean?
- Only one eye can see
- Ex. peripheral vision
What is the binocular field?
- Where left/right visual fields overlap (central portion)
- Projection of binocular field onto 2 retinas and its relation to the crossing of fibres in optic chiasm (topographical organization)
- Left half of visual world is represented in right half of brain and vice versa (regardless of what eye it originates in)
Which part of the retina receives info from the right visual hemifield?
- Temporal left retina
- Nasal right retina
What is periphery vision and what is it mediated by?
- Monocular
- Mediated by most medial portion of nasal retina
What does the centre of the visual field project to?
- Foveal region of retina
What does the right lateral geniculate nucleus receive information from?
- Left visual field
- Nasal left retina
- Temporal right retina
How do axons containing info about the superior portion of the visual field travel?
- Sweep around lateral horn of ventricle in temporal lobe before reaching occipital lobe
- “Meyer’s loop”
How do axons containing info about inferior portions of the visual field travel?
- Travel in parietal lobe
Where is the fovea represented in the visual cortex?
- Posterior part
- Large portion
- Everything else (more peripheral) in anterior regions
Do inputs of the two eyes converge?
- Yes, at cortical levels
Where does the V1/striate cortex project to?
- Others of cerebral cortex (extrastriate) involved in complex visual perception
Where does the dorsal stream travel?
- Leads from striate cortex to parietal lobe
- Spatial aspects of vision
- Analysis of motion and positional relationships
Where does the ventral stream travel?
- From striate cortex to inferior part of temporal lobe
- Responsible for high resolution form vision and object recognition
