Physiology of vision Flashcards
Perception of light
- photons of shorter wavelengths have more energy
- visible light occupies small portion of EM spectrum
- light travels in straight lines
- sunlight contains all wavelengths
- objects absorb some wavelengths and reflect others, they present as the colour reflected
Lens system
- in vertebrates
- focuses light from objects in environment onto the retina at rear of eye
- retina is where sensory cells are located
- transmits information to the brain
Ciliary bodies
thickened anterior parts of choroid (between sclera and retina)
- produce aqueous humour
Lens
adjustable focus mediated by ciliary bodies
Optics of the eye
- cornea refracts light entering the eye
- parallel light rays hit biconcave lens, refracted to a point beyond the lens called PRINCIPLE FOCUS
- principle focus distance is the distance between the lens and the principle focus
- dioptres are unit of refractive measurement
Lens; focusing images
- Further the object is, the smaller the distance between the lens and the focal point because light rays are parallel
- Closer object’s refracted light rays are not parallel so distance increases
- Increases curvature of lens (convexity) decreases distance
Accommodation to focus images
Process by which the curvature of the lens is increased to focus on a near object
- ciliary bodies contract
- distance between ciliary bodies decrease
- lens ligaments relax
- active process
Ciliary bodies role in accomodation
- Lens is attached to ciliary body via zonulas
- Zonulas are made of inelastic fibres
- Ciliary bodies contract, tension released on zonulas
- more convex shape formed
Iris
- pigmented and opaque
- contains constrictors of the pupil
- and dilators of the pupil
- controls amount of light let into the eye
Quantity of light control
- Iris; smooth muscle in mammals, striated in birds and reptiles
- autonomic innervation
- radial muscle fibres dilate pupil
- circular muscle fibres constrict pupil
Types of pupil shapes
- Vertical slit; in some carnivores. Allows greater light control. Enhances distance judgement
- Rectangle; in herbivores. Sheilds eye from sunlight, greater span of vision for predators
Vitreous humour
a gelatinous substance between the lens and the retina
- not continually replaced
- retains globe shape
Aqueous humour
a clear liquid that nourishes the cornea and the lens
* Bathes lens and cornea providing nutrients
* Drains into venous plexus at the filtration angle
* Production and drainage must remain balanced
Schlemm’s canal
at the junction between the iris and the cornea
- drains the aqueous humour
Sclera
outer layer of eye
- tough connective tissue
- merges with cornea
Cornea
transparent layer at front, for physical protection, refraction of light and highly sensitive
Choroid
vascularised middle layer of the eye
Retina
- innermost layer containing sensory cells and
neurons
Cell types in retina
- 2x sensory cell types; rods and cones
- rods; sensitive for vision in low light
- cones; colour vision
- bipolar cells; connect sensory cells and ganglion cells
- horizontal cells; connect groups of sensory cells
- amacrine cells; forms connections between ganglion cells
Layers of retina
3x layers
- Outermost layer; closest to choroid. Rods and cones
- Middle layer; conatins the interneuron cell types (bipolar, horizontal and amacrine)
- Innermost layer; ganglion cells that are connected to the optic nerve
The Optic Disc
Axons of the ganglion cells converge to form the optic nerve
- Optic disc is the point where the optic nerve leaves the eye & blood vessels enter
- Blind spot
Macula Lutea & Fovea Centralis
- Macula lutea; yellow area near posterior pole marks location of fovea centralis
- Fovea centralis; rod-free cone-packed area
- NO blood vessels
- Maximum visual accuity
Retinal vessels
- supply the bipolar and ganglion cells
Choroid plexus
- supplies the rods and cones
Segments of rods and cones
- Outer segment; modified cilia that form saccules and discs containing photosensitive compounds
- Inner segments; contains a nuclear region rich in mitochondria
- Synaptic zone; synapses with bipolar or horizontal cells
Rods vs. cones
- Cones have narrower tip compared to base
- Rods have long outer segments of even thickness
- Rods are more numerous usually and more sensitive
- Cones are more abundant near fovea
- Both have photopigments
Photopigments
- Photoreceptor cells hyperpolarise in response to light
- Light removes cGMP so cyclic nucleotide-gated ion channels are closed
- Rods and cones contain; Retinal and Opsins
- cis-retinal is converted to all-trans-retinal in light
- this activates transducin to convert cGMP to GMP
Retinal
- aldehyde of vitamin A
- 11-cis-retinal is converted to all-trans-retinal in light
Opsins
- modifies retinals sensitivity to differing wavelengths of light
Eye movements
-Saccadic eye movements (rapid) – take lots of snapshots which the brain interprets as a stable image
- Vestibulo-ocular reflex enables maintenance of an image despite rapid head movement
Physiological nystagmus
When image extends past range of ocular motion a correcting saccade brings the eye back
Pathological nystagmus
When the eye moves and saccades when the head is still indicating a problem with the vestibular system
Colour perception
- Depends on the different types of opsins and different cone types
- Can be di, tri… chromats, refering to number of types of cones
Visual Pathways
- the right side of each retina collects light from the left visual field and vice versa
- medial ganglia cross over at optic chiasm
- lateral ganglia do NOT cross over
- ## contralateral processing; both sides of the brain process info from both eyes
Retinal Convergence
- many rods can converge to just a single ganglion cells
- individual cones can synapse with individual ganglion cells
Vision processing and Ganglion cells
- ganglion cells only detect a difference in light intensity between the centre and periphery of their receptive fields
