Vision Flashcards
What are the 4 steps required in order to see an object?
- Amount of light entering eye is regultaed - Iris aperture control
- Objects pattern must fall on vision receptors - Accommodation
- Energy from waves of photons must be transduced into APs - Phototransduction
- Brain must interpret signals - Visual pathways
Which two muscles are involved in regulation of light enterin the eye?
Two muscles in iris aperture control
Circular iris sphincter muscle
- Constriction = less light
- Parasympathetic control
Radial iris sphincter muscle
- Dilation = more light
- Sympathetic control
What is the photopupillary reflex?
Pupillary constriction and dilation occurs in response to light changes and changes when gaze shifts between objects of varying distance.
Photopupillary reflex - pupillary constriction in response to light
- Autonomic reflex arc
- Consensual
- Parasympathetic fibres in oculomotor nerves → ciliary ganglia → pupillary constriction
What is accommodation?
FOcusing - lens curvature adjusts such taht external image falls exactly on retina.
Near object
- Contraction of ciliary muscle
- Parasympathetic
- Round lens
Far
- Relaxatin of ciliary muscle
- Sympathetic control
- Flat lens
What are 4 types of accommodation abnormalities and how are they caused?
Myopia
- Nearsightedness
- Greater than normal curvature on lens causes image to before retina
Hyperopia
- Farsightedness
- Less than normal curvature on lens causes image to fall past retina
Astigmatism
- Irregular cornea (covering on pupil+lens) causes irregular pattern of vision
Presbyopia
- Stiffening of lens with aging
- Increased difficulty with near vision
What is stereoscopic vision?
Abiltiy to judge distance of objects - overlapping visual fields to look at objects from different angles.
What is required to correct myopia/hyperopia?
Myopia requires a diverging lens to make the image fall further back
Hyperopia requires a converging lens to make the image fall shorter onto the retina
What is the near response? 3 steps.
- Convergence of eyes
- Constriction of pupil (block out peripheral light)
- Accommodation of lens (ciliary muscle contracts)
Where are cones most densley packed?
At the fovea - center of macula lutea
What ist he retinal pathway?
Photorecptors → Bipolar cells → Ganglion cells → Optic nerve
What role do horizontal and amacrine cells play in the retinal pathway?
They are not involved in the pathway itself, but sharpen/modify teh signal
Describe the function and sensitivity of rods and cones/
ROds - 100 million/retina
- Detect prescence of photons, whatever wavelength
- Very sensitive, work even in dim light
- Edges of retina have rods for low-res motion detection
Cones - 3 million/retina
- Three types: Blue, gree and red cones
- Contain pigments sensitive to specific wavelength of light
- Less sensitive, work only in brighter light
What is the blind spot?
Optic disc - where axons of ganglion cells converge.
What is rhodopsin?
Opsin (protein) bound to cis form of retinal (Vitamin A)
What is the process of phototransduction in light and dark?
No light → depolarisation → glutamate release
Light → hyperpolarisation → stops glutamate release
Dark
- cGMP levels are high
- Na+ channel is open, depolarising cell
- Ca2+ channel open in response to depolarisation and glutamate is released
Light
- Light energy converts retinal from cis form to trans form
- Rhodopsin (retinal + opsin) dissociates
- Free opsin activates transducin
- Transducin activates phosphodiesterase (PDE) which breaks down cGMP
- Low cGMP = Na+ channels close, causing hyperpolarisation of cell
- Ca2+ channels close, glutamate isn’t released
When light stops, retinal goes back to cis form and binds with opsin.
What is the role of bipolar cells in phototransduction?
Dark
- Photoreceptors depolarise, release glutamate
- Glutamate depolarises OFF bipolar cells
- Glutamate hyperpolarises ON bipolar cells
- Results = signal is OFF, ganglion cells NOT stimulated
Light
- Photoreceptors hyperpolarised, glutamate is not released
- No glutamate = hyperpolarised OFF bipolar cells
- No glutamate = depolarised ON bipolar cells
- Result = signal is ON, ganglion cells are stimulated.
What is an optic chiasm?
Ganglions converge to form optic nerve.
Two optic nerves combine to form optic chiasm.
Where do optic tract axons end?
Lateral geniculate nucleus in the thalamus
What are superior and inferior colliculi responsible for?
Superior colliculi - visual reflex
Inferior colliculi - auditory reflex
What are the 1st, 2nd and 3rd order neurons of the visual projection pathway?
1st - Bipolar cells
2nd - Retinal ganglion cells
3rd - Neurons from lateral geniculate nucleus in thalamus project to visual cortex
What effect do optic nerve, optic chiasm and optic tract lesions have on vision?
Optic nerve - ipsilateral blind eye (same side, fully blind in 1 eye)
Optic chiasm - laterla half of both eyes gone
Optic tract - opposite half of vision gone (e.g. right optic tract = left half of both eyes)
