special senses (vision) Flashcards
1
Q
briefly describe the development and evolution of the eye
A
- patch of light sensitive cells
- eye cup (protection, directional sense of light source)
- fluid filled (causes refraction, better interpretation)
- cornea / camera (focus, specialised to function, protection, converging)
- compound (100’s of eyes, better directionality / radius)
2
Q
describe human vision as a sense and its accessory structures
A
- dominant sense (70% of all sensory receptors in eyes)
- half cerebral cortex = processing visual information
- accessory structures (protect the eye, aid in function), eyebrows, eyelids, conjunctiva (fluid), lacrimal apparatus (tears), 6 extrinsic muscles, 4 rectus muscles, 2 oblique muscles
3
Q
describe the components of the fibrous avascular outer layer
A
- sclera: tough connective tissue, makes up ‘white’ of eye
- cornea: transparent, front of eye, allows light to enter (anterior / posterior segments)
- A: aqueous humour (thin, plasma), supplies nutrients / oxygen to lens, cornea and retina, removes wastes
- P: vitreous humour (thicker, jelly), intra-ocular pressure (structure), holds neural layer of retina against pigmented layer
4
Q
describe the components of the vascular pigmented middle layer
A
- choroid: vascular, pigmented, under sclera, stops reflection of light if it reaches back of eye, tapetum lucidum (shiny, see in dark)
- lens: focuses light on the retina (behind cornea)
- ciliary body: ciliary muscles (attached to lens by zonular fibres)
- iris: colour, regulates light entering, between cornea and lens, part of ciliary body, adjusts diameter of pupil, sphincter pupillae (inner) and dilatory pupillae (outer)
5
Q
describe the components of the inner layer (retina)
A
- retina: pigmented layer
- fovea: area of greatest visual acuity, where light from centre of visual field strikes retina
- optic disc: site where optic nerves leaves eye, nerves / BV supplying eye pass through, no photoreceptors
- neural layer: photoreceptors (R/C) transduce light energy, bipolar and ganglion cells
6
Q
what are the different photoreceptors in the neural layer
A
- R: very sensitive to light, respond to dim light, more numerous, more at periphery, single pigment (grey tones), low acuity (many converge to one ganglion = no sharp images)
- C: low sensitivity, respond to bright light, fovea centralis (more), react quicker, 1 of 3 pigments, colour, high acuity (one per ganglion in fovea)
7
Q
what is colour blindness
A
- x-linked recessive
- deficiency of colour vision
- red-green is most common
8
Q
describe the signal pathway of light entering the eye
A
- cornea - aqueous humor - lens - vitreous humour - neural layer of retina - photoreceptors
- graded potential - bipolar cells - graded potential ganglion cells - action potential along optic nerve
9
Q
describe the what visual pigments are
A
- part of photoreceptor, flattened membranous plates contain visual pigments
- made up of 1 retinal molecule and 1 opsin protein (rhodopsin, red, green, blue)
- change shape as they absorb light
- depending on the type of opsin bound, retinal absorbs different wavelengths of the visible spectrum
10
Q
describe the entire pathway of light from eye to brain
A
- R / C activated by light
- transmitters released from R / C (graded potentials)
- communicate to bipolar cells in retina
- varying degrees of convergence occur (determines visual acuity)
- ganglion cells generate action potentials in their axons
- two optic nerves combine at optic chiasm (in front of brainstem), partial crossing over of info
- travel in optic tracts to terminate in nucleus of thalamus = lateral geniculate nucleus
- thalamic neurons form optic radiation, projects to primary visual cortex in occipital lobe (image information interpreted)
11
Q
what is the differences in visible spectrums
A
- humans: wavelengths of 380-760 nm, 3 colour sensitive cone cells (red, blue, green)
- mantis shrimp: 16 colour sensitive cells (UV / visible spectrum
- insects / birds: can see UV colours
12
Q
what are the different colour perceptions in different species
A
- dogs: dichromatic colour perception (2 colour cells, yellow and blue)
- sharks: monochromacy, nocturnal vision, probably no colour vision
- tetrochromats: rare humans, better than normal colour vision, 4th cone (tetrochromatic vision)
13
Q
how is light refracted by the eye
A
- cornea + lens = convex
- lightwaves hence converge to retina
- causes retinal image to be upside down and reversed from left to right
- light from top of an object = lower retinal surface
- light from bottom of an object = upper retinal surface
14
Q
how does the eye focus on objects
A
- light is refracted - entering cornea, entering lens and leaving lens
- close: PS contracts ciliary, lens rounded, focal point on fovea
- distant: S relaxes ciliary, lens flattens, tightens, focal point on fovea
15
Q
what are problems associated with refraction
A
- myopia: nearsightedness, image of distant object is projected in front of retina, fix (diverging concave lens)
- hyperopia: farsightedness, lens cannot refract enough to focus close image on retina, fix (converging convex lens)
