special senses (vision) Flashcards
briefly describe the development and evolution of the eye
- 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)
describe human vision as a sense and its accessory structures
- 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
describe the components of the fibrous avascular outer layer
- 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
describe the components of the vascular pigmented middle layer
- 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)
describe the components of the inner layer (retina)
- 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
what are the different photoreceptors in the neural layer
- 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)
what is colour blindness
- x-linked recessive
- deficiency of colour vision
- red-green is most common
describe the signal pathway of light entering the eye
- cornea - aqueous humor - lens - vitreous humour - neural layer of retina - photoreceptors
- graded potential - bipolar cells - graded potential ganglion cells - action potential along optic nerve
describe the what visual pigments are
- 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
describe the entire pathway of light from eye to brain
- 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)
what is the differences in visible spectrums
- 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
what are the different colour perceptions in different species
- 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)
how is light refracted by the eye
- 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
how does the eye focus on objects
- 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
what are problems associated with refraction
- 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)
what are different diseases affecting the eyes
- cataract: change in colour of lens (usually transparent)
- glaucoma: aqueous humor doesn’t drain, pressure, compress optic nerve / blindness
- conjunctivitis: conjunctiva = swollen, irritated, infected
- retinal detachment: less visceral humor = less fluid, fluid between anterior / posterior segments = sagging
- ocular migraine: visual disturbance, self resolve without medication
describe the changes that are caused by the iris
- low light: increased sympathetic stimulation, dilator pupillae contracts (dilation)
- intense light: increased parasympathetic stimulation, sphincter pupillae contracts (constriction)
describe the process of photo-transduction cascade (in dark vs light)
dark: rod cell on
1. retinal binds to opsin and accumulates (on)
2. cGMP binds with Na channels, keeping them open and allowing influx of Na
3. Na causes depolarisation of cell, entering of Ca causes release of neurotransmitter
4. neurotransmitter causes bipolar cell hyper polarise (close Ca channels) and no AP is generated
light: rod cell off
1. retinal absorbs light, changes structure and shape
2. activates transducin which activates PDE and breaks down cGMP and closes Na channels (hyper polarisation)
3. pigment bleaching (retinal and opsin dissociate)
5. stops release of neurotransmitter and causes depolarisation of bipolar cell (opens Ca channels)
6. AP along optic nerve
