49 Visual System & Retina Flashcards
describe how images are formed by the eye
- light rays bend (refract) when they cross from ar into the curved surface of the cornea filled with aqueous humor
- rays are bent to converge on the retina and photoreceptors in the back of the eye
- refraction power of the cornea is ~ 42 diopters
describe what occurs during near vision
- during near vision, incoming light rays can no longer be considered parallel
- therefore, a stronger refractive power is needed to focus the light rays on the retina
- this is achieved by constriction of the circular ciliary muscle, which reduces its diameter and relaxes the suspensory ligaments
- the lens becomes more round/fatter, which increases its refractive power
describe what occurs during far vision
- during far vision, light rays originating from a distant object can be considered parallel
- the focus of light rays during far vision largely depends on the high refractive power of the cornea and only a small refractive power of the flat lens
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during far vision, the ciliary muscle which is a circular muscle around the lens, is relaxed
- this causes tightening of the suspensory ligaments (zonule fibers) which flattens the lens and minimizes the refractive power
describe refractive plasticity and what occurs with aging
the variability of the refractive power of the lens between far vision (13 D) and near vision (26 D) is called refractive plasticity
the lens loses its elasticity during aging, thereby reducing the ability to focus on near objects, a condition called presbyopia
describe the dark current
- photoreceptors are depolarized during darkness (in the absence of stimulus, aka light)
- during darkness, the visual pigment, which is coupled to a G-protein (transducin) is in its inactive state
- in the dark, the G-protein does not activate the enzyme cGMP phosphodiesterase
- as a consequence, there is plenty of cytoplasmic cGMP available, which keeps the cGMP fated channels in the photoreceptor membrane open
- this allows a continuous inward current of Na ions, which keeps the photoreceptors depolarized in the dark
- similar to other sensory neurons, photoreceptors release their neurotransmitter (glutamate) while they are depolarized during darkness
describe the phototransduction process
- there is absorption of light by the visual pgiment, which causes a conformational change of the retinal molecule from its inactive 11-cis isomer to its active all-trans isomer
- after activation, the enzyme cGMP phosphodiesterase (PDE) is activated via the G-protein
- the enzyme causes a breakdown of cGMP which causes closing of the cGMP gated channels
- as a consequence, photoreceptors are hyperpolarized during the light, which reduces/terminates the release of their neurotransmitter (glutamate) during hyperpolarization
describe emmetropia
in order to produce a sharp picture of parallel rays entering the eye ball, the refractive power of the optical apparatus of the eye has to match the length of the eye ball; emmetropia, which is normal sightedness
describe myopia (near sightedness)
- in myopia, the focus of parallel light rays, as in far vision, is anterior to the photosensitive elements of the retinal photoreceptors
- objects positioned closer to the eye, as in near vision, can still be focused on the retina, even without the contribution of the usual mechanisms for near accomodation
describe hyperopia (farsightedness)
- in hyperopia, the focus of parallel light rays would be beyond the retina, in case the ciliary muscle is relaxed and the lens has its lowest refractive power
- objects in the far distance from the eye can still be brought into focus by activating the mechanisms for near accomodation, which increases the refractive power of the eye
describe the regional differences in retinal structure
- periphery
- predominantly monopigmental rods
- with large receptive fields
- night-time vision
- predominantly monopigmental rods
- central
- predominantly multi-pigmental cones
- with small receptive fields
- predominantly multi-pigmental cones
- fovea
- highest density of cones
- no rods
- no blood vessels, bipolar, or ganglion cells -> cones only
describe the chart relating to regional difference in retinal structure
describe papilledema
- swelling of the optic disk
- due to increased intracranial pressure
- pressure transmitted to optic nerve sheath
- optic disk appears elevated and white, not yellow as is normal
- associated with:
- tumors arising in brainstem and cerebellum
- hematoma
- cerebral edema, TBI
- increased CSF
describe what is seen
bilateral papilledema with elevated disk margins
describe retinal detachment
- part of the retina pulls away from retinal pigment epithelium that provides oxygen and nutrients
- can lead to vision loss in area of detachment (scotoma)
- early symptoms include sudden appearance of floaters and flashes and reduced vision
