10.1 accommodation Flashcards
what is accommodation ?
- increase in dioptric power of the eye with an effort to focus at near
what is accommodation caused by ?
- mostly caused by a change in shape of the crystalline lens
- lens becomes increasingly convex
what is near response triad ?
the three things that happen together
- pupil meiosis
- accommodation
- convergence
what are the different ways of accommodation in eye ?
- in some animals - lens is rigid and lens shifts forward and back in the eye
- cornea shape can change
what is the difference between accommodated and unaccommodated lens ?
accommodated - lens is thicker and surface is more convex and this gives the increase in dioptric power of eye which allows us to focus at near
how is lens suspended in the eye ?
- ciliary process come out from ciliary body and these are attached to lens zonules which
how is lens suspended in the eye ?
- ciliary process come out from ciliary body and these are attached to lens zonules which are attached to lens capsule
what is lens surrounded by ?
- lens is surrounded by the lens capsule , an acellular elastic membrane
- the elastic membrane allows lens to change shape
what attaches the lens capsule to the ciliary process ?
- sensory ligaments known as zonules attach the lens capsule to the ciliary process
how does lens get its convex shape during accommodation ?
- in response to a near stimulus the ciliary muscle contracts
- contraction of the ciliary muscle relaxes the tension on the zonules( stretchy wires that hold lens in shape ) and the lens takes on a more convex shape
explain the pathway that causes ciliary muscle contraction ?
- degree of defocus of the retinal image is assessed by the visual cortex (which makes an assesment of how sharp the image is)
- signal is sent from visual cortex to the edinger-westphal nucleus ( part of 3rd nerve nucleus )
- parasympathetic preganglionic fibres travel within the 3rd nerve to the ciliary ganglion
- postganglionic fibres after the ciliary ganglion that innervate the ciliary body and cause the ciliary muscles to contract
what happens as ciliary body moves inwards from contraction of the ciliary muscle ?
- lens moves inwards as tension on zonules is relaxed and lens can take more rounded shape
what is Helmholtz theory of accommodation ? right theory
- ciliary muscle contraction releases tension on zonules and lens take on a more convex shape
what is Tscherning theory of accommodation ?
- zonular tension increased , lens cortex pushed around nucleus to give more convex shape
what is Coleman theory of accommodation ?
- vitreous pressure causes change in posterior lens surface shape that is transmitted to the anterior surface
what Schachar theory of accommodation ?
- ciliary muscle contraction causes relaxation of some zonules and increased tension on equatorial zonules
- peripheral lens flattens and central lens steepens
what is presbyopia ?
- accommodation declines with age until it becomes clinically significant ( presbyopia )
- at age 50 accommodation is 0 - 0 ability to increase power of our lens - values measured are depth of focus
what happens to lens with age ?
- lens increases in mass with age
- equatorial lens epithelial cells start to lay down new cells that become fibres - theses fibres increase mass of lens
- increase in thickness of lens and anterior shift with age - more anterior than posterior
- human ciliary muscle maintains its mobility throughout life
what is ciliary muscle apex ?
ciliary muscle apex is displaced anteriorly and inward with age ( result of increased lens mass ) moving it closer to the lens : less effective
what is disaccommodation ?
the return of the lens to its state used for viewing distant object
what is decline in accommodation with age caused by ?
caused by increasing lens mass
