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Neural Biology 1 > ciliary body and accommodation > Flashcards

ciliary body and accommodation Flashcards

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1
Q

recap of lens ?

A
  • lens is an elliptical structure
  • lies behind the iris
  • attached to the ciliary body vis suspensory ligaments
  • centre of front surface is known as anterior pole
  • centre of back surface is known as posterior pole
  • running around the lens is the equator
  • the lens consists of 3 concentric layers ( capsule , epithelium , stroma )
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2
Q

why is lens capsule highly elastic ?

A
  • not due to highly elastic connective tissue

- it contains collagen which is coiled into super helices

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3
Q

what is the area between the equator and the anterior pole ?

A
  • germinative zone - this is where the epithelial cells undergo mitosis displacing cells towards the equator
  • epithelial cells already at the equator are pushed inwards by the new arrivals forming new lens fibres
  • therefore new lens fibres are formed at the equator due to epithelial cells
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4
Q

what happens to epithelial cells once they get to the lens stroma ?

A

they elongate and become very long cells which are hexagonal in shape and joined to one another by tight junctions and desomosmes and gap junctions which run from the anterior surface of lens to the posterior surface

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5
Q

what is important about lens fibres ?

A

they don’t have many organelles but have a high concentration of proteins known as crystallines which give the lens a high refractive index

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6
Q

what is the uvea ?

A
  • middle coat of eye
  • consists of iris, ciliary body and choroid
  • iris is attached to the ciliary body merges with the choroid where the retina begins at the ora serrata
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7
Q

what are the two distinct regions of the ciliary body ?

A
  • pars plicata - which has pleated/ folded surface and contains ciliary process
  • pars plana - smooth surface
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8
Q

what happens at the ora serrata?

A
  • where ciliary body would stop

- retina and the choroid begin

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9
Q

explain the dissection of human anterior eye seen from behind ?

A
  • retina lines the posterior surface of eye
  • retina merges with the ciliary body which is known as ora serrata
  • smooth part of ciliary body known as pars plana merges with the folded pars plicata
  • folded parts of pars plicata are known as ciliary process
  • moving inwards you have the iris which forms pupil
  • superimposed on iris and pupil is the lens
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10
Q

why is the junction between retina and ciliary body called the ora serrata?

A
  • junction between retina and ciliary body is not smooth, its serrated
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11
Q

what are dentate bay in the ora serrata?

A

bits of ciliary body sticking onto the retina

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12
Q

dentate process?

A
  • bits of retina that project into the pars plana

- important because it’s where suspensory ligaments attach - which attach lens to ciliary body

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13
Q

what is it called pars plicata ?

A
  • it has ciliary processes

- it has a lot of folds

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14
Q

what does a ciliary process look like in a histological section compared to dissection ?

A
  • thin, long structure sticking up
  • histological section is just a 2 dimension representation of complex 3 dimension object
  • dissection of human eye shows the ridge like nature of ciliary proceses
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15
Q

how do ciliary processes change with age ?

A
  • young human
  • 70-80 ridges which are even making up pars plicata
  • 2mm long
  • half mm wide
    1mm hight
  • as we age they become fatter and taller and irregular
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16
Q

what are the cells that line the surface of ciliary processes ?

A
  • epithelial cells
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17
Q

what is ciliary body covered by ?

A
  • 2 layered epithelium
  • inner un pigmented epithelium - closest to inside of eye
  • outer pigmented epithelium - closest to outside of eye
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18
Q

what does inner and outer refer to ?

A
  • inner and outer refer to proximity to the inside of the eye
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19
Q

what happens at the ora serrata when the retina starts ?

A
  • the inner un pigmented epithelium stops and is replaced by the retina
  • outer pigmented epithelium carries on and becomes retinal pigment epithelium
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20
Q

where is the ciliary body not covered by epithelial cell layer ?

A

at the tips of the ciliary processes where both layers are un pigmented and the processes appear as white hairs ( cilia ) - this is why they are called ciliary processes

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21
Q

what is in the ciliary body ?

A
  • double epithelium covers the whole of ciliary body
  • goes over ridged pars plicata and pars plana
  • stroma of ciliary body
  • ciliary muscle - large area of muscle particularly pars plicata
  • supraciliary layer - outside of which you have the sclera
  • drainage angle where you have sclera merging with the cornea via the limbal region - you have canal of schlemm which is a drainage channel
  • scleral spur- bit of sclera sticking into the trabecular meshwork
22
Q

what is supraciliary layer ?

A
  • thin layer- boundary layer
  • consists of few strands of collagen derived from the ciliary muscle and sclera as well as few fibroblast and melanocytes
  • has some significance in the drainage of aqueous
23
Q

what is ciliary muscle ?

A
  • smooth muscle- it control shape of lens in the process of accommodation
  • different fibres have different orientations
24
Q

how many muscle orientations are there in the ciliary muscle ?

A
  • three different orientation of ciliary muscle
25
Q

what are the three orientations of ciliary muscle ?

A
  • longitudinal - longitudinal fibres of smooth ciliary muscle insert into the scleral spur anteriorly and at the ora serrata posteriorly
  • oblique fibres - run from longitudinal fibres towards the ciliary processes
  • circular fibres - run all the way around the ciliary body forming a sphincter muscle around the lens
26
Q

what is different about smooth muscle fibres of the ciliary body?

A
  • has some characteristics of skeletal muscle
  • parallel myofibrils
  • some dense structure representing z lines
  • contracts and relaxes faster than most smooth muscle
27
Q

what are the properties of ciliary muscle ?

A
  • although ciliary muscle is smooth, it has some special properties ( dense innervation, parallel myofibrils, structure resembling z lines ) that allow it to contract relatively quickly
  • innervation of ciliary muscle is primarily parasympathetic
  • ciliary muscle deteriorates with age
28
Q

what is ciliary muscle stroma ?

A
  • made of dense connective tissue
  • collagen which separates the ciliary muscle from epithelium
  • highly vascular
  • fenestrated capillaries in the stroma - involved in exchange of substances
29
Q

how are lens and ciliary body connected ?

A
  • connected by suspensory ligaments
30
Q

what are suspensory ligament/ zonules fibres made up of ?

A
  • cysteine rich components of the elastic system called fibrillin
  • arise from dentate processes where they merge non pigmented epithelium of ciliary body
  • run over the surface of pars plana
  • they run between ciliary process
  • attached to the floor of valleys between the ciliary process via tension fibres
  • after leaving the ciliary valleys the suspensory ligaments change direction to insert into lens capsule
31
Q

where most of the eye’s refractive power?

A
  • most of the eye’s total refractive power (60D) is provided by the cornea(45D) due to the difference between the refractive index of air and cornea
32
Q

what is the lens refractive power ?

A

15D

- lens serves primarily to fine focus the image through the process of accommodation

33
Q

what happens in emmetropic eye is at rest ?

A

at rest the human eye is usually emmetropic ( parallel light rays from distant objects are focussed on the retina)

34
Q

where are close object focused in a emmetropic eye ?

A

close objects are focussed behind the retina

35
Q

what does the eye do t bring close objects into focus?

A

in order to bring close objects into focus the eye must increase its refractive power which is known as accommodation

36
Q

what happens in unaccommodated lens ?

A
  • lens is under tension and pulled flat by zonules
  • eye is focused for distant objects
  • close objects are focused in a plane behind the retina
37
Q

what happens in an accommodated lens ?

A
  • ciliary muscle contracts
  • tension on the lens is released and it take up a rounded shape resulting in an increased dioptric power of the lens- enabling the lens to bring closer objects into focus on the retina
  • lens is elastic
38
Q

how ciliary muscle contraction causes a release in tension on the lens during accommodation ?

A
  • an impulse is sent along the parasympathetic nervous system towards the ciliary muscle
  • ciliary muscle contracts
  • when longitudinal fibres contract the ciliary body will be moved towards the front of the eye
  • when circular muscle fibres contract they move ciliary body towards the lens
  • the end result of ciliary muscle contraction is that the ciliary body moves both forward and inwards and that releases tension on lens
39
Q

what is difference between when ciliary body is relaxed and when ciliary body contracts ?

A
  • as lens is under tension when ciliary muscle is relaxed
  • as soon as ciliary muscle contracts and moves the ciliary body towards the lens tension is released , as lens is elastic it take up round shape - more convex and focus close objects
  • contraction of ciliary body= tension released
    relaxed ciliary body = under tension
40
Q

summary of accommodation ?

A
  • in the relaxed eye ( focussed at distance ) , the ciliary muscle is relaxed. this puts tension on the lens pulling it flat
  • parasympathetic stimulation of the ciliary muscle cause it to contract which due to the arrangement of the ciliary muscle causes the ciliary muscle to move forwards and inwards towards the lens
  • therefore tension is released on the lens. due to the elastic capsule , it take a rounder shape
41
Q

what are other ways to focus closer objects?

A
  • direct deformation of lens ( bird) - ring of muscle around the lens which contracts to squeeze lens and make it fatter
    quicker accommodation
  • elongation of eye ( fogs)
  • alteration of corneal curvature ( birds )
  • movement of lens without shape change ( fish, amphibia)
42
Q

what is presbyopia ?

A

loss of accommodative ability with age

43
Q

what are the causes of presbyopia?

A
  • increased sclerosis of lens fibres- they can’t change shape as tension is put on or released
  • loss of elasticity of lens capsule
44
Q

explain the Glasser and Campbell study regarding the causes of presbyopia?

A
  • the ciliary body, with the lens attached was stretched and the focal point of the lens determined with laser beams
  • in young lens - tension was put on lens and change where lens was focused
  • in old people - ability to stretch the lens to change focus was reduced
  • no matter how much the lens was stretched the focal length could not be changed
45
Q

what is the neural pathway of accommodation reflex ?

A

. stimulus ( close object) -acts on receptors
. sensory neuron takes this information to the CNS
. CNS produces a response which it sends via motor neuron to the effector which is ciliary muscle
. response - accommodation

46
Q

what system innervates skeletal muscle ?

A

somatic nervous system

47
Q

what muscle does ANS innervate?

A

smooth muscle
cardiac muscle
gland cell

48
Q

what is iris muscle controlled by ?

A

as the iris muscles are smooth they are controlled by the ANS

49
Q

what is ciliary muscle controlled by ?

A

ciliary muscle are controlled by parasympathetic (ANS)

50
Q

explain neural pathway of accommodation in detail?

A
  • the degree of defocus of the retinal image is assessed by the cortex
  • a signal is sent to the EWN
  • parasympathetic preganglionic fibres travel within the 3rd nerve to ciliary ganglion
  • postganglionic fibres within the short ciliary nerves innervating the ciliary body and cause the ciliary muscles to contract
51
Q

what happens when we fixate close objects ?

A
  • the near triad ( near reflex )
  • ciliary muscle contraction ( focus)
  • pupil constriction ( increased depth of field )
  • eye convergence
  • improve quality of image

Neural Biology 1 (58 decks)

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