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Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image (focus) on an object as its distance changes. Accommodation acts like a reflex, but can also be consciously controlled. Mammals, birds and reptiles vary the optical power by changing the form of the elastic lens using the ciliary body (in humans up to 15 diopters). Fish and amphibians vary the power by changing the distance between a rigid lens and the retina with muscles. The young human eye can change focus from distance to 7 cm from the eye in 350 milliseconds. This dramatic change in focal power of the eye of approximately 12 diopters (a diopter is 1 divided by the focal length in meters) occurs as a consequence of a reduction in zonular tension induced by ciliary muscle contraction. The amplitude of accommodation declines with age. By the fifth decade of life the accommodative amplitude has declined so the near point of the eye is more remote than the reading distance. When this occurs the patient is presbyopic. Once presbyopia occurs, those who are emmetropic (do not require optical correction for distance vision) will need an optical aid for near vision; those who are myopic (nearsighted and require an optical correction for distance vision), will find that they see better at near without their distance correction; and those who are hyperopic (farsighted) will find that they may need a correction for both distance and near vision. The age-related decline in accommodation occurs almost universally, and by 60 years of age, most of the population will have noticed a decrease in their ability to focus on close objects. It is normally accompanied by a convergence of the eyes to keep them directed at the same point, sometimes termed accommodation convergence reflex. Theories of mechanism Helmholtz - The most widely held theory of accommodation is that proposed by Hermann von Helmholtz in 1855. When focusing at near the circular muscle fibers of the ciliary muscle contract decreasing the equatorial circumlenticular space which reduces zonular tension and allows the lens to round up and increase in optical power lens zonules. When viewing a distance object the circular ciliary muscle fibers relax which increases the equatorial circumlenticular space causing an increase in zonular tension. The increase in zonular tension causes the surfaces of the lens to flatten and the optical power of the lens to decrease. Helmholtzâ€™s theory of accommodation is inconsistent with the well-documented flattening of the anterior peripheral surfaces of the lens and negative shift of spherical aberration that occurs during human in vivo accommodation. Schachar - Ronald Schachar has contributed scientific insight into the mechanism of human accommodation, indicating that focus by the human lens is associated with increased tension on the lens via the equatorial zonules. Moreover, the evidence supporting the Schachar hypothesis disproves the older theory concerning the mechanism of accommodation of von Helmholtz. Schachar found that when the ciliary muscle contracts, equatorial zonular tension is increased. The increase in equatorial zonular tension causes the central surfaces of the crystalline lens to steepen, the central thickness of the lens to increase (anterior-posterior diameter), and the peripheral surfaces of the lens to flatten. While the tension on equatorial zonules is increased during accommodation, the anterior and posterior zonules are simultaneously relaxing. As a consequence of the changes in lens shape during human in vivo accommodation, the central optical power of the lens increases and spherical aberration of the lens shifts in the negative direction. Because of the increased equatorial zonular tension on the lens during accommodation, the stress on the lens capsule is increased and the lens remains stable and unaffected by gravity. The same shape changes that occur to the crystalline lens during accommodation are observed when equatorial tension is applied to any encapsulated biconvex object that encloses a minimally compressible material (volume change less than approximately 3%) and has an elliptical profile with an aspect ratio â‰¤ 0.6 (minor axis/major axis ratio). Equatorial tension is very efficient when applied to biconvex objects that have a profile with an aspect ratio â‰¤ 0.6. Minimal equatorial tension and only a small increase in equatorial diameter causes a large increase in central curvature. This explains why the aspect ratio of a vertebrate crystalline lens can be used to predict the qualitative amplitude of accommodation of the vertebrate eye. Vertebrates that have lenses with aspect ratios â‰¤ 0.6 have high amplitudes of accommodation; e.g., primates and falcons, while those vertebrates with lenticular aspect ratios > 0.6 have low amplitudes of accommodation; e.g. owls and antelopes. The decline in the amplitude of accommodation eventually results in the clinical manifestation of presbyopia; i.e., when the near focal point of the eye is more remote than the near reading distance. It has been widely suggested that the age-related decline in accommodation that leads to presbyopia occurs as a consequence of sclerosis (hardening) of the lens. However, the lens does not become sclerotic until after 40 years of age. In fact, the greatest decline in the amplitude of accommodation occurs during childhood, prior to the time that any change in hardness of the lens has been found. The decline in accommodative amplitude, rapid in childhood and slow thereafter, follows a logarithmic pattern that is similar to that of the increase in the equatorial diameter of the lens, which is the most likely basis for the accommodative loss. As the equatorial diameter of the lens continuously increases over life, baseline zonular tension simultaneously declines. This results in a reduction in baseline ciliary muscle length that is associated with both lens growth and increasing age. Since the ciliary muscle, like all muscles, has a length-tension relationship, the maximum force the ciliary muscle can apply decreases, as its length shortens with increasing age. This is the etiology of the age-related decline in accommodative amplitude that results in presbyopia. Any procedure that can prevent equatorial lens growth or increase the effective distance between the lens equator and the ciliary muscle can potentially increase the amplitude of accommodation. Catenary - D. Jackson Coleman proposes that the lens, zonule and anterior vitreous comprise a diaphragm between the anterior and vitreous chambers of the eye. Ciliary muscle contraction initiates a pressure gradient between the vitreous and aqueous compartments that support the anterior lens shape in the mechanically reproducible state of a steep radius of curvature in the center of the lens with slight flattening of the peripheral anterior lens, i.e. the shape, in cross section, of a catenary. The anterior capsule and the zonule form a trampoline shape or hammock shaped surface that is totally reproducible depending on the circular dimensions, i.e. the diameter of the ciliary body (MÃ¼ellerâ€™s muscle). The ciliary body thus directs the shape like the pylons of a suspension bridge, but does not need to support an equatorial traction force to flatten the lens. Induced effects of accommodation When someone accommodates to a near object, they also converge their eyes and constrict their pupils. The combination of these three movements (accommodation, convergence and miosis) is under the control of the Edinger-Westphal nucleus and is referred to as the near triad. Although, it is clear that convergence allows to focus the object's image on the retina, the functional role of the pupillary contraction remains less clear. Arguably, it may increase the depth of field by reducing the aperture of the eye, and thus reduce the amount of accommodation needed to bring the image in focus on the retina. There is a measurable ratio between how much convergence takes place because of accommodation (AC/A ratio, CA/C ratio). Abnormalities with this can lead to many orthoptic problems. Accommodative dysfuncti
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Answers:Everybody is different but here is what works for me. Close your eyes and try to concentrate on being completely relaxed. Concentrate on breathing through your nose and try to literally feel your shoulder and back muscles relaxing. Practice at home before you go to sleep. It takes time to master relaxation. Once you are at the doctor try not to think about the exam. I know that sounds impossible but it's not. Positive thinking helps. You are there because the doctor will help you, so try helping the doctor by trust. It's important. You need to trust him/her.
Answers:The ciliary muscle acts much like a sphincter muscle. What that means is when your brain sends a signal to stimulate and innervate the ciliary muscle the contraction that occurs actually DECREASES the distance between the muscle and the lens. When the ciliary muscle contracts it actually RELIEVES tension on the suspensory ligaments. Draw a circle on a piece of paper. This will represent the ciliary muscle. Now, inside this circle draw another circle. This will represent the lens. Now draw lines from the outer circle to the inner circle. These will represent the suspensory ligaments. Now imagine that by innervating the ciliary muscle you will see the outer circle act as a sphincter and draw CLOSER to the inner circle. If the outer circle draws closer to the inner circle then the ligaments connecting them will have to loosen. It does seem counter intuitive until you think it through. I just now looked at the diagram to which you referrred. Because this diagram is a lateral view it is difficult to realize the actual sphincter type arrangement of the ciliary muscle. The crude drawing which I had you make would be viewing the system from an anterior position and allows a better representation of the sphincter nature of the muscle.
Answers:The orbit is surrounded by layers of soft, fatty tissue which protect the eye and enable it to turn easily. Three pairs of extraocular muscles regulate the motion of each eye. There are six extraocular muscles which act to rotate an eye about its vertical, horizontal, and antero-posterior axes: 1.the medial rectus (MR), 2.the lateral rectus (LR), 3.the superior rectus (SR), 4.the inferior rectus (IR), 5.the superior oblique (SO), 6.and the inferior oblique muscle movements A given extraocular muscle moves an eye in a specific manner, as follows: medial rectus (MR) moves the eye toward the nose lateral rectus (LR) moves the eye away from the nose superior rectus (SR) primarily moves the eye upward and secondarily rotates the top of the eye toward the nose inferior rectus (IR) primarily moves the eye downward and secondarily rotates the top of the eye away from the nose superior oblique (SO) primarily rotates the top of the eye toward the nose and secondarily moves the eye downward inferior oblique (IO) primarily rotates the top of the eye away from the nose and secondarily moves the eye upward The primary muscle that moves an eye in a given direction is known as the agonist. A muscle in the same eye that moves the eye in the same direction as the agonist is known as a synergist, while the muscle in the same eye that moves the eye in the opposite direction of the agonist is the antagonist. According to Sherrington s Law, increased innervation to any agonist muscle is accompanied by a corresponding decrease in innervation to its antagonist muscle(s). muscle innervations Each extraocular muscle is innervated by a specific cranial nerve (C.N.): medial rectus (MR) cranial nerve III lateral rectus (LR) cranial nerve VI superior rectus (SR) cranial nerve III inferior rectus (IR) cranial nerve III superior oblique (SO) cranial nerve IV inferior oblique (IO) cranial nerve III The following can be used to remember the cranial nerve innervations of the six extraocular muscles: LR6(SO4)3. That is, the lateral rectus (LR) is innervated by C.N. 6, the superior oblique (SO) is innervated by C.N. 4, and the four remaining muscles (MR, SR, IR, and IO) are innervated by C.N. 3. anatomical arrangement All of the extraocular muscles, with the exception of the inferior oblique, form a cone within the bony orbit. The apex of this cone is located in the posterior aspect of the orbit, while the base of the cone is the attachment of the muscles around the midline of the eye. This conic structure is referred to as the annulus of Zinn, and within the cone runs the optic nerve (cranial nerve II), and within the optic nerve are contained the ophthalmic artery and the ophthalmic vein. The superior oblique muscle, although part of the cone-shaped annulus of Zinn, differs from the recti muscles in that before it attaches to the eye it passes through a ring-like tendon, the trochlea (which acts as a pulley), in the nasal portion of the orbit. The inferior oblique, which is not a member of the annulus of Zinn, arises from the lacrimal fossa in the nasal portion of the bony orbit and attaches to the inferior portion of the eye. ductions When considering each eye separately, any movement is called a duction. Describing movement around a vertical axis, abduction is a horizontal movement away from the nose caused by a contraction of the LR muscle with an equal relaxation of the MR muscle. Conversely, adduction is a horizontal movement toward the nose caused by a contraction of the MR muscle with an equal relaxation of the LR muscle. Describing movement around a horizontal axis, supraduction (elevation) is a vertical movement upward caused by the contraction of the SR and IO muscles with an equal relaxation of the of the IR and SO muscles. Conversely, infraduction (depression) is a vertical movement downward caused by the contraction of the IR and SO muscles with an equal relaxation of the SR and IO muscles. Describing movement around an antero-posterior axis, incycloduction (intorsion) is a nasal or inward rotation (of the top of the eye) caused by the contraction of the SR and SO muscles with an equal relaxation of the IR and IO muscles. Conversely, excycloduction (extorsion) is a temporal or outward rotation (of the top of the eye) caused by the contraction of the IR and IO muscles with an equal relaxation of the SR and SO muscles. versions When considering the eyes working together, a version or conjugate movement involves simultaneous movement of both eyes in the same direction. Agonist muscles in both eyes which work together to move the eyes in the same direction are said to be yoked together. According to Herring s Law, yoked muscles receive equal and simultaneous innervation. There are six principle versional movements: dextroversion (looking right) levoversion (looking left) supraversion or sursumversion (looking up) infraversion or deorsumversion (looking down) dextrocycloversion (rotation to the right) levocycloversion (rotation to the left) vergences A vergence or disconjugate movement involves simultaneous movement of both eyes in opposite directions. There are two principle vergence movements: convergence (looking nasally or inward crossed-eyes ) divergence (looking temporally or outward wall-eyes ) cardinal positions of gaze The cardinal positions are six positions of gaze which allow comparisons of the horizontal, vertical, and diagonal ocular movements produced by the six extraocular muscles. These are the six cardinal positions: up/right right down/right down/left left up/left In each position of gaze, one muscle of each eye is the primary mover of that eye and is yoked to the primary mover of the other eye. Below, each of the six cardinal positions of gaze is shown, along with upward gaze, downward gaze, and convergence strabismus (heterotropia) Normally, when viewing an object, the lines of sight of both eyes intersect at the object; that is, both eyes point directly at the object being viewed. An image of the object is focused upon the macula of each eye, and the brain merges the two retinal images into one. Sometimes, however, due to some type of extraocular muscle imbalance, one eye is not aligned with the other eye, resulting in a strabismus, also called a heterotropia or simply tropia. (Occasionally, this ocular deviation is referred to as a squint, although this term is not very discriptive and no longer is commonly used.) With strabismus, while one eye is fixating upon a particular object, the other eye is turned in another direction. As a result, the person either experiences diplopia (double vision), since two different objects are imaged onto the maculas of both eyes, or else the person s brain learns to suppress (turn off) the image of the strabismic eye to maintain single vision. If the vision in the strabismic (deviating) eye is suppressed (turned off) for too long, that eye very well may develop amblyopia or a lazy eye condition, meaning that the visual acuity in that eye no longer is as good as the visual acuity in the other eye which is used all the time. In this case, when the normal eye is covered, forcing the strabismic eye to fixate, the strabismic eye usually does not point exactly straight ahead, and the image of the object being viewed does not fall upon the macula but, rather, at some eccentric point away from the macula. Thus, this is referred to as eccentric fixation. (Note that amblyopia does not infer that an eye is lazy because it turns and does not align with the other eye. Rather, amblyopia refers to decreased visual acuity in one eye compared to the other eye, and the most common cause of amblyopia is eccentric fixation in a strabismic eye.) If the strabismus occurs sometimes, but not all the time, it is said to be interm
Answers:1) What is accommodation ? Accommodation is to change the focal length of the lens by changing the curvature of the eyelens. 2) Why does our eye need accommodation? Normally, when our ciliary muscles are relaxed, parallel rays form distant objects will converge onto the retina. If our eye is maintained at the above state, and a near object is put before it, light rays will converge behind the retina. As the sharp image is behind the retina, our brain can only detect a blurry image. To bring the image into focus, our eye does accommodation. 3) How does our eye accommodate? Remember that the cornea provides 2/3 of the refractive power and the lens only provide 1/3 ? However, our eye changes the curvature of the lens, rather than the cornea. The curvature of the cornea cannot be changed. . Changing the thickness of the lens means changing the focal length of the lens. Our eye lens is made of an elastic lens capsule and filled with compressible? lens substance. It is suspended by Zonular ligaments to the ciliary muscles. In the normal resting state: our ciliary muscle is relaxed the elastic lens tends to become thick aqueous & vitreous humour push outward on the sclerotic coat ligaments become taut / tensed lens pulled into a thin shape short focal length This is what happens when a near object is brought to our eye: sphincterlike action of circular muscle fibres + contraction of longitudinal muscle fibres contraction of ciliary muscle distance between edges of ciliary body decreases relaxation of suspensory ligament lens becomes thicker focal length shortens light rays converge earlier; image formed on retina