Human Physiology and Structure- Week 7, Lecture 1 PDF

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The structure of the eye...

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Human Physiology and Structure- Week 7, Lecture 1 Main points:        

External and internal structures of the eye Circulation of aqueous humor Pupillary reflex and controlling light entering the eye Accommodation of the lens Movement of eyes Focusing of light on the retina Common refractive errors and correction of errors Visual acuity test

External structures of the Eye:

The Conjunctiva:      

The thin membrane Covers the sclera and inner surface of eyelids Fine blood vessels nourish the sclera and eyelids On the coloured part of the eye there is a clear tissue in front of it and its called cornea. The role of the conjunctiva is that it’s a thin membrane that consists of tiny blood vessels in it and provides blood supply to the surface of eye. Keeps the surface of the eye moist and protects it from infection

Internal surface of the Eye:

Cellular Structures of the Cornea and Sclera: Cornea:     

Transparent Normally without blood supply Acts like a window Allows entry of light in the eye Stroma is extracellular material with some cells

Sclera:    

Opaque Gives shape to the eye Dense in collagen fibres Has few cells

Cellular Structures of the Iris, Ciliary body and Lens:

Iris: Gives colour to the eyes   

Acts as a diaphragm Varies sizes of the pupil Has sphincter and radial muscles

Ciliary body: 

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Produces aqueous humor (the fluid that fills the space between the cornea and the lens) Supports the lens Has ciliary muscles

Lens:    

Double convex shape Normally transparent Encased in elastic lens capsule Focuses light on the retina

Suspensory ligaments (Zonules)  

Elastic fibres; extend from the ciliary processes to the lens equator Hold the lens in its place

Cellular Structures of the Choroid and Retina: Choroid:   

Provides blood supply to the eye and retina Shields the retina from extraneous light Has pigmented cells called melanocytes (red arrows)

Retina: 

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Composed of a layer of pigmented epithelium (blue arrow, single layered of cells) and multilayered neural retina has multiple specialised neurons. Senses light and converts it into visual impulse and visual stimulus (these are located adjacent to the pigment epithelium) Close connection between the retinal pigment epi

The retina:

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Has two types of photoreceptors, light sensing cells, cones and rods Macula (Macula lutea) in the central retina contains mostly cones Retinal structures are displaced to one side in the macula to allow light to focus directly on the photoreceptors and detailed vision Fovea- a 0.4mm pit in the macula contains only cones that provide detailed (central) and colour vision. Peripheral retina contain only rods Density of rods decreases and cones increases towards the macula

Circulation of Aqueous Humor: 

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The iris incompletely divides the anterior segment into anterior (space between the cornea and iris) and posterior (space behind the iris) chamber that are continuous through the pupil. Aqueous humor: o Produced by the ciliary processes o Fills the anterior and posterior chambers o Drained into the venous blood via the canal of Schlemm (scleral venous sinus) o Also diffuses through the vitreous humor in the posterior segment o Nourishes the cornea and lens o Its circulation maintains intraocular pressure

Iris controls the amount of light entering the eye: 

The iris regulates the amount of light entering into the eye by varying pupil size

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The iris has circular (sphincter) and radial (dilator) smooth muscle fibres that allow it to act like a reflexively activated diaphragm to vary pupil size. For near vision and in bright light, the circular muscles contract ant the pupil constricts, allowing regulated light to enter the eye. For distant vision and in dim light, the radial muscles contract and the pupil dilates, allowing more light to enter the eye. Pupillary constriction, a reflex action, is mediated by the parasympathetic fibres of the oculomotor nerve

Ciliary muscles control the shape of the lens (accommodation):

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Near accommodation for close vision: Contraction of the ciliary muscles moves the ciliary processes towards the lens that reduces elastic tension in the suspensory ligaments and the lens capsule, as a result the lens bulges and its curvature increases. Far accommodation for distant vision: Relaxation of the ciliary muscles stretches the elastic suspensory ligaments that in turn stretches the lens capsule and the lens becomes flatter. Tensile forces from the choroid and sclera also contribute to constriction and relaxation of the ciliary muscles. Operation of the ciliary muscles is mainly controlled by the parasympathetic fibres of the oculomotor nerves.

Cornea and Lens focus the light entering the eye on the retina:

Light passes from air to the eye through the cornea, aqueous humor, lens and vitreous humor and falls on the retina.  During the passage, light is bent three times- as it enters the cornea, and as it enters and leaves the lens.  The cornea is responsible for the most light refraction in the eye; it is the fixed refractory apparatus of the eye (ie. Its thickness is constant therefore its refractory power does not change). The lens is highly elastic and its curvature can actively change to allow fine focussing of light on the retina; it is the adjustable refractory apparatus of the eye. The humors are of minimal importance in light refraction. Light falling on the retina passes through the entire thinckness of the neural retina and excites the photoreceptors that abut the pigmented epithelium. Light falling on the central retina is focused on the fovea, the central pit in the macula, responsible for detailed (central) and colour vision. Visual signal generated by an object on the retina is inverted an horizontally flipped. 

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Ocular muscles control Eye movement:

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The movement of the eyeball, located in the bony orbit in the skull, is controlled by bundles of six extra-ocular muscles (as shown). These muscles facilitate movement of the eyeball above and below the horizon, on the left and right (medially and laterally), diagonally above and below on the left and right, and at the horizon. Convergence, medial rotation of the eyeballs by the medial rectus muscles, is controlled by somatic motor fibres of the oculomotor nerves and is important for focusing light on the retina. Degree of convergence depends upon the distance of the object being viewed.

How do eyes work for focusing light on the retina?

Human eye are best adapted for distant vision. For near vision, eyes must make adjustments. Distant vision: From the distant object, parallel rays of light enter the eye, there is      

Non or minimum convergence of eyeballs Pupils are dilated Ciliary muscles relaxed Suspensory ligaments stretched Lens flattened (narrow), and Light is focused on the retina

Near vision: from the near object, light rays enter the eye at an angle 

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Eyeballs converge depending upon the distance of the object (the closer the object, the greater convergence) Pupils are constricted Ciliary muscles contracted (budge)

Suspensory ligaments relaxed, and The lens accommodates to increase its curvature (bulged) to focus light on the retina

Convergence of eyeballs, adjustment of pupils, and accommodation of lenses must occur simultaneously to restore focus. Far and Near points of Vision: 

The far point of vision: the point from which light entering the eye is focused on the retina without adjustment of the ens. For normal (emmetropic) eyes the far point of vision is 6m (20 feet).

Light from an object at or beyond the far point of vision is focused on the retina by the fixed refractory apparatus (cornea and humors) and the at-rest lens. 

The near point vision: the point from which light entering the eye is focused on the retina with maximum adjustment of the lens. In young adults with normal (emmetropic) vision the near point of vision is 10cm (4 inches) from the eye.

Light from objects at the near point of vision is focused on the retina by the fixed refractory apparatus, pupillary reflex and accommodation of the lens. Constriction of the pupil prevents the most divergent light rays from entering the eye.  

Objects close than the near point of vision are visible but out of focus and fuzzy. In children, the near point of vision is close because of greater elasticity of the lens. The near point of vision recedes with age due to gradual loss of elasticity and accommodation of the lens.

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In many people over 40 years of age, the lens does not accommodate easily, a condition known as presbyopia. Presbyopia can be corrected with eyeglasses.

Refractive errors:   

When light entering the eye is not focused on the retina, this leads to blurred on the distorted vision or refractive errors. Refractive errors can occur from an over-accommodating (hyper-refractive) or underaccommodating (hypo-refractive) lens or structural abnormalities of the cornea and eyeballs. Myopia and hyperopia are common refractive errors.

Myopia (near or short sightedness): occurs when light from distant objects is focused in from of the retina. A linger than normal eyeball, steeper cornea and/or hyper-refractive lens (the light from distant objects doesn’t get focused onto the retina but focuses before the retina) can lead to myopia. People with myopia usually have clear near vision by blurred distant vision. Hyperopia (far sightedness): occurs when light from the near objects is focused behind the retina. Hyperopia can result from shorter than normal eyeball, flatter cornea and/or hypo-refractive lens. People with hyperopia have clear distant vision but blurred near vision. The focus point of light is behind the retina Correction of refractive errors:

Visual acuity assessment: Visual acuity tests:    

Measures the ability of the eye to see fine detail (central vision). Visual acuity is determined by measuring the smallest size print that a person can read. Visual acuity can be measured for both distant and near vision. Distance visual acuity is the most common test.

Distance visual acuity:  

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Uses a visual acuity chart to test distance visual acuity The chart uses capital letters of different sizes to test vision in literate adults (Snellen or LogMAR visual acuity chart) The test is conducted from a distance of 6m (20 feet, in the USA). The test results is indicated as a fraction (e.g. 6/12) The tip number of the fraction indicates the test distance. The bottom number represents the size of the letter seen. The larger bottom number, the large the letter seen o the chart, poorer the vision (e.g. 6/48 indicates poorer vision than 6/12). Normal visual acuity is recorded a 6/6 in Australia and as 20/20 in the USA.

Processes examined by an Optometrist:      

Visual acuity Pupil function Ocular motility Intraocular pressure- glaucoma Ophthalmoscopy- health of internal structures Visual fields- how well the visual processing/pathway works...