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- Identify the structures of the eye, limited to cornea, iris, pupil, lens, retina, optic nerve, ciliary muscles, suspensory ligaments and blind spot
Note: ciliary body in the diagram is the ciliary muscles
Each eye is set in a bony socket called the orbit. Only the front of the eye isn’t surrounded by bone.
Your eye is attached to your skull by muscles (shown by the branches that extend up and down from the eyeball in the diagram).
The iris is the coloured part of your eye.
The gap through which light enters the eye is called the pupil.
The suspensory ligaments hold the lens in place and contract and relax to change the shape of the lens so that light can be focused onto the retina. The ciliary muscles control the tension in the suspensory ligaments, determining whether the suspensory ligaments are contracted or relaxed.
The aqueous humour contains salts to nourish the lens.
The main body of the eye (called the vitreous chamber) contains the semi-solid fluid called the vitreous humour. This supports the eyeball by helping it retain its shape.
Next is the retina – this is the part that’s actually sensitive to light (it contains the light receptor cells). Some receptor cells are sensitive to light of different colours, helping to build up a coloured image.
When light falls onto the retina, impulses are sent down the optic nerve. Note that there are no receptor cells in the part where the optic nerve leaves the eye, so this part is called the blind spot.
- Describe the function of each part of the eye, limited to:
- Cornea – refracts light
The convex shape of the cornea allows the cornea to refract light to enter the eye through the pupil.
- Iris – controls how much light enters the pupil
The iris is the coloured part of the eye. It contains pigments that absorb light to prevent it from getting through to the retina. The gap through which light enters the eye is called the pupil. Thie size of the pupil (i.e. the aperture of the eye) can be adjusted to control how much light enters the eye. If too much light enters the eye, this can damage the retina.
- Lens – focuses light onto retina
The suspensory ligaments hold the lens in place. The suspensory ligaments and ciliary body work together to change the shape of the lens, so that light can be focused onto the retina.
- Retina – contains light receptors, some sensitive to light to different colours
The retina contains light receptors. There are two types of receptor cells in the retina: rod cells and cone cells. Rod cells are sensitive to dim light but can’t detect colours. Cone cells, on the other hand, are sensitive to colour but are only functional in bright light. There are three types of cone cells – ones that detect red light, blue light and green light (the three primary colours of light).
- Optic nerve – carries impulses to the brain
When light falls on the retina, the receptor cells generate electrical impulses called action potentials. These impulses are sent down the optic nerve to the brain, where the information is processed by the brain to produce the image that we see.
- Explain the pupil reflex in terms of light intensity and antagonistic action of circular and radial muscles in the iris
The iris contains two types of muscles – circular muscles and radial muscles. These control the contraction and relaxation of the pupil to change how much light enters the eye.
Note: the circular muscles and radial muscles are antagonistic – as one contracts, the other relaxes.
Radial muscles run outwards from the edge of the pupil and circular muscles circle the pupil.
In bright light (high light intensity), the radial muscles relax and the circular muscles contract, making the pupil smaller. This means less light can enter the eye.
In low light, the opposite occurs – radial muscles contract and the circular muscles relax, causing the pupil to dilate. This means more light can enter the eye.
This action of the muscles in the iris is called pupil reflex or iris reflex.
This is an example of a reflex action (because we do not need to make the conscious decision to do it – our body does it by itself.)
- Explain accommodation to view near and distant objects in terms of the contraction and relaxation of the ciliary muscles, tension in the suspensory ligaments, shape of the lens and refraction of light
Accommodation is the ability of the eye to change the shape of its lens to alter its focus from distant to near objects, and vice versa.
To focus light on the fovea, light must be refracted by the eye. This is done by the cornea and the lens. The cornea does most of the refracting, and the lens makes finer adjustments to get a clear, sharp, focused image.
Note that the image formed on the retina is inverted (upside down), but the brain interprets it so that we perceive it the right way up.
Light rays coming from a distance will be almost parallel, so they need to be refracted less.
The lens is held in place by a ring of suspensory ligaments. The tension in the suspensory ligaments, and thus the shape of the lens, is altered by the ciliary muscle.
When focusing on distant objects, the ciliary muscle is relaxed and the pressure in the vitreous humour causes the suspensory ligaments to be pulled tight. This causes the lens to be stretched thin, so the light is refracted less, allowing it to focus on the retina.
When focusing on nearby objects, the ciliary muscle contracts, allowing the suspensory ligaments to relax, so the lens gets thicker (it bulges), refracting the light more.
Notes submitted by Sarah
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