Special Senses: Vision (Physiology)

Question 1

Describe the basic structure of the eye

Answer 1

Cornea

• Collagen rich connective tissue containing blood vessels and nerves.
• Modified to admit light and responsible for greatest refraction of light.

Ciliary body

• Muscular ring around the lens.
• Changes lens thickness and secretes a fluid called the aqueous humor.

Iris

• Pigmented tissue with two muscular layers.
• Adjusts pupillary diameter.

Aqueous humor

• Fluid reabsorbed by the canal of Schlemm.
• Maintains the shape of the eyeball.

Lens

• Transparent tissue rich in elastic fibres.
• Focuses light rays on to the fovea.

Vitreous humor

• Transparent jelly like material.
• Maintains the shape of the eyeball.

Retina

• Thin transparent layer containing photoreceptors.
• Detects light entering the eye.

Fovea centralis

• Small area of the retina.
• Focused image is recreated here.

Optic nerve

• Point which this nerve leaves the eye (optic disc).
• Relays signals from the retina to the CNS.

Question 2

Explain the process of refraction by the lens and why it is important

Answer 2

• To form a focused image the lens needs to refract light rays entering the eye.
• How much the light rays need to be refracted depend on how far the object is away from the eye.
• When light is emitted from an object it is scattered in different directions. However, when given distance to do so, the light rays tend to reorganise themselves so they are parallel.
• The closer the object, the more scattered the light rays, and therefore the light rays need more refraction. To increase its refractive power the lens becomes thicker.
• Light rays from distant objects are parallel and therefore require less refraction. To do this the lens becomes flat and thin.
• The image is upside down and reversed left to right when received on the retina. Signals arriving at the visual cortex from the retina are interpreted so that the image is perceived correctly.

Question 3

Describe how accommodation occurs and why

Answer 3

• Accommodation is the process by which the lens changes shape. This allows light rays to be focused on the fovea centralis to create a focused image.
• The ciliary muscle enable the lens to do this.
• When viewing a distant object the ciliary muscles relax (due to sympathetic stimulation) so the suspensory ligaments become taut. This pulls the lens thin so that the light rays are refracted less.
• When viewing a near object the ciliary muscles contract (due to parasympathetic stimulation) so the suspensory ligaments slacken. This allows the lens to become thicker so light rays are refracted more.

Question 4

Describe the near response, why it is required, and how it occurs

Answer 4

• For close objects, not only does the lens need to become thicker but both pupils constrict and the eyes cross slightly (converge). These changes are collective referred to as the near response.
• Constriction occurs in order to reduce edge effects. The lens is not as effective at bending light rays around its edges because its thinner here. If scattered light rays strike these weaker parts of the lens they may bot be sufficiently bent to strike the fovea, causing the edges of the image to blur. The pupils therefore constrict to eliminate the scattered rays hitting the weakest parts of the lens.
• If a close object is focused on, activation of parasympathetic nerves innervates the iris, causing the circular muscles to contract and constrict the pupil. If a distant object is then focused on, the activation of sympathetic nerves causes dilation of the pupil.
• Convergence of the eyes occurs since the brain is producing a single image from information gathered by two eyes. It is therefore important that the image falls on the same part of each retina to help with integration. The position of the fovea on the retina means they must turn inwards when observing a near object, which is made possible by the extraocular muscles of the eye. The superior colliculi of the brain control eye movements.

Question 5

Describe the pupillary responses to light and the mechanisms responsible

Answer 5

• When light is directed into one eye the pupil constricts, known as the pupillary light reflex. The pupil of the other eye also constricts at the same time, known as the consensual light reflex.
• These light responses are controlled by the pretectal nuclei of the midbrain and the sympathetic and parasympathetic innervations have the same effect.
• The actual circuitries of the light reflexes and near responses are different. It is possible to lose the light responses but not the near response and vice versa.

Question 6

Describe the structure of the retina and events occurring here

Answer 6

• The retina is the only part of the brain that can be viewed without dissection.
• The optic disc is where the nerve fibres from all over the retina come together and leave the back of the eye as the optic nerve. There are no photoreceptors at this point and is therefore referred to as the blind spot.
• The pigmented epithelium of the retina absorbs light to prevent it from being reflected back into the eye.
• The bipolar cells allow the ganglion and photoreceptor cells to synapse and therefore communicate.
• The ganglion cells at the front relay signals to the optic nerve so that they might reach the visual areas of the brain, not only the area for image formation (visual cortex) but also the midbrain to help control pupillary diameter.

Question 7

Outline depth perception

Answer 7

• This is how the eyes work together to judge how far objects are away.
• Carried out by judging the position of other points relative to a fixation point.
• The fixation point is the fovea. When objects are further away, the image will be on the nasal side of the fovea and closer objects will be on the temporal side.

Question 8

Describe the pathways by which signals from the eyes reach the appropriate areas of the brain

Answer 8

• The visual cortex is the part of the brain which interprets nerve impulses coming from the eyes to form an image.
• The optic nerve leaves the back of each eye and crosses over at the optic chiasm. Half of the fibres from each nerve cross over to the opposite side of the brain which helps integrate the signals from each eye to form a single image.
• Since the right side of the brain controls motor responses on the left side of the body and vice versa, each side of the brain can see what is on the side of the body it controls.
• Some fibres which have crossed over also extend to the pretectal nuclei of the midbrain (mediate the pupillary aspects of the near response and the light reflexes) and the superior colliculi (controls eyeball orientation).