AP2 3.1: Vision and the Eye

Question 1

The five special senses

Answer 1

vision, smell, taste, hearing, and equilibrium (balance) are the senses that have specialized organs containing specialized receptor cells, which carry their impulses by way of specialized somatic and visceral afferents.

Question 2

The other sense

Answer 2

The other sense, touch, is a somatic sense that does not have a specialized sense organ. Instead, touch uses general receptors composed of modified dendrites of sensory neurons. Touch includes pressure, vibration, pain, heat, and the combined information is carried in general somatic afferents and general visceral afferents.

Question 3

organ responsible for sight

Answer 3

The organ responsible for sight is the eye, and it consists of three layers. The outer fibrous layer includes the sclera and cornea.

Question 4

sclera

the Eye

Answer 4

The posterior portion is known as the sclera. also known as the “white of the eye.

Question 5

cornea

the Eye

Answer 5

The anterior portion is known as the cornea. The cornea is the transparent part of the eye where light enters.

Question 6

Aqueous humor

the Eye

Answer 6

Aqueous humor fills the spaces between the cornea and the lens.

Question 6

The middle layer of the eye

the Eye

Answer 6

The middle layer of the eye includes choroid, the ciliary muscle, and the iris.

Question 7

choroid

the Eye

Answer 7

The darkly-colored posterior choroid prevents light from dispersing throughout the eye. The choroid is highly vascular and supplies blood to the other layers of the eye.

Question 8

ciliary body

the Eye

Answer 8

The anterior ciliary body changes the shape of the lens, allowing it to focus.

Question 9

iris

the Eye

Answer 9

The iris is anterior to the ciliary body and contains the circular colored portion of the eye. The iris controls the amount of light let into the pupil, a hole in the center of the iris. The iris uses its muscle fibers to contract or dilate based on the amount of light in the environment.

Question 10

Vision Pathway

Answer 10

Light passes into eye moving, progressively through the cornea, aqueous humor, lens, and vitreous humor to the posterior surface of the eye on the retina.

Question 10

vitreous body

the Eye

Answer 10

The interior of the eye, posterior to the lens, is called the vitreous body. It is a chamber filled with vitreous fluid, which helps to hold the retina firmly to the choroid.

Question 10

retina

the Eye

Answer 10

The inner sensory layer includes the retina. The retina contains containing two types of photoreceptors, cells that are sensitive to light. The more numerous cells are rods, which are stimulated in dim light. Rods are more sensitive to light but do not generate sharp or color images. The cones operate in bright light, helping to generate sharp color images.

Question 11

lens

the Eye

Answer 11

The lens is located posterior to the iris and pupil.

Question 11

Photoreceptors in the retina

Vision Pathway

Answer 11

Photoreceptors in the retina send a signal through the optic nerve on to the optic chiasm, located at the base of the hypothalamus.

Question 12

optic tracts

Vision Pathway

Answer 12

The medial fibers of the optic nerve cross to the other side when they reach the optic chiasm where the optic tracts are formed. The optic tracts terminate in the thalamus in a region called the lateral geniculate nucleus (LGN). The information then continues from the thalamus through the optic radiations to the primary visual area of the occipital lobe. Finally, in the occipital lobe, the incoming sensory information is interpreted as vision.

Question 13

binocular visual field

Answer 13

Overlapping information in the nasal visual fields allows for 3-D vision, called the binocular visual field.

Question 13

nasal visual field

Vision Pathway

Answer 13

Each eye receives information from the left and right sides of the body. The nasal visual field of each eye is closest to the nose.

Question 13

visual field

Answer 13

Each eye has a left visual field (purple in Figure 3.4) and a right visual field (orange in Figure 3.4).

Question 14

peripheral vision field

Vision Pathway

Answer 14

The peripheral vision field is on the lateral side of each eye.

Question 15

Each eye has a left and a right visual field

Answer 15

Sensory information gathered from the left side of the body (bilateral left visual fields) will eventually be interpreted in the right side of the brain and is reversed for the right visual fields. Information from the left visual field of each eye is gathered onto the ride side of each retina. The information from each retina is carried through the optic nerves to the optic tract. Only the medial fibers of the optic nerves cross at the optic chiasm. The left side of the brain receives information from the medial side of the left eye and the lateral side of the right eye (bilateral right visual fields). The situation is reversed for the right side of the brain, with the right side of the brain receiving information from the bilateral left visual fields.

Question 16

Eye Muscles

Eye Muscles

Answer 16

Eye movements are controlled by six muscles, called the extrinsic eye muscles. The medial rectus, inferior rectus, superior rectus, and inferior oblique are all innervated by the third cranial nerve, the oculomotor nerve.

Question 16

medial rectus

Eye Muscles

Answer 16

The medial rectus turns the eye medially.

Question 17

inferior rectus

Eye Muscles

Answer 17

The inferior rectus moves the eye medially and depresses it.

Question 18

superior rectus

Eye Muscles

Answer 18

The superior rectus moves the eye medially and elevates it.

Question 19

inferior oblique

Eye Muscles

Answer 19

The inferior oblique is responsible for eye elevation, lateral movement, and external rotation.

Question 20

lateral rectus

Eye Muscles

Answer 20

Finally, the sixth muscle, the lateral rectus, is controlled by the sixth cranial nerve, the abducens nerve, and turns the eye laterally.

Question 20

superior oblique

Eye Muscles

Answer 20

The fifth muscle is the superior oblique, which is controlled by the fourth cranial nerve, the trochlear nerve. The superior oblique is responsible for eye depression, lateral movement, and internal rotation.

Question 21

Visual System Pathology

Answer 21

Visual impairments occur from damage to any part of the vision pathway.

Question 22

Cranial nerve damage

Visual System Pathology

Answer 22

Cranial nerve damage can also cause vision problems. Several different cranial nerves supply the muscles to move the eye. For example, the abducens nerve is responsible for the lateral movement (left and right) of the eyes. Damage to the abducens nerve would cause diplopia, or double vision. One test for abducens nerve damage is to have a person look left and right while keeping the head still. If the lateral rectus does not receive nervous signals from the abducens nerve, the eye is unable to rotate laterally when looking to the same side. The affected eye deviates medially when looking forward because of the imbalance in muscle tone between the medial and lateral recti.

Question 22

Damage inside the brain

Visual System Pathology

Answer 22

Damage inside the brain, such as damage from a CVA or a brain tumor, can also cause vision impairments. For example, if the right optic nerve is damaged, the right eye will be unable to see.

Question 22

Cataracts

Visual System Pathology

Answer 22

Cataracts are lenses that slowly become hardened and cloudy over time. Cataracts make vision look blurry because the light is unable to enter the lens clearly to be refracted onto the retina.