Physiology of Vision

Question 1

What is vision?

Answer 1

Vision is the perception of light reflected by various objects.

Question 2

What can the eyes and visual pathways in the CNS determine about an object?

Answer 2

The eyes and visual pathways in the CNS can determine an object’s size, shape, and color. They can also interpret the object’s distance, rate, and direction of movement.

Question 3

What is the basic unit of light that stimulates photoreceptors in the retina?

Answer 3

The basic unit of light that stimulates photoreceptors in the retina is a photon.

Question 4

What is the refractive index of air and water?

Answer 4

Air has a refractive index of 1, meaning light passing through air is not significantly bent. Water has a higher refractive index than air and can significantly refract light.

Question 5

What is the difference between a convex lens and a concave lens?

Answer 5

A convex lens has a surface that bulges outward in the middle region and causes light rays to bend inward, or converge as they pass through. A concave lens is thicker on the edge and depressed in the middle region, causing light rays to diverge or spread out (unfocused).

Question 6

What is the emmetropic state of the eye?

Answer 6

The emmetropic state is when the eye is relaxed and focusing on distant objects. In this state, the lens is in its normal flattened shape, and parallel light rays are minimally refracted by the cornea and focused on the retina.

Question 7

What happens to the lens when light rays from objects closer to the eye need more refraction?

Answer 7

When light rays from objects closer to the eye need more refraction (greater than the cornea can provide), the lens becomes more thickened (accommodation). This refracts light more than a flattened lens, so more light rays are focused on the retina.

Question 8

What allows for lens shape changes when viewing distant objects?

Answer 8

The ciliary body surrounding the lens relaxes when viewing distant objects, allowing for lens shape changes.

Question 9

What happens when the ciliary body contracts?

Answer 9

When the ciliary body contracts, it moves closer to the lens and the suspensory ligaments slacken. This allows the lens to change to a more thickened state, increasing its refractive power.

Question 10

What are the two events that accompany accommodation to allow for the ability to focus on near objects?

Answer 10

The two events that accompany accommodation are pupillary constriction, which limits the amount of scattered light that makes objects appear blurry, and convergence, which is the process by which eyeballs move more medially to direct light rays onto the photoreceptor-dense region of the fovea.

Question 11

What is the near point of accommodation?

Answer 11

The near point of accommodation is the closest point at which the eye can focus on an object. This increases with age as the lens becomes less flexible.

Question 12

What is presbyopia?

Answer 12

Presbyopia is a condition where an individual’s near point of accommodation is 10–20 inches or greater. This usually occurs in individuals in their 5th decade and can cause difficulty reading. It can be corrected with reading glasses or bifocals.

Question 13

What is hyperopia and how is it corrected?

Answer 13

Hyperopia, or farsightedness, occurs when the eyeball is too short or the cornea is too flat. The lens is unable to accommodate (become thick enough) to focus light on the retina, instead focusing behind the retina and causing blurry vision when looking at close objects. It can be corrected with convex lenses, which cause more light to converge on the retina.

Question 14

What is myopia and how is it corrected?

Answer 14

Myopia, or nearsightedness, occurs when the distance between the cornea and lens is too great or the cornea is too curved. The lens is unable to flatten enough and incoming light is focused in front of the retina, blurring objects viewed at a distance. It can be corrected with concave lenses, which diverge incoming light before it contacts the lens, redirecting the focus onto the retina.

Question 15

What is astigmatism?

Answer 15

Astigmatism is a condition where the curvature of the lens or cornea is irregular. This causes light rays to not be evenly refracted, resulting in blurred vision at all distances.

Question 16

What is LASIK and how is it used to correct astigmatism?

Answer 16

LASIK, or laser-assisted in situ keratomileusis, is a surgical procedure that uses a laser to remodel the cornea and smooth out irregularities. If astigmatism involves only the corneal surface, LASIK can be used to treat it. If astigmatism is due to deeper problems such as abnormal lens curvatures, a special form of LASIK may be used that induces asymmetry in the corneal surface to compensate for deeper irregularities. LASIK is also a popular procedure for the correction of myopia and hyperopia.

Question 17

What are the two types of photoreceptors in the retina and what are their functions?

Answer 17

The two types of photoreceptors in the retina are rods and cones. Cones function best in bright light for processing high-resolution color vision. Rods do not detect colors and are most sensitive in low light and as a component of peripheral vision.

Question 18

What is the role of retinal ganglion cells in the retina?

Answer 18

Retinal ganglion cells are located in the anteriormost region of the retina. Their axons form the optic nerve (CN II).

Question 19

What is rhodopsin and where is it found?

Answer 19

Rhodopsin is a pigment found in the cylindrical outer segments of rods. It is composed of the protein opsin and the pigment retinal. Rhodopsin absorbs light, and all rods contain this pigment.

Question 20

What is the difference between rhodopsin in rods and iodopsin in cones?

Answer 20

Rhodopsin, found in rods, is composed of the protein opsin and the pigment retinal. Iodopsin, found in cones, is composed of retinal and the protein photopsin. Photopsin is similar to opsin but has a slightly altered structure that allows it to absorb different wavelengths of light.

Question 21

How is light transduced into an electrical signal in photoreceptors?

Answer 21

The transduction of light into an electrical signal begins when a photon encounters a disc in the outer segment of a rod or cone. In the absence of stimulation (in the dark), photoreceptor cells are depolarized and continuously release neurotransmitters into synapses with other neurons. In the presence of light, the photoreceptor becomes hyperpolarized and stops releasing neurotransmitter, altering the activity of neighboring retina cells and sending information to the brain.

Question 22

What happens in the photoreceptor cells in the dark?

Answer 22

In the dark, opsin and cis-retinal combine to form rhodopsin in the disc membrane of the rod. The G protein complex transducin and the phosphodiesterase enzyme (PDE) are inactive. Sodium ion channels in the plasma membrane of the outer segment are opened by the second messenger cyclic guanosine monophosphate (cGMP), which is bound to them. Sodium ions flow down the concentration gradient into the cell and depolarize the cell (from −70 mV to −40 mV).

Question 23

What happens when light strikes a photoreceptor?

Answer 23

When light strikes a photoreceptor, it converts retinal to trans-retinal, which separates from opsin. This activates the G protein complex transducin, which in turn activates the phosphodiesterase enzyme (PDE). As a result, sodium ion channels close and the photoreceptor hyperpolarizes.

Question 24

What is rhodopsin and what happens to it in the presence of light?

Answer 24

Rhodopsin is a pigment found in rods that is composed of the protein opsin and the pigment retinal. In the presence of light, rhodopsin is bleached when trans-retinal dissociates from opsin, rendering it unable to respond to light.

Question 25

What is the process of dark and light adaptation in the retina?

Answer 25

Dark and light adaptation allow for adjustments in the amount of light present in the environment. In the dark, when light is suddenly reduced, cones can no longer function and rods are slow to regenerate enough rhodopsin to function. In the presence of light, when light is suddenly increased, it bleaches rods and cones, resulting in blinding glare. Rods become nonfunctional as rhodopsin is bleached as fast as it is regenerated, while cones can regenerate functional pigments faster and are able to respond within a few minutes as their sensitivity decreases.

Question 26

What is color blindness and what is the most common form?

Answer 26

Color blindness is a condition where a person lacks a functional gene for one or more cone pigments. The most common form involves a missing or defective gene for red or green pigment, causing affected people to have difficulty distinguishing red from green, with both colors appearing grayish-brown.

Question 27

Why is color blindness more common in males?

Answer 27

Color blindness is more common in males because the genes for red and green pigments are located on the X chromosome. Males have only one copy of the X chromosome, so a missing or defective gene results in color blindness. Females have two copies of the X chromosome, so if the second copy is normal, females will have normal vision.

Question 28

How is color blindness tested?

Answer 28

The most common test for color blindness uses Ishihara plates, which are collections of dots with a number embedded in a different color. For example, a person with red-green color blindness will not be able to detect the number 74 and will see 21 instead on a particular plate.

Question 29

What is the consensual pupillary response?

Answer 29

The consensual pupillary response is the constriction of both pupils in response to light entering only the left or right pupil. This happens because some axons terminate in the midbrain and communicate with the oculomotor nerve that innervates the pupillary muscle in both eyes. When light is detected in one eye, the pupil is constricted in both eyes reflexively to prevent damage to the retina, optic nerve, and brainstem.

Question 30

What is stereoscopic vision?

Answer 30

Stereoscopic vision, or depth perception, is the perception of the distance an object is from the eyes. This is possible because the eyes face anteriorly and much of the visual field for one eye overlaps with that of the other eye, a condition known as binocular vision. The brain compares these overlapping images to determine the distance an object is from the eyes.

Question 31

How does the brain process images?

Answer 31

Image processing by the brain involves more than just the detection and conscious awareness of visual stimuli. Retinal axons project to the lateral geniculate nucleus and primary visual cortex where the image from the retina is mapped. Visual information follows two main pathways from the primary visual cortex: the dorsal pathway, which leads to the parietal lobe and is involved in the interpretation of motion, and the ventral pathway, which ends in the inferior region of the temporal lobe and is involved in the processing of colors and forms of objects. Many other regions of the brain are associated with visual memory and behavioral decision making based on visual cues.

Question 32

What is color blindness and how common is it?

Answer 32

Color blindness is a condition where a person lacks a functional gene for one or more cone pigments. The most common form involves a missing or defective gene for red or green pigment, causing affected people to have difficulty distinguishing red from green, with both colors appearing grayish-brown. About 8–10% of males have some form of color blindness, compared to fewer than 1% of females. This is more common in males because the genes for red and green pigments are located on the X chromosome. Males have only one copy of the X chromosome, so a missing or defective gene results in color blindness. Females have two copies of the X chromosome, so if the second copy is normal, females will have normal vision.