T18. SPECIAL SENSES II

Question 1

What is the primary function of the eyes in terms of energy conversion?

Answer 1

Eyes transduce light energy of a limited part of the electromagnetic spectrum to nerve impulses by excitation of photoreceptors.

Question 2

How can the anatomy of the eye be compared to a camera?

Answer 2

Lenses: convex (lens), cornea and humors; Diaphragm: iris and pupil; Photosensitive film: retina; Picture development: CNS.

Question 3

Trace the path of light through the eye to the retina.

Answer 3

Light passes through cornea → anterior chamber → pupil → lens → retina (photoreceptors).

Question 4

What muscles are involved in pupil dilation and constriction?

Answer 4

Radial muscles contract for dilation; Circular muscles contract for constriction.

Question 5

What is light refraction and what happens to the image?

Answer 5

Light bends when passing through different media; image is flipped upside down.

Question 6

How does curvature affect refraction in the eye?

Answer 6

Changing lens curvature allows fine control of focus by bending light more or less.

Question 7

How is the visual field projected onto the retina?

Answer 7

Right side of external world → left retina; Left side → right retina.

Question 8

What is the function of the lens in the eye?

Answer 8

It refracts light and helps focus images on the retina by modifying its convexity (accommodation).

Question 9

What structures suspend and adjust the curvature of the lens?

Answer 9

Suspensory ligaments and ciliary bodies; curvature is modified by ciliary smooth muscle.

Question 10

What is accommodation in the eye?

Answer 10

Ability of lens to focus on objects at different distances by changing shape.

Question 11

How does the ciliary muscle affect lens shape for near vision?

Answer 11

Contraction of ciliary muscle → suspensory ligaments relax → lens thickens → close vision.

Question 12

How does the ciliary muscle affect lens shape for distant vision?

Answer 12

Relaxation of ciliary muscle → suspensory ligaments tighten → lens thins → distant vision.

Question 13

What is presbyopia?

Answer 13

Loss of lens accommodation ability with age.

Question 14

What is visual acuity?

Answer 14

Sharpness of vision, the ability to distinguish two closely spaced dots.

Question 15

What is hyperopia?

Answer 15

Farsightedness: distant images focus behind retina; due to short eyeball; corrected with convex lenses.

Question 16

What is myopia?

Answer 16

Nearsightedness: distant images focus in front of retina; due to elongated eyeball; corrected with concave lenses.

Question 17

What is astigmatism?

Answer 17

Asymmetry between cornea and lens; corrected by cylindrical lenses.

Question 18

What part of the eye is light-sensitive?

Answer 18

Retina.

Question 19

What cells are responsible for color vision and night vision?

Answer 19

Cones for color vision; Rods for black-and-white and night vision.

Question 20

Where are photopigments located in rods and cones?

Answer 20

In the flattened discs of their outer segments.

Question 21

What do photoreceptors signal to in the retina?

Answer 21

They signal through bipolar cells → ganglion cells → optic nerve fibers → cerebral cortex.

Question 22

What are the layers of the retina involved in vision?

Answer 22

Photoreceptors → Bipolar cells → Ganglion cells.

Question 23

What are the functions of the retinal pigment epithelium?

Answer 23

Phagocytizes outer discs, absorbs scattered light, delivers nutrients to rods/cones, participates in visual cycle.

Question 24

What are the two main parts of rods and cones?

Answer 24

Outer segment (with discs) and inner segment.

Question 25

What is rhodopsin?

Answer 25

Pigment in rods that absorbs green light best.

Question 26

What happens when rhodopsin absorbs light?

Answer 26

It dissociates into retinaldehyde (retinal, from vitamin A) and opsin – called bleaching reaction.

Question 27

What form does retinal take in rhodopsin and after bleaching?

Answer 27

In rhodopsin: 11-cis; after bleaching: all-trans.

Question 28

How is retinal restored to 11-cis form?

Answer 28

By cis-trans isomerase in the pigment epithelial cells.

Question 29

What are key functions of the retinal pigment epithelium again?

Answer 29

Phagocytizes outer discs, absorbs light, delivers nutrients, visual cycle participation.

Question 30

What happens during dark adaptation?

Answer 30

Rods/cones regenerate visual pigments after ~20 minutes in dark; eyes adapt to low light.

Question 31

What is dark adaptation in vision?

Answer 31

Dark adaptation is the process by which the eyes adjust to darkness after being in the light, involving the regeneration of rhodopsin photopigments in rods and cones.

Question 32

How long does it take for eyes to adapt to darkness during dark adaptation?

Answer 32

Approximately 20 minutes.

Question 33

What happens to rhodopsin in the presence of light?

Answer 33

Rhodopsin dissociates, activating a second messenger system.

Question 34

What G-protein is activated during rhodopsin dissociation?

Answer 34

Transducin.

Question 35

What enzyme does transducin activate in the visual phototransduction pathway?

Answer 35

Phosphodiesterase.

Question 36

What does phosphodiesterase do in the phototransduction cascade?

Answer 36

It converts cGMP to GMP.

Question 37

What is the effect of cGMP being converted to GMP in rods and cones?

Answer 37

It causes the closure of Na+ channels, leading to hyperpolarization.

Question 38

Why does hyperpolarization of photoreceptors affect neurotransmitter release?

Answer 38

It stops the release of inhibitory neurotransmitters, allowing excitation of bipolar cells.

Question 39

What is the state of Na+ channels in photoreceptors in darkness?

Answer 39

They are open, keeping the photoreceptor depolarized.

Question 40

What kind of neurotransmitters are released in the dark?

Answer 40

Inhibitory neurotransmitters.

Question 41

How does light affect Na+ channels in photoreceptors?

Answer 41

Light causes them to close via the second messenger system.

Question 42

What happens to photoreceptors when Na+ channels close?

Answer 42

They become hyperpolarized.

Question 43

What is the final result of photoreceptor hyperpolarization in light?

Answer 43

Inhibition on bipolar cells is lifted, allowing them to stimulate ganglion cells.

Question 44

What do bipolar cells do when photoreceptors are hyperpolarized?

Answer 44

They stimulate ganglion cells.

Question 45

What happens to the visual signal in the dark?

Answer 45

Photoreceptors inhibit bipolar cells, so the visual signal is blocked.

Question 46

How do cones differ from rods in sensitivity to light?

Answer 46

Cones are less sensitive to light but provide color vision and higher visual acuity.

Question 47

What photopigments are found in cones?

Answer 47

Photopsins (protein portion) and retinal.

Question 48

What type of vision do cones allow?

Answer 48

Color vision and greater visual acuity.

Question 49

What are the three types of cones and their pigment sensitivity?

Answer 49

S cones (blue, short wavelength), M cones (green, medium wavelength), L cones (red, long wavelength).

Question 50

Where is vision sharpest in the retina?

Answer 50

At the fovea centralis.

Question 51

Why is acuity highest at the fovea centralis?

Answer 51

Because some retinal layers are pushed aside, and cones have a 1:1 ratio with ganglion cells.

Question 52

What type of cells do many rods converge onto?

Answer 52

A single ganglion cell.

Question 53

What does rod convergence onto a ganglion cell improve?

Answer 53

Light sensitivity.

Question 54

What pathway do ganglion cell axons follow?

Answer 54

They synapse on the lateral geniculate nucleus of the thalamus.

Question 55

What retinal information crosses at the optic chiasm?

Answer 55

Information from the medial (nasal) portion of the retina.

Question 56

Where do neurons from the thalamus project in the brain?

Answer 56

To the occipital lobe cortex.