Physiology of Special Senses: Vision

Question 1

Describe eyebrows

Answer 1

• short, coarse hairs that overlie the supraorbital margins of the skull.
• Function - Shading the eye
  & Preventing perspiration from reaching the eye

Question 2

What are the muscles of the eyebrows. What is there role?

Answer 2

• Orbicularis muscle – depresses the eyebrows
• Corrugator muscles – move the eyebrows medially

Question 3

Describe the palpebrae/eyelid

Answer 3

• Protect the eye anteriorly from foreign objects.

Question 4

What is the palpebral fissure?

Answer 4

the medial and lateral commissures. separates eyelids

Question 5

What are the muscles of the eyelids. What is there role?

Answer 5

• The orbicularis muscle encircles the eye, and the eye closes when it contracts.
• the levator palpebrae superioris muscle, which raises that eyelid to open the eye.

Question 6

What is the conjunctiva?

Answer 6

• A transparent mucous membrane
• Lines the eyelids as the palpebral conjunctiva and folds back over the anterior surface of the eyeball as
  the bulbar conjunctiva.

Question 7

What is conjunctivitis?

Answer 7

• Inflammation of the conjunctiva results in reddened, irritated eyes.
• Pinkeye, a conjunctival infection caused by bacteria or viruses, is highly contagious.

Question 8

What is the lacrimal appartus?

Answer 8

(tear) consists of the lacrimal gland and the ducts that drain lacrimal secretions into the nasal cavity.

Question 9

Describe the lacrimal gland.

Answer 9

• It lies in the orbit above the lateral end of the eye.
• continually releases lacrimal secretion (tears) into the superior part of the conjunctival sac through several small excretory ducts.

Question 10

How do tears come out?

Answer 10

1. Blinking spreads the tears downward and across the eyeball to the medial commissure, where they enter the paired lacrimal canaliculi.
2. From the lacrimal canaliculi, the tears drain into the lacrimal sac and then into the nasolacrimal duct, which empties into the nasal cavity at the inferior nasal meatus.

Question 11

What is in the lacrimal fluid?

Answer 11

mucus, antibodies, and lysozyme, an enzyme that destroys bacteria

Question 12

What are the 3 layers of the eyeball wall?

Answer 12

the fibrous, vascular, and inner layers.

Question 13

Describe the regions of the fibrous layer of the eyeball.

Answer 13

1. Sclera

• protects and shapes the eyeball and provides a sturdy anchoring site for the extrinsic eye muscles.

2. Cornea

• odified to form the transparent cornea, which bulges anteriorly from its junction with the sclera.
• crystal-clear cornea forms a window that lets light enter
  the eye, and is a major part of the light-bending apparatus of the eye.
• well supplied with nerve endings, most of which are pain receptors

Question 14

Describe the regions of the vascular layer of the eyeball.

Answer 14

1. Choroid - nourish all eye layers.

• Its brown pigment, produced by melanocytes, helps absorb light, preventing it from scattering and reflecting within the eye

2. Ciliary Body

• Ciliary muscles make up most of the ciliary body and consist of interlacing smooth muscle bundles that control lens shape
• Ciliary processes which secrete the fluid that fills the cavity of the anterior segment of the eyeball.
• Ciliary zonule encircles the circumference of the lens and helps hold it in its upright position.

3. Iris

• made up of two smooth muscle layers, allow it to act as a reflexively activated diaphragm to vary pupil size:

Question 15

What are the differences in the iris between close vision & bright light, and distant vision & dim light?

Answer 15

• In close vision and bright light, the sphincter pupillae (circular muscles) contracts and the pupil constricts. Parasympathetic fibers control constriction
• In distant vision and dim light, the dilator pupillae (radial muscles) contracts and the pupil dilates, allowing more light to enter.

Question 16

What is the role of the pigmented layer of the retina?

Answer 16

• Absorb light and prevent it from scattering in the eye
• Act as phagocytes participating in photoreceptor cell renewal
• Store vitamin A needed by the photoreceptor cells

Question 17

What is the role of the neural layer of the retina?

Answer 17

• composed of three main types of neurons: photoreceptors, bipolar cells, and ganglion cells (AP) and leave as the optic nerve

Question 18

What is the difference between rods and cones?

Answer 18

• Rods are our dim-light. They are more numerous and far more sensitive to light than cones, but they do not provide sharp images or color vision.
• Cones are our vision receptors for bright light and provide high-resolution color vision.

Question 19

What are ganglion cell axons?

Answer 19

• Run along the inner surface of the retina
• Leave the eye as the optic nerve

Question 20

Describe the optic disc

Answer 20

• Is the site where the optic nerve leaves the eye
• Is a weak spot in posterior wall of the eye
• It is not reinforced by the sclera
• Lacks photoreceptors (the blind spot)

Question 21

What is the difference between the anterior segment and posterior segment?

Answer 21

• The anterior segment filled with aqueous humor
• The larger posterior segment behind the lens. is filled with vitreous humor that binds tremendous amounts of water.

Question 22

What is refraction?

Answer 22

refraction is the redirection of a wave as it passes from one medium to another.

• The redirection can be caused by the wave’s change in speed or by a change in the medium

Question 23

How many times is light bent when passing into the eye

Answer 23

1. Entering the cornea
2. Entering the lens
3. Leaving the lens

Question 24

How is the lens positioned during distant vision?

Answer 24

the sphincter like ciliary muscles are completely relaxed, and tension in the ciliary zonule stretches the lens flat. Consequently, the lens is as thin as it gets and is at its lowest refractory power when the ciliary muscle is at rest

Question 25

How is the lens positioned during close vision?

Answer 25

the ciliary muscles contract, pulling the ciliary body anteriorly toward the pupil and inward, releasing tension in the ciliary zonule. No longer stretched, the elastic lens recoils and bulges, providing the shorter focal length needed to focus the image of a close object on the retina. Parasympathetic fibers of the oculomotor nerves control the contraction of the ciliary muscles.

Question 26

Describe problems of refraction.

Answer 26

• Hyperopia or farsightedness, occurs when the parallel light rays from distant objects focus behind the retina.
• close objects appear blurry, and convex corrective lenses are needed to converge the light more strongly for close vision.

Question 27

When does phototransduction begin?

Answer 27

when light activates visual pigments in retinal photoreceptors

Question 28

Which pigments are embedded in areas of the plasma membrane that form discs.

Answer 28

Photopigments/visual pigments

Question 29

How do we see other colors besides blue, green, and red?

Answer 29

Cones’ absorption spectra overlap, and our perception of intermediate hues, such as yellow, orange, and purple, results from differential activation of more than one type of cone at the same time.

Question 30

Describe the process of Signal transmission in the retina

Answer 30

1. Light hyperpolarizes photoreceptors, which then stop releasing their inhibitory neurotransmitter (glutamate).
2. Voltage-gated Ca2+ channels close in synaptic terminals
3. With a lack of IPSPs, bipolar cells depolarize and open voltage-gated Ca2+ channels onto ganglion cells, releasing neurotransmitters.
4. Once the signal reaches the ganglion cells, it is converted into an AP due to the EPSPs.
5. This AP is transmitted to the brain along the ganglion cell axons that make up the optic nerve.

Question 31

What is the visual pathway to the brain?

Answer 31

1. Medial fibers of the optic nerve (from retinal ganglion cells) decussate at the optic chiasm
2. Most fibers of the optic tracts continue to the lateral geniculate body of the thalamus
3. Optic radiations travel from the thalamus to the primary visual cortex in the occipital lobes, where conscious perception of visual images (seeing) occurs

OR

2. optic tract fibers end in superior colliculi (initiating visual reflexes controlling the extrinsic muscles of the eyes), pretectal nuclei (involved with pupillary light reflexes), and to the suprachiasmatic nucleus of the hypothalamus (“timer” to set our daily biorhythms)