[W8] The eye Flashcards
1
Q
what percentage of external input of information delivered to the brain is processed by the visual pathway?
A
up to 80%
2
Q
what are the 6 accessory structures of the eye?
A
- extrinsic eye muscles
- eyebrows
- eyelashes
- eyelids (palpebrae)
- conjunctiva
- lacrimal glands
3
Q
what is the proper name for the eyelids?
A
palpebrae
4
Q
Where are the Meibomian glands located, and what do they secrete?
A
- Meibomian glands are located within the tarsal plates of the eyelid
- release an oily secretion at the edge of the eyelid.
5
Q
What is the common tendinous ring (of Zinn)?
A
A ring of fibrous tissue surrounding the optic nerve at the apex of the orbit. It serves as the common origin of the four recti muscles of the extraocular muscle group.
6
Q
Name the seven extrinsic (extraocular) muscles.
A
- Levator palpebrae superioris
- Superior rectus
- Inferior rectus
- Medial rectus
- Lateral rectus
- Superior oblique
- Inferior oblique
7
Q
What is the conjunctiva?
A
A continuous, transparent lining of the eye made of specialized stratified columnar epithelium.
8
Q
Does the conjunctiva cover the cornea?
A
No, the conjunctiva does not cover the surface of the cornea.
9
Q
What is the function of goblet cells in the conjunctiva?
A
Goblet cells produce mucus to lubricate and moisten the eye.
10
Q
What is conjunctivitis (pink eye), and what causes it?
A
Conjunctivitis is inflammation of the conjunctiva caused by infectious agents or irritants (e.g., pollen).
11
Q
What is the function of the lacrimal glands?
A
The lacrimal glands continuously secrete tears, which are spread across the conjunctiva and cornea by blinking.
12
Q
Why is blinking important?
A
Blinking is an important defense mechanism that helps distribute tears and protect the eye.
13
Q
What is the average blink rate per minute?
A
The average blink rate is 15-20 blinks per minute.
14
Q
How often does a person blink on average?
A
Roughly once every 5-7 seconds.
15
Q
What percentage of waking hours is spent with eyes closed due to blinking?
A
On average, 10% of waking hours are spent with eyes shut due to blinking.
16
Q
What are the main components of lacrimal fluid?
A
Water, sodium ions, antibodies, and lysozyme.
17
Q
What are the 3 functions of lacrimal fluid?
A
Lubricates, cleanses, and moistens the anterior surface of the eye.
18
Q
What are the three layers of the pre-corneal tear film?
A
Lipid layer (superficial)
Aqueous layer (middle)
Mucus layer (inner)
19
Q
What are the 4 functions of the tear film?
A
- Prevents corneal drying
- Transports oxygen and nutrients to the cornea
- Maintains a clean and smooth cornea
- Protects against infection
20
Q
What is the normal pH range of tears?
A
7.3 – 7.7 (mean value 7.4).
21
Q
When is tear pH at its lowest?
A
On awakening.
22
Q
How much can tear pH vary?
A
Between 5.2 and 9.3, depending on age and diseases.
23
Q
What determines the buffer capacity of tears?
A
Bicarbonate ions, proteins, and mucins.
24
Q
Why are tears more acidic in contact-lens wearers?
A
Due to the impediment of carbon dioxide efflux.
25
In which conditions are tears more alkaline?
Dry eye, severe ocular rosacea, and lacrimal stenosis.
26
What 5 structures are found within the skull orbit along with the eye?
- The lacrimal gland
- extraocular eye muscles
- blood vessels
- cranial nerves
- orbital fat, which cushions the posterior and lateral sides of the eye.
27
What is the main arterial supply to the orbit and eye?
The ophthalmic artery, a branch of the internal carotid artery.
28
What does the central retinal artery supply?
The inner layers of the retina (travels in the optic nerve).
29
What do the short and long posterior ciliary arteries supply?
The ciliary body, iris, and choroid (they pierce the sclera).
30
What do the lacrimal arteries supply?
The lacrimal gland, conjunctiva, and eyelids.
31
What do the medial palpebrae arteries supply?
The eyelids.
32
What do the muscular arteries supply?
The skeletal muscles of the orbit and smooth muscles of the eyeball.
33
What do the dorsal nasal arteries supply?
The lacrimal sac.
34
What does the central retinal vein drain?
Blood from the inner layers of the retina.
35
Where does the central retinal vein travel?
in the optic nerve.
36
What do the vorticose veins drain?
Blood from the choroid.
37
How many vortex veins are there?
4
38
What does the superior ophthalmic vein drain?
Blood from the central retinal vein, superior vortex veins, and other orbital structures.
39
Where does the superior ophthalmic vein drain into?
The cavernous sinus.
40
What does the inferior ophthalmic vein drain?
Blood from the lower structures of the eye, including inferior vortex veins and surrounding tissues.
41
Where does the inferior ophthalmic vein drain into?
The cavernous sinus or sometimes the pterygoid plexus.
42
What is the location of the cavernous sinus?
On either side of the pituitary gland within the cranial cavity.
43
What is the drainage pathway from the ophthalmic veins?
Superior and inferior ophthalmic veins → cavernous sinus → internal jugular vein.
44
what are the 2 parts of the fibrous tunics?
sclera
cornea
45
what are the 3 parts of the vascular tunic?
choroid
ciliary body
iris
46
what are the 2 parts of the retina?
pigmented layer
neutral layer
47
What happens when light rays pass between two media of different densities?
They are refracted.
48
What is the refractive index?
A number that represents the comparative density of a medium.
49
What must light pass through before reaching the photoreceptor cells?
Air, cornea, aqueous humour, crystalline lens, vitreous humour, and the inner layers of the retina.
50
Which structures play a significant role in the refraction of light for vision?
The cornea and crystalline lens.
51
What does the anterior cavity of the eye contain?
Aqueous humour.
52
What separates the anterior cavity from the posterior cavity?
The crystalline lens.
53
Into what is the anterior cavity subdivided?
Anterior chamber and posterior chamber.
54
What separates the anterior chamber from the posterior chamber?
The iris.
55
What does the posterior cavity of the eye contain?
Vitreous humour, a gel-like substance.
56
What is the function of the vitreous humour?
It helps maintain the eye shape and supports the retina.
57
What is aqueous humour and how is it produced?
A transparent fluid produced from blood plasma by the ciliary processes of the ciliary body in the posterior chamber.
58
What does aqueous humour provide to the eye?
Nutrients and oxygen to the avascular lens and cornea.
59
How is aqueous humour resorbed from the anterior chamber?
Through the trabecular meshwork at the junction of the sclera and cornea into the scleral venous sinus (canal of Schlemm).
60
Where does the aqueous humour drain after passing through the scleral venous sinus?
Into nearby veins.
61
What maintains normal intraocular pressure (IOP)?
When the formation rate of aqueous humour is equal to the drainage rate.
62
What is the lens of the eye like in terms of structure?
Avascular, transparent, biconvex structure.
63
What is the lens composed of?
Precisely arranged layers of cells that have lost their organelles and are filled with a protein called crystallin.
64
What holds the lens in place?
The lens is held in place by the suspensory ligaments (zonule of Zinn), which attach it to the ciliary muscle.
65
What is the function of the lens?
It focuses incoming light onto the retina.
66
What is involved in focusing incoming light onto the retina?
The lens.
67
What happens when an object is more than 20 feet away?
- Extraocular muscles direct the eyes forward.
- Intraocular ciliary muscles relax.
- The ciliary body moves away from the lens.
- Tension on the suspensory ligaments increases.
- The lens flattens, decreasing refractive power.
68
What happens when an object is less than 20 feet away?
- Extraocular muscles contract to move the eyes medially (converge).
- Intraocular ciliary muscles contract.
- The ciliary body moves closer to the lens.
- Tension on the suspensory ligaments decreases.
- The lens becomes more spherical, increasing refractive power.
69
What is the process of making the lens more spherical to view close-up objects called?
Accommodation.
70
What happens to the lens shortly after age 40?
- The lens becomes less elastic and less able to become spherical.
- Weakening of the ciliary muscles occurs.
71
What difficulty does a person experience as a result of these changes in the lens?
- The lens may not be able to return to a more spherical shape needed for near vision.
- Vision becomes blurry at a near distance.
- Reading close-up words becomes more difficult, and eye strain occurs when working on a computer.
72
What is this age-related change in vision called?
Presbyopia.
73
What is the typical treatment for presbyopia?
- Reading eyeglasses with convex lenses.
- Multifocal or bifocal lenses, or contact lenses.
74
What is the outer fibrous white coat of the eyeball?
The sclera (posterior 5/6th).
75
What is the sclera continuous with anteriorly?
The transparent cornea.
76
What is the border between the cornea and sclera called?
The limbus (corneoscleral junction).
77
Why is the sclera opaque?
Due to the irregularity of the collagen fibers (dense connective tissue).
78
What are the 2 functions of the sclera?
- It maintains the shape of the eye.
- It gives attachment to the extrinsic muscles of the eye.
79
What is the cornea like in terms of structure?
- Transparent (due to its avascularity and parallel arrangement of corneal collagen).
- Dome-shaped (spherical).
80
How does the cornea receive nutrients and oxygen?
- From the aqueous humour for the inner layers of the cornea.
- From lacrimal fluid for the surface corneal epithelium.
81
What are the 3 functions of the cornea?
- Light rays pass through it to reach the retina.
- It refracts (bends) light rays to focus them on the retina.
- Provides protection through the corneal ‘blink’ reflex.
82
What are the 5 layers of the cornea?
- Epithelium
- Bowman's membrane
- Stroma
- Descemet’s membrane
- Endothelium
83
What is the middle layer of the eye called?
The uvea.
84
What does the uvea house?
An extensive array of blood vessels, lymph vessels, and the intrinsic muscles of the eye.
85
What are the three distinct regions that make up the uvea?
- Choroid
- Ciliary body
- Iris
86
What is the choroid?
- The most extensive region of the uvea (posterior 5/6th).
- Composed of areolar (loose) connective tissue.
87
What does the choroid house?
A network of capillaries and melanocytes.
88
What is the function of the choroid’s blood vessels?
They supply blood to the retina (the outer layer containing photoreceptors).
89
What do the melanocytes in the choroid do?
They produce melanin, which absorbs extraneous light and prevents it from scattering within the eye.
90
Where is the ciliary body located?
Immediately anterior to the choroid.
91
What is the ciliary body composed of?
Ciliary processes and the ciliary muscle.
92
What do the ciliary processes do?
They secrete aqueous humour.
93
What is the role of the ciliary muscle?
- It anchors the crystalline lens via the suspensory ligaments (zonule of Zinn).
- It is involved in accommodation.
94
What is the most anterior region of the vascular tunic?
The iris, which is the coloured portion of the eye.
95
What is the iris composed of?
Two layers of smooth muscle fibers, melanocytes, and an array of vascular and nervous structures.
96
What is located in the center of the iris?
The pupil, an opening that allows light to enter the eye and reach the retina.
97
How does the iris control the amount of light entering the eye?
By controlling pupil size (diameter) using two smooth muscle layers:
- The sphincter pupillae (concentric pattern)
- The dilator pupillae (radial pattern)
98
What happens when the sphincter pupillae muscle contracts?
The pupil becomes smaller.
99
What happens when the dilator pupillae muscle contracts?
The pupil becomes larger.
100
What happens in the pupillary reflex in bright light?
- Sensory input from the photoreceptors of the retina is sent to the brain.
- The brain initiates nerve signals along the parasympathetic division of the autonomic nervous system (ANS).
- The sphincter pupillae muscle contracts, causing the pupil diameter to decrease.
101
What happens in the pupillary reflex in low light levels?
- Nerve signals are initiated along the sympathetic division fibers of the ANS.
- The dilator pupillae muscle contracts, causing the pupil diameter to increase.
102
What are the two layers of the innermost lining of the eye?
An outer pigmented layer and an inner neural layer.
103
What is the ora serrata?
The margin between the photosensitive posterior retinal region and the non-photosensitive anterior retinal region.
104
What does the non-photosensitive portion of the retina do?
It continues anteriorly to cover the ciliary body and the posterior surface of the iris.
105
Where is the pigmented layer (RPE) located in relation to the choroid?
The pigmented layer is immediately internal to the choroid and is attached to it
106
What are the 4 functions of the retinal pigmented epithelium (RPE)?
- Provides vitamin A for the photoreceptors (neural layer).
- Absorbs extraneous light to prevent it from scattering within the eye (a function shared with the choroid).
- Forms the blood-retinal barrier (BRB), allowing for diffusion of nutrients and oxygen from the choroidal vessels to the neural layer.
- Aids in the reformation of rhodopsin, the photopigment in rods, essential for vision in low light.
107
What are photoreceptors and what are the two types?
- Photoreceptors are cells that detect light.
- The two types are rods (for low light vision) and cones (for color vision and visual acuity).
108
What are bipolar cells and how do they function?
- Bipolar cells have dendrites that synapse with rods and cones.
- Their axons synapse with the dendrites of ganglion cells.
109
What are ganglion cells and what is their role?
Ganglion cells' axons extend into and through the optic disc, forming the optic nerve.
110
What are horizontal cells and what is their function?
- Horizontal cells are located between photoreceptors and bipolar cells.
- They regulate the electrical signals from photoreceptors to other cells.
111
What is the macula lutea?
A rounded, yellowish region just lateral to the optic disc.
112
What is the fovea centralis and what is its function?
- A depression within the macula lutea.
- It contains the highest proportion of cones and almost no rods.
- It is the area of sharpest vision.
113
What is the peripheral retina?
- The most extensive region of the retina.
- It contains primarily rods and functions most effectively in low light.
114
What is the optic disc and why is it called the blind spot?
- The optic disc contains no photoreceptors.
- It is where the axons of ganglion cells extend from the back of the eye as the optic nerve.
- It is commonly called the blind spot.
115
What is MD (macular degeneration) and what are its symptoms?
- MD is a degenerative condition affecting the macula lutea.
- Patients experience central vision blurriness.
116
At what age does macular degeneration most commonly occur?
While it can occur in younger patients, it is more common in patients over 55 years old.
117
What causes age-related macular degeneration (AMD)?
- The formation of extracellular deposits (drusen) between the retina and choroid.
- This disrupts the function of the retinal pigment epithelium (RPE).
- Leads to atrophy and loss of photoreceptors in the macula, resulting in central vision loss.
118
Is there a cure for AMD?
There is no cure, but its progression can be slowed.
119
How can the progression of AMD be tracked?
Self-monitoring with the use of an Amsler grid can help track the progression.
120
What lifestyle changes can support eye health in patients with AMD?
- Smoking cessation and dietary changes.
- A diet rich in green leafy vegetables, fish, and omega-3 fatty acids can support eye health.
121
How do rods compare to cones in terms of shape?
Rods are longer and narrower than cones.
122
What are the 4 components of a rod?
- An outer segment (with discs).
- An inner segment (with mitochondria).
- A cell body (with the nucleus).
- A synaptic terminal.
123
How many rods are there in each eye and where are they located?
- Each eye contains more than 100 million rods.
- They are primarily located in the peripheral retina.
124
What do rods help with?
- They are activated by dim (low-intensity) light, which aids in night vision.
- They do not provide color recognition.
125
What is the photopigment in rods?
- The photopigment in rods is called rhodopsin.
- Rhodopsin is involved in the transduction of dim light and is most sensitive to light at a 500-nm wavelength.
126
How many cones are there in each eye and where are they concentrated?
- Cones occur at a density of less than 10 million per eye.
- They are concentrated in the fovea centralis.
127
What are the 4 components of a cone?
- An outer segment (with discs).
- An inner segment (with mitochondria).
- A cell body (with the nucleus).
- A synaptic terminal.
128
What do cones help with?
- They are activated by high-intensity light.
- They provide color recognition and precise visual sharpness.
129
What is the photopigment in cones?
The photopigment in cones is called photopsin.
130
How many types of photopsin are there, and what are their absorption wavelengths?
There are 3 different photopsin proteins:
- Blue cones: 420 nm.
- Green cones: 531 nm.
- Red cones: 558 nm.
131
What is color blindness?
Color blindness is an inherited genetic trait that occurs when an individual has an absence or deficit in one type of cone cell.
132
What is the most common form of color blindness?
The most common form is the X-linked recessive trait, which affects the red and green cone cells, resulting in red-green color blindness.
133
How is color blindness detected?
The Ishihara test is used for color blindness detection.
134
Where do the optic nerves converge?
The optic nerves converge at the optic chiasm, which is located anterior to the infundibular stalk (the connection between the hypothalamus and the posterior pituitary).
135
What happens to nasal retinal axons at the optic chiasm?
Nasal retinal axons cross into the contralateral optic tract.
136
What happens to temporal retinal axons at the optic chiasm?
Temporal retinal axons remain on the same side (ipsilateral) and enter the optic tract.
137
What is the primary destination of visual information after it travels around the midbrain?
Visual information primarily terminates in the lateral geniculate nucleus (LGN) of the thalamus. A small subset of fibers also travels to the pretectal area and superior colliculus.
138
What functions are associated with the pretectal area and superior colliculus?
- Pretectal area: Involved in the pupillary light reflex and lens accommodation reflex.
- Superior colliculus: Responsible for reflexive movements of the extrinsic eye muscles (e.g., turning the eyes and head in response to a visual stimulus).
139
What happens after the LGN in the visual pathway?
Axons from the LGN form the optic radiations, which project to the primary visual cortex in the occipital lobe.
140
Where do the lower and upper optic radiation fibers terminate?
- Lower optic radiation fibers terminate in the lower half of the visual cortex.
- Upper optic radiation fibers terminate in the upper half of the visual cortex.
141
How are retinal images processed by the visual system?
- Retinal images are inverted vertically and laterally.
- The nasal hemiretina fibers cross, so optic tracts, LGN, optic radiations, and the visual cortex receive input from the contralateral visual field.
142
