Fundamentals Flashcards
(135 cards)
1
Q
What is the Cloquet Canal?
A
● A remnant of hyaloid vasculature between the lens
and the optic nerve head
2
Q
What is a bergmeister papilla?
A
● Remnants of hyaloid vasculature near to or attached to the optic nerve head
3
Q
What is a mittendorf dot?
A
● A remnant of hyaloid vasculature that remains
attached to the posterior surface of the crystalline lens
4
Q
What are axenfeld loops?
A
● Ciliary nerves visible in the sclera near the limbus
5
Q
Fast-twitch fibers have what type of neuromuscular junctions and are important in what type of eye movements?
A
● En plaque neuromuscular junctions
● Important saccadic movements
6
Q
What are properties of tonic type muscle fibers?
A
● En grappe neuromuscular junction
● Important in smooth pursuit movements
7
Q
The hyaloid artery is a branch of what artery?
A
● Primitive dorsal ophthalmic artery
8
Q
Which portion of lacrimal gland should be biopsied?
A
● Orbital Portion
● Avoid the palpebral portion to prevent scarring of the ductules
9
Q
What are the characteristics of the corneal epithelium and the stroma regarding lipids and water?
A
● Epithelium is lipophilic
● Stroma is hydrophilic
10
Q
Describe the three orders of sympathetic chain.
A
● Hypothalamus to ciliospinal center of Budge-Waller
● Ciliospinal center to superior cervical ganglion
● Superior cervical ganglion to iris dilator muscle
11
Q
What is the significance of blue light?
A
● Phototoxic compound A2E, damages cytochrome
oxidase and leads to RPE death and photoreceptor death
● Important mechanism of ARMD
12
Q
What is the volume of adult orbit?
A
● 30 cm^3
13
Q
What is the original AREDS formula?
A
● Vitamin C – 500 mg
● Vitamin E – 400 IU
● Beta carotene – 15 mg
● Zinc oxide – 80 mg
● Cupric oxide – 2 mg
14
Q
What is the AREDS 2 formulation?
A
● Vitamin C – 500 mg
● Vitamin E – 400 IU
● Lutein – 10 mg
● Zeaxanthin 2 mg – 10 mg
● Zinc oxide – 80 mg
15
Q
What are the types of crystallins of the lens?
A
● Alpha – largest, 1/3 of lens protein
● Beta – predominant, 55% of water-soluble lens protein
● Gamma - smallest
16
Q
What are the actions of pilocarpine (muscarinic agonist)?
A
● Miosis
● Accommodation – the lens becomes more spherical
shaped, adds refractive power to overall refractive
state via circular muscle constriction
● Increased aqueous outflow - contraction of
longitudinal muscle causes tension on scleral spur
which opens trabecular meshwork
17
Q
What is the most common site of scleral rupture?
A
● Superonasal quadrant near the limbus
● Circumferential arc parallel to corneal limbus in the
quadrant opposite of side of impact
● Insertion of rectus muscles
● Equator of globe
18
Q
What is the light sensitive molecule of phototransduction which is regenerated via visual cycle?
A
● 11-cis retinal
19
Q
What is the number of axons in a normal adult optic nerve?
A
● 1.1 million (largest number at 16 weeks gestation, 3.7 million)
20
Q
What are the functions of the Muller cells?
A
● Muller cells – their end-footplates form the internal
limiting membrane and external limiting membrane.
● They provide a buffer to the extracellular space
● They play a role in Vitamin A metabolism of cones
21
Q
What are the main neural cells of inner nuclear layer?
A
● Bipolar, horizontal, and amacrine cells
22
Q
What are the 3 waves of neural crest cells that are responsible for the development of the anterior chamber?
A
● First wave: corneal endothelium, initially 2-3 cell layer
● Second wave: iris and pupillary membrane
● Third wave: corneal stroma, sclera
23
Q
What structures are derived from the neuroectoderm?
A
● Retinal Pigment Epithelium
● Neurosensory Retina
● Optic nerve, axons, glia
24
Q
What structures derived from cranial neural crest cells?
A
• Corneal stroma
• Corneal endothelium
• Iris pigment epithelium
• Sclera(also mesoderm)
• Trabecular meshwork
• Bones/cartilage
• Ciliary ganglion
• Choroidal stroma
• Extraocular muscle sheaths/tendons
• Fat (also mesoderm)
• Melanocytes (uveal and epithelial)
• Meningeal sheaths of optic nerve
• Vasculature and connective tissue
25
What structures are derived from the surface ectoderm?
● Lens
● Vitreous (also mesoderm)
● Conjunctiva epithelium
● Lacrimal drainage system
● Lacrimal gland
● Epithelium, glands, cilia of skin of eyelids, and caruncle
● Sweat glands
26
What structures are derived from the mesoderm?
● Extraocular muscle fibers
● Fat (also from neural crest)
● Iris stroma
● Iris sphincter
● Iris dilator
● Sclera (temporal portion)
● Vitreous (also surface ectoderm)
● Vascular endothelium
27
What is the parafoveal width?
● 0.5mm wide
● Area where ganglion cell layer, inner nuclear layers,
and outer plexiform layer are the thickest
28
How do you treat edrophonium (Tensilon) overdose during MG workup?
● Intravenous atropine 0.5mg
29
Describe rubella cataracts.
● Pearly white nuclear opacifications
● Rubella infection in 1st or 2nd trimester
● Patients with cataracts virtually never develop
glaucoma, and vice versa
● Retention of cell nuclei within lens fibers
30
Describe effects/side effects of retrobulbar lidocaine.
● Ocular anesthesia/akinesia
● Works within 5 minutes and lasts 1-2 hours
● Suppression of cough
● Decreased respiratory rate
● Drowsiness
31
What are the glands of Moll?
● Apocrine sweat glands
● Can forms cysts – apocrine hidrocystomas
● Eccrine
32
What kind of glands are Krause, Wolfring, and Lacrimal glands?
● Exocrine glands
● Gland secretes products into a duct
33
What kind of glands are meibomian glands and goblet cells?
● Holocrine glands
● Secretion by disruption of plasma membrane and
destruction of cell
34
The Tenon capsule fuses anteriorly with what structure and how far from the limbus?
● Intermuscular Septum
● 3 mm posterior to corneal limbus
35
After how many months of steroid use will steroid-induced IOP be permanent?
● 18 months or more
36
What are the branches of the ophthalmic artery after it has entered the muscle cone?
● Central Retinal Artery
● 2 long posterior ciliary arteries
37
What muscles are innervated by the superior branch of cranial nerve 3?
● Superior rectus and levator
38
What structures are innervated by the inferior branch of cranial nerve 3?
● Medial rectus
● Inferior rectus
● Inferior oblique
● Pupil
39
What is the average corneal dioptric power?
• 43D
• 49D anterior surface and -6D posterior surface
40
What testing is indicated in a person with optic nerve hypoplasia?
● MRI to evaluate for pituitary hypoplasia
● Septo-optic dysplasia (De Morsier Syndrome)
41
What are the three types of exocrine glands?
● Merocrine/eccrine
● Apocrine
● Holocrine
42
What are merocrine/eccrine glands and where are they located?
● Exocrine glands where cells secrete their substances by
exocytosis (freely released)
● Eyelid (sweat glands)
43
What are apocrine glands and what are some ocular examples?
• Exocrine glands where cells “bud” or “pinch off” plasma
membrane-bound vesicles
• Examples: glands of Moll
44
What are holocrine glands and what are some ocular examples?
● Entire cell disintegrates to release its substance
● Examples: meibomian glands, goblets
45
Cells of the basal layer of the corneal epithelium are attached to each other by what structures?
● Desmosomes
46
The basal layer of corneal epithelium is attached to the basal lamina by what structures?
● Hemidesmosomes
47
Superficial corneal epithelial cells attach to each other by what structures?
● Occlusion of zonular fibers (occluding zonules)
48
What is posterior embryotoxon?
● Thickened, prominent, anteriorly displaced Schwalbe line
● 15% - normal patients
● Associated most commonly with Axenfeld-Rieger
49
PAX6 mutations are associated with what conditions?
● Aniridia
● Peters anomaly
● Autosomal Dominant Keratitis
● Dominant Foveal Hypoplasia
50
What are the characteristics of photoreceptors?
● Outer segments of rods and cones contact apical
processes of RPE, no intercellular connections between
outer segments and RPE
● Rod photoreceptor outer segments have multiple discs
● Microtubules of rod outer segment cilium have “9 plus 0” configuration
● Rod discs are discrete structures
● Cone discs are attached to cell membrane (important difference)
51
What is the thickness of the sclera at different locations?
● Thinnest just posterior to EOM insertion = 0.3mm
● Thickest around optic nerve = 1.0 mm
● Anterior to muscle insertion = 0.6 mm
52
Where does cranial nerve V2 exit the skull?
● Foramen rotundum
53
Where does CN V3 exit the skull?
● Foramen ovale
54
What structure divides superior orbital fissure into two?
● Annulus of Zinn
55
What structures are outside the Annulus of Zinn?
● Lacrimal Nerve
● Frontal Nerve
● Trochlear Nerve
● (“LFTs”)
● Superior ophthalmic vein
56
What structures pass inside the Annulus of Zinn
● Abducens nerve
● Nasociliary nerve
● Optic nerve
● Superior and inferior branch of CN3
● Ophthalmic artery
57
Where do the upper retinal fibers travel in optic tract?
● Medially
58
Where do the lower retinal fibers travel in the optic tract?
● Laterally (Low = Lateral)
59
Where do macular retinal fibers travel in the optic tract?
● Dorsolateral position as they approach the lateral geniculate body
60
How thick, what type of collagen, and where does the Descemet membrane come from?
● Type 4 collagen
● Thickness 10 microns
● Secreted by endothelium
61
Describe the path of the ophthalmic artery.
● Ophthalmic artery is branch of internal carotid artery
just after it emerges from cavernous sinus
● Gives rise to the central retinal artery, lacrimal artery,
posterior ciliary arteries
● As it enters the annulus of Zinn, ophthalmic artery runs
inferior to the optic nerve
62
Describe the sizes of the perifovea, parafovea, fovea, and foveola.
● Perifovea = most peripheral portion of macula, 1.5 mm wide
● Parafovea = directly surrounding fovea, ganglion cell
layer is thickest, 0.5 mm wide
● Foveola = 350 microns, all cones, 4 mm temporal and
0.8 mm inferior
● Fovea 1.5 mm in diameter
63
What is the blood supply to different portions of optic nerve?
● Nerve fiber layer – CRA
● Prelaminar short posterior ciliary arteries (SPCA),
recurrent choroidal arteries
● Laminar – SPCA, branch of circle of Zinn-Haller
● Retrolaminar – pial vessels and SPCA
● Anterior intraorbital – branches of CRA pial vessels,
branches of ophthalmic artery
● Posterior intraorbital – pial vessels, branches of
ophthalmic artery
● Intracanalicular – Ophthalmic artery
● Intracranial – ophthalmic artery, ICA
64
What is the importance of ultraviolet A rays?
● 320 – 400 nm
● Penetrate lens, generate oxygen free radicals and
insoluble proteins
● Glutathione is major oxygen scavenger, diminishes
with age
65
What is the importance of ultraviolet B rays?
● <320 nm
● Largely filtered out by cornea and vitamin C in the
aqueous humor
66
What are the average internal volumes of the adult eye?
● 6.5 mL to 7 mL total
● Largest compartment = vitreous (5-6 mL)
● Anterior Chamber = 200 microliters
● Posterior Chamber = 60 microliters
67
Describe Leber Hereditary Optic Neuropathy.
● Single nucleotide base change “G” to “A” nucleotide
position 117788 in ND-4 gene
● Abnormality of NADH dehydrogenase of mitochondria
● Males 10-30 years
● Acute, severe (<20/200) painless, monocular vision
loss, central scotoma
● Elevation of disc, peripapillary telangiectasia, retinal
arteriolar tortuosity
● Fellow eye becomes involved weeks to months later
68
What ligament helps turns the course of levator palpebrae superioris from horizontal to vertical?
● Whitnall’s ligament (superior transverse ligament)
● Analogous in lower lid = Lockwood’s ligament
69
Describe the growth of the anterior and posterior lens capsules.
● Anterior capsule thickens with age
● Posterior does not
70
Describe the cycle of rhodopsin.
● When light is absorbed by 11-cis-retinal it is converted
to all-trans-retinal. It is then transported to the retinal
pigmented epithelium where it is recycled to
11-cis-retinal
71
What are the flattest 2 quadrants of peripheral cornea?
● Superior and nasal
72
The posterior medial canthal tendon attaches to which bone?
● Posterior lacrimal crest of lacrimal bone
73
The anterior medial canthal tendon attaches to which bone?
● Anterior lacrimal crest of maxillary bone
74
Fibers from which cranial nerve III subnuclei cross to innervate contralateral muscle?
● Superior rectus
75
Fibers from which cranial nerve III subnuclei are fused and supply muscles in both eyes?
● Levator subnuclei
76
Which cranial nerve has the longest intracranial course?
● Cranial Nerve 4
77
What structures are attached to lateral orbital tubercle?
● Ligament of lateral rectus
● Suspensory ligament of eyeball
● Lateral palpebral ligament
● Aponeurosis of levator muscle
● Whitnall ligament
78
What are the ocular manifestations of Gardner’s Syndrome (familial adenomatous polyposis)?
● CHRPE-like lesions
● Lesions are smaller, more ovoid, less uniform in
pigmentation, multiple, bilateral
● Lesions are hyperplasia of RPE, therefore hamartomas
● CHRPE is hypertrophy of RPE
79
What molecule creates the viscosity of the vitreous?
● Hyaluronic acid (also manufactured to maintain the
anterior chamber as a viscoelastic device)
80
Rhodopsin is a combination of what molecules?
● Opsin + 11-cis-retinaldehyde
81
What are some common mydriatics/cycloplegics and their duration of action?
● Phenylephrine: 3-5h
● Tropicamide: 4-6h
● Cyclopentolate: 2 days
● Homatropine: 3 days
● Scopolamine: 4-7 days
● Atropine: 7-14 days
82
What are two mechanisms of aqueous humor formation?
● Active secretion: pressure independent, Na-K-ATPase,
carbonic anhydrase II
● Passive secretion: diffuse ands ultrafiltration
● Diffusion – related to concentration and charge
● Ultrafiltration – pressure dependent, depends on IOP,
BP, blood osmotic pressure
83
Which membrane surface of RPE has Na-K-ATPase?
● Apical
● ATPase transports ions from subretinal space to RPE cells
● Net flow of ions across this apical surface creates
electrical potential – can be measured – basis of
electrooculogram
84
What is the inheritance pattern, gene, and key testing characteristics of Best disease?
● Autosomal dominant
● Mutation in VMD2 gene/ BEST1 gene, codes
bestrophin, a transmembrane chloride channel on
basolateral membrane of RPE
● Abnormal EOG, normal ERG
85
What are the 3 predominant glycosaminoglycans (GAGs) in the corneal stroma?
● Keratin sulfate
● Chondroitin sulfate
● Dermatan sulfate
86
How does pilocarpine increase aqueous outflow?
● Contracts longitudinal portion of ciliary muscle which
attaches to scleral spur, contracture causes opening of
the trabecular meshwork
87
How does epinephrine lower IOP?
● Improves trabecular outflow and possibly increasing
uveoscleral outflow.
● No longer available in US as adverse effects are common
88
What is unique about the ionic (sodium and potassium) compositions of the lens?
● Low sodium
● High potassium
● Due to Na+/K+/ATPase pump that pumps sodium out
and potassium into lens
89
When would you want to avoid succinylcholine during surgery?
● During ruptured globe repairs
● Causes depolarization of the extraocular muscles and
leads to unwanted contraction. This can cause
extrusion of intraocular contents through an ocular laceration.
● Also in a patient with history of malignant
hyperthermia
90
What are the differences between Miochol and Miostat?
● Miochol = acetylcholine, acts within seconds, short
duration, rapidly broken down by acetylcholinesterase
in the anterior chamber
● Miostat= carbachol, maximal action in 5 minutes, not
easily broken down by Ach-ase, thus lasts much longer
(24h), more effective in lowering IOP
● Both used for miotic effect, to determine if there is
vitreous prolapse into AC, pupil is often peaked against
the taut strand of vitreous
91
What are Blessig-Iwanoff cysts?
● Very thin and cystic appearing lesions in peripheral
retina near the ora serrata
92
Posterior ethmoid air cells drain into what structure?
● Superior meatus
93
Is Betaxolol a beta-1 antagonist, beta-2 antagonist, or mixed?
● Beta-1 selective antagonist
● Not as effective in lowering IOP compared to
non-selective beta blockers
94
What is unique about carteolol?
● It has intrinsic sympathomimetic activity
● Causes slight to moderate activation of receptors
● Reduces effect on cardiovascular and respiratory systems
95
What is Kearns-Sayre Syndrome?
● Chronic progressive external ophthalmoplegia
(maternal inheritance)
● Severe retinitis pigmentosa
● Heart block
96
Which layer of the ciliary epithelium maintains the blood aqueous barrier?
● Nonpigmented epithelium via tight junctions
● In contrast, pigmented ciliary epithelium is “leaky”
97
What is Weill-Marchesani Syndrome?
● Ectopia lentis (anterior)
● Microspherophakia
● Short Limbs
● Brachydactyly
98
What are three disorders associated with lens ectopia?
● Marfan’s (superotemporal)
● Homocystinuria (inferonasal)
● Weill-Marchensani syndrome (small lens, anterior
displacement)
99
What wavelength is Rhodopsin most sensitive to?
● 510 nm (green light)
● Less absorption to blue and yellow light
● Cannot absorb longer wavelengths (red)
100
What wavelength does an excimer laser emit and type of laser is it?
● Emits light at 193 nm (UV)
● Argon fluoride laser
101
What frequency of light does a Nd:YAG laser emit?
● 1064 nm wavelength
102
What is the wavelength of light of the Femtosecond laser?
● 1053 nm (infrared)
103
What is the mechanism of action of lidocaine?
● Increases threshold for electrical excitability and eventually blocks nerve conduction by blocking axonal
sodium channels
104
What is the shape of the normal cornea?
● Prolate (flattens from center to periphery up to 4.00D)
● Refractive surgery – hyperopic ablation makes the
cornea more prolate
● Myopic ablation makes the cornea more oblate
(central cornea flatter
105
Which subnuclei of CN3 is fused and supplies axons to muscles in both eyes?
● Levator subnuclei
106
What CN3 subnuclei supplies fibers that cross to innervate the contralateral muscle?
● Superior rectus subnuclei
107
What type of collagen makes up the Bowman layer?
● Type 1 and 5
108
Interconnections between what cells make up the inner plexiform layer?
● Bipolar
● Amacrine
● Ganglion
● IN a BAG (IN for inner plexiform)
109
What separates the greater and lesser wings of sphenoid?
● Superior orbital fissure
110
When does tear production first occur?
● 20 or more days after birth
111
Valacyclovir is a prodrug of what drug?
● Acyclovir
112
Famciclovir is prodrug of what other antiviral drug?
● Penciclovir
113
Where is the ciliary ganglion located?
● Approximately 1 cm anterior to annulus of Zinn
between optic nerve and lateral rectus muscle
114
List 4 direct acting cholinergic agonists.
● Pilocarpine
● Miochol (acetylcholine)
● Miostat (carbachol)
● Bethanechol
115
What is the origin of the cilioretinal artery and what percentage of people have macular-supplying cilioretinal artery?
● 20-30% of people
● Cilioretinal artery arises from short posterior ciliary artery
rather than central retinal artery (CRA)
● Not everyone who has cilioretinal artery have collateral
circulation to their macular supplied by the cilioretinal
artery
● Below is picture of CRA occlusion with patent cilioretinal
artery. Picture shows retinal retinal ischemic whitening in
the distribution of CRA but preservation of normal retinal
transparency in the zon4 supplied by cilioretinal artery
116
What is the most common cause of cilioretinal artery occlusion and why?
● Most common occurs in the setting of central retinal
vein occlusion
○ Due to associated increased hydrostatic pressure
→ reduce blood flow in the cilioretinal artery to
the point of stagnation
117
What is a potentially fatal side effect associated with systemic mannitol and urea?
● Subarachnoid hemorrhage
118
What is the blood supply to various regions of optic nerve?
● Nerve fiber layer: Central retinal artery (CRA)
● Prelaminar nerve: Short posterior ciliary arteries and
recurrent choroidal arteries
● Lamina: Short posterior ciliary arteries or branches of
arterial circle of Haller and Zinn (circle of Zinn-Haller).
CRA does not supply this region
● Retrolaminar nerve: Mainly by pial vessels and short
posterior ciliary vessels. Some help from CRA and
recurrent choroidal arteries
● Intra-orbital region: Proximally by pial vascular
network and branches of ophthalmic artery. Distally by
branches CRA
● Intracanalicular region: Ophthalmic artery
● Intracranial region: Branches of both internal carotid
artery and ophthalmic artery
119
What is the primary route of ATP production in the crystalline lens?
● Anaerobic glycolysis in metabolically active cells in
anterior portion. Lens has very low oxygen tension
120
What cellular configuration do most cells in crystalline lens have and where do they attain this shape?
● Ribbon-like
● Equatorial
● Metabolically active cells of crystalline lens are found
in single layer of cuboidal epithelial cells just posterior
to the anterior lens capsule. Greatest activity of these
epithelial cells occurs in “germinative zone” which is
pre-equatorial zone of lens.
● Newly formed cells migrate from germinative zone to
equator to form ribbon-like configuration
121
Which metabolic pathway is NOT used significantly to metabolize glucose in corneal stroma?
● HMP shunt
● Glucose is metabolized in the cornea by 3 pathways:
TCA cycle, glycolysis, HMP shunt
● Corneal epithelium and endothelium use HMP shunt
35-65% of their glucose
● Corneal stroma/keratocytes do not have
6-phosphogluconate dehydrogenase (key enzyme in
HMP shunt pathway)
● TCA cycle is more active in endothelium and epithelium
122
What serves as the basis of electrooculogram?
● Trans-RPE potential
● Apical surface of RPE has Na+/K+/ATPase that
transports ions from subretinal space into RPE cells.
Net flow of ions across this apical surface creates
electrical potential that is measure by
electrooculogram
123
What are the main components of the vitreous?
● Vitreous components: 99% water, type 2 collagen,
hyalocytes, hyaluronic acid.
● Fibrils are continuous with ILM of retina and basal
lamina of nonpigmented epithelium of pars plana
124
What effect does pilocarpine have on a patient’s refractive error?
● Induces miosis and accommodation. Makes the patient more myopic
125
What contributes to the drastic increase in optic nerve size behind the lamina cribrosa?
● Meningeal sheaths, myelination of axons, presence of oligodendroglia
● Optic nerve anterior to lamina cribrosa: 1.5 mm (h) x
1.75 mm (v)
● Optic nerve behind lamina cribrosa: 3mm in diameter
126
How many short posterior ciliary arteries enter the globe around the optic nerve?
● 20 short posterior ciliary arteries enter the globe in a
ring around the optic nerve
● 10 short posterior ciliary nerves enter the globe in a
similar fashion
127
What are the 3 stages of development of human vitreous?
● Primary vitreous: hyaloid artery, vasa hyaloidea propia,
mesenchymal cells, tunica vasculosa lentis
● Secondary vitreous: begins to form 9th week of
gestation, Cloquet’s canal, Bergmeister papilla,
Mittendorf dot (m/c nasal)
● Tertiary vitreous: zonular fibers which hold lens in place
128
What traits are associated with increased risk of developing primary angle closure glaucoma?
● Race: Inuit (highest), African-American (lowest)
● Age: >40 (lens increases in thickness and moves forward)
● Family History: 1st degree relatives
● Refraction: Hyperopia
● Anterior chamber depth: <2.5 mm
129
The orbital and palpebral portions of the lacrimal gland are separated by what structure?
● Levator aponeurosis
130
What is the longest extraocular muscle (muscle and tendon)?
● Superior Oblique
● Active muscular portion of medial, superior, lateral and
inferior rectus muscles and levator: 40-41 mm
● Active muscular portion:
○ Inferior oblique: 37 mm
○ Superior oblique: 40 mm
● Longest tendon: superior oblique (20mm)
● Shortest tendon length: inferior oblique (no tendon)
131
Retrobulbar block provides akinesia by blocking which cranial nerves?
● CN3 and CN4, ciliary nerve branches of CNV1
● Does not affect CN 4
132
Where do the EOMs penetrate the tenon capsule?
● 10 mm posterior to their insertion
133
What is the optic vesicle?
● The space between outer and inner layers of optic cup
● This becomes subretinal space. This potential space is
prone to disease (i.e. serous retinal detachment)
134
What disease does NADH dehydrogenase deficiency cause?
● Leber hereditary optic neuropathy (nucleotide 11778)
135
What disease does α-galactosidase A mutation cause?
● Fabry’s disease
● Accumulation of ceramide trihexoside
