Anatomy of the Eye Flashcards
What are the 3 layers of the eyeball?
- Outer fibrous layer
- Middle vascular(pigmented) layer
- Inner layer
Describe the components of the fibrous coat?
- Sclera – white fibrous layer covering 5/6 of the eye
- Cornea – transparent structure forming the anterior 1/6 of the outer coat
Describe the components of the eyebeall?
- Choroid
- outer pigmented and inner vascular and invests 5/6 of the eye
- Nourishes the retina and darkens the eye. - The ciliary body
- Thickened portion of the vascular coat between choroid and iris
- Consists of ciliary ring, ciliary processes and ciliary muscles - Iris
- Thin contractile circular pigmented diaphragm with central aperture the pupil
What is the internal nervous coat?
Consists of the retina
> outer pigmented
> inner nervous
Posterior part – photosensitive
Anterior part – not photosensitive
What is the optic disc?
- Consists of optic nerve fibres
- formed by axons of ganglion cells which connect to rods and cones
- Has no receptors hence not photosensitive
Name the 3 tunics of the eye?
- The outer Fibrous coat
- inelastic coat
- transparent cornea (anteriorly)
- opaque sclera (posteriorly). - The middle vascular
pigmented coat:
- The Uvea: choroid, ciliary body and iris - The inner nervous coat
- The Retina, extends forwards to within 6 mm of the limbus.
What are the components of the outer coat?
- Sclera- opaque (white)
- Cornea - transparent
What is the sclera?
Dense fibrous tissue
- opaque + white
What pierces the sclera?
- Optic nerve
- the sclera is fused with the dural sheath of the nerve - Venae vorticosae
- ciliary arteries, nerves and veins
NB. At the lamina cribrosa is where these vessels pierce the sclera
What is the junction between the sclera and cornea called?
What is found here?
Corneoscleral junction or limbus
- scleral venous sinus is found here
What is the cornea?
Location?
Function?
The transparent cornea is largely responsible for the refraction of the light entering the eye
- It is in contact posteriorly with the aqueous humor.
Describe the blood supply of the cornea?
How is it nourished?
The cornea is avascular and devoid of lymphatic drainage
- It is nourished by diffusion from the aqueous humor and from the capillaries at its edge.
Describe the nerve supply of the cornea?
Long ciliary nerves from the ophthalmic division of the trigeminal nerve
What is the function of the cornea?
- The cornea is the most important refractive medium of the eye.
- the tear film maintains the environment for the corneal epithelial cells
Describe cornea transplants?
• Transplants: common & successful
- no blood vessels so no antibodies to cause rejection
• Nourished by tears & aqueous humor
What is the limbus?
Junction between cornea and sclera
What is the function of the limbus.
- Nourishes peripheral cornea
- Assists in corneal wound healing
- Pathway for aqueous outflow (contains trabecular meshwork and canal of schlemm)
Describe the transition zone of the limbus?
• anterior: bowman’s terminal-
Descemet’s terminal
• posterior: scleral spur / iris root
• 1.5~2.0mm width
What does the limbus look like?
• Semitransparant zone
• White sclera
What is the conjunctiva?
Where is it?
is a translucent mucous membrane which lines the posterior surface of the eyelids and anterior aspect of the eyeball (up to the edge of the cornea)
What does a normal conjunctiva look like?
Pink
Smooth
Thin
Transparent
• There are normally large deep blood vesells that run vertically
What is formed at the reflections between the eyeball and eyelid?
conjunctiva forms two sacs
- the superior and inferior fornices.
What are the functions of the conjunctiva?
- The conjunctiva helps lubricate the eye by producing mucus and tears, although a smaller volume of tears than the lacrimal gland
- it also contributes to immune surveillance and helps to prevent the entrance of microbes into the eye
What are the parts of the conjunctiva?
- Bulbar conjunctiva.
- Palpebral conjunctiva.
- Forniceal conjunctiva.
Describe the palpebral conjunctiva?
It is richly vascular, extremely thin and strongly bounded to the tarsal plate
The palpebral conjunctiva is divided into?
- Marginal
- Tarsal
- Orbital
Describe the marginal palpebral conjunctiva?
- Extends from the lid margin to about 2mm back of the lid upto the sulcus subtarsalis
- Actually a transitional zone between skin and the conjunctiva proper
- Lacrimal puncta open in the marginal zone
Describe bulbar conjunctiva?
- It is transparent and lies loose over the
underlying structures and thus can be moved easily - It is separated from the anterior sclera by episcleral tissue and tenon’s capsule
Describe the blood supply of the conjunctiva?
•Arteries supplying the conjunctiva are derived from 3 sources. They are:
1. Marginal arcade of the eyelid
2. Peripheral arterial arcade of the eyelid
3. Anterior ciliary artery
• The palpebral conjunctiva and the fornices are supplied by branches from the marginal and peripheral arcades of the artery
Describe the venous drainage of the conjunctiva?
- The veins from conjunctiva drain into the venous plexus of eyelids which in turn drain into the superior and inferior ophthalmic veins.
- A cicumcorneal zone of limbus drain into the anterior cilliary veins
Describe the lymphatic drainage of the conjunctiva?
- lateral side drain into the periauricular lymph nodes
- medial side drain the submandibular lymph nodes
Describe the nerve supply of the conjunctiva?
•BULBAR CONJUNCTIVA:
- long ciliary nerve
- Naso ciliary nerve
- Ophthalmic division of trigeminal nerve
•SUP.PALPEBRAL AND FORNICEAL CONJUNCTIVA:
-Frontal and lacrimal branches of ophthalmic division of trigeminal nerve
What are clinical conditions involving the conjunctiva?
- Conjunctivitis
- Malignant lymphoma
- Conjunctival papilloma
The middle layer (tunica media) of the eye comprises of what 3 structures?
•Choroid
•Ciliary body
•Iris
What do the structures of the middle layer of the eye make?
The choroid, ciliary body and iris together form the uveal tract
- this layer contains blood vessels
Note: Pupil is the small opening in front of it
What is the choroid?
- is the vascular layer of the eye containing connective tissues
- lying between the retina and the sclera
- It is composed of an outer pigmented layer and an inner highly vascular layer
What is the function of the choroid?
Provides oxygen and nourishment to the outer layers of the retina
What structure does the choroid help form?
The uveal tract
- along with the ciliary body and iris
What is the blood supply of the choroid?
are branches of the ophthalmic artery and enter the eyeball without passing with the optic nerve.
What is the ciliary body?
Relations?
- ciliary body is continuous posteriorly with the choroid, and anteriorly it lies behind the peripheral margin of the iris
- is composed of the ciliary ring, the ciliary processes, and the ciliary muscle
What is the ciliary ring?
The ciliary ring is the posterior part of the body
- its surface has shallow grooves = the ciliary striae
What are the ciliary processes?
The ciliary processes are radially arranged folds, or ridges, to the posterior surfaces of which are connected the suspensory ligaments of the lens
What is the ciliary muscle?
composed of meridianal and circular fibers of smooth muscle.
- The meridianal fibers run backward from the region of the corneoscleral junction to the ciliary processes
- The circular fibers are fewer in number and lie internal to the meridianal fibers.
What is the nerve supply of the ciliary muscle?
is supplied by the parasympathetic fibers from the oculomotor nerve
- After synapsing in the ciliary ganglion, the postganglionic fibers pass forward to the eyeball in the short ciliary nerves.
Describe the action of the ciliary muscle?
Contraction of the ciliary muscle, especially the meridianal fibers, pulls the ciliary body forward
- This relieves the tension in the suspensory ligament, and the elastic lens becomes more convex
- This increases the refractive power of the lens
Describe the action of suspending ligaments?
• The suspensory ligaments are either taut or relaxed based on the action of the ciliary muscles.
• The tension on the ligaments changes the shape of the lens, depending on the distance of the object being viewed.
• This process is called “accommodation”
What is the iris?
What is it made of?
What is it’s function?
Colored part of the Eye
• it is the most anterior portion of the vascular tunic
• Made up of radial muscle & circular muscle
• Controls the amount of light entering the eye
• the opening in the middle of the iris is called the “pupil,” which appears as the dark center of the eye.
What is the function of the iris?
Regulates the amount of light entering the eye so that there is enough light to stimulate the cones, but not enough to damage them
What are the 2 sets of muscles that make up the iris?
Circular > sphincter pupillae
Radial > dilator pupillae
Note: have opposite affects (antagonistic)
What is the function of the muscles of the iris?
By contracting and relaxing these muscles the pupil can be constricted and dilated
Describe what happens when the iris is exposed to bright light?
Parasympathetic nerve impulse
Circular muscles contract
Radial muscles relax
Pupil constricts
Less light enters eye
Describe the iris when it is exposed to dim light?
Sympathetic nerve impulse
Circular muscles relax
Radial muscles contract
Pupil dilates
More light enters eve
Describe the nerve supply of the muscles of the iris?
- The sphincter pupillae - supplied by
parasympathetic fibers from the oculomotor nerve
> After synapsing in the ciliary ganglion, the postganglionic fibers pass forward to the eyeball in the short ciliary nerves. - The dilator pupillae - is supplied by sympathetic fibers
> which pass forward to the eyeball in the long ciliary nerves.
Describe the action of the muscles of the iris?
- The sphincter pupillae - constricts the pupil in the presence of bright light and
during accommodation. - The dilator pupillae - dilates the pupil in the presence of light of low intensity or in the presence of excessive sympathetic activity such as occurs in fright.
Describe the nerve supply of the iris?
under control of the autonomic nervous system
- Sympathetic Nerve > pupil dilation
- Parasympathtic Nerve > pupil constriction
Note: The drug atropine inhibits the parasympathetic nerve, causing pupil to dilate
Describe the reflex arc mechanism of the iris?
Stimulus = more light
> receptor = rods and cones
> sensory neuron
> coordinator = brain
> motor neuron
> effector = iris muscles
> response = pupil constricts
What are the chambers of the eye?
- The anterior chamber - in front of the iris
- The posterior chamber - immediately behind the iris
> These two chambers which communicate through the pupil are filled with clear aqueous humour. - The vitreous cavity - filled by gel-like structure
Aqueous humor of anterior cavity?
Production?
Drainage?
Replacement rate?
Filled with aqueous humor
• produced by ciliary body
• continually drained by scleral venous sinus
• replaced every 90 minutes
• drainage of aqueous humor from eye to bloodstream
What is a glaucoma?
Causes?
Consequences?
-increased intraocular pressure
- that could produces blindness
- problem with drainage of aqueous humor
What is the aqueous humor?
Describe its course?
• Clear liquid produced by the ciliary body and secreted onto the posterior chamber
• flows into the anterior chamber through the pupil.
• It is absorbed in the sclera venous sinus, the Schlemm’s canal
- a circular venous channel at the junction between the cornea and the iris
What is the intraocular pressure of the eye?
What’s considered a normal IOP?
What causes increased IOP?
The pressure within the eye is maintained at a steady level by continuous formation & drainage of aqueous.
- The intraocular pressure is normally
10 - 21 mmHg
- High IOP almost always due to an obstruction of aqueous outflow
> increased IOP is ocular hypertension
Describe the secretion and drainage of aqueous?
> Aqueous is secreted by the ciliary epithelium into the posterior chamber
flows through anterior chamber (through the pupil)
drained through the anterior chamber angle
What is the function of the aqueous humor?
- Supplies nutrients to avascular cornea and lens
- maintains intraocular pressure
Describe the relationship between aqueous humour and glaucoma?
Consequences?
If the normal cycle of production and absorption is disturbed so that the amount of fluid increases then intraocular pressure will increase
- This condition is called Glucoma and can lead to a variety of visual problems
E.g. such as blindness due to compression of the retina and its blood supply
What is the lens?
biconvex optically transparent intraocular structure
- The crystalline lens is the only structure continuously growing throughout the life
- Separates the anterior 1/5th of the eyeball and posterior 4/5th
- Changeable refractive media.
- contains Capsule, epithelium and lens fibers.
Note: Congenital anomalies and effect of systemic diseases are seen within the lens
What is a cataract?
Opacity of lens
What is the function of the lens?
- focuses image on retina
> suspensory ligament and ciliary muscles control curvature to focus images on retina - divides interior of eyeball into anterior cavity and posterior cavity
What is accommodation?
Refers to the ability of the eye to alter its focus so that clear images of both close and distant objects can be formed on the retina
- The lens shape can be altered by suspensory ligaments and the ciliary muscles : This adjusts the focus
Describe how the lens accommodates distant objects?
- Light rays are almost parallel so do not need much refraction to focus onto the retina.
- The lens therefore needs to be thin and
“weak” (i.e. have a long focal length).
> Flattened (less convex) for distant images
Describe how the lens accommodates close objects?
- Light rays are likely to be diverging, so need more refraction to focus them onto the retina.
- The lens therefore needs to be thick and “strong” (i.e. have a short focal length).
> Rounded (more convex) for close images
What is the posterior cavity?
• posterior to lens cavity filled with vitreous humor (jellylike)
• Holds retina in place
• formed once during embryonic life
Clinical correlates: floaters are debris in vitreous of older individuals
Where is the inner sensory tunic/retina?
Posterior ¾ of eye ball only
anterior margin - ora serrata retina (rods only)
What is the optic disc?
attachment of optic nerve / blind spot
- no photoreceptors
What is the fovea centralis?
near the middle of the retina
- highest concentration of cones
- region of highest visual acuity
Describe the blood supply to the retina?
derived from the central retinal artery and vein, and from the choroid.
- The retinal vessels enter and leave the eye through the optic nerve and run in the nerve fibre layer.
Note: A major arterial and venous branch, forming an ‘arcade’, supplies each of the retinal quadrants
What are the cells of the retina?
Contains photoreceptor cells (rods and cones) and associated interneurones and sensory neurones.
What is the inner sensory tunic/retina?
Functions?
- superficial layer of pigment epithelium (melanin)
- non-visual portion
- absorbs stray light & helps keep image clear deeper layer of neurons
What are the layers of the inner sensory tunic/retina?
- rods/cones are photoreceptor layer
- bipolar neuron layer
- ganglion cell layer
What are rods?
rod shaped cells
• shades of gray in dim light
• nocturnal vision - black and white vision, great sensitivity in dim light
• 120 million rod cells
• discriminates shapes & movements, distributed along periphery of retina
What are cones?
cone shaped cells
• sharp, color vision at all higher light intensities
• 6 million
• Fovea centralis - widest distribution at center of retina
What is visual transduction?
Where does it start?
• This is the process by which light initiates a nerve impulse.
• Detection of light is carried out on the membrane disks of a rod cell
- These disks contain thousands of molecules of rhodopsin (photoreceptor molecule)
What is rhodopsin?
Function?
Composed of?
- Definition
Rhodopsin is a pigment of the photoreceptor cells in the retina. - Function
It is responsible for the first events in the perception of light. - Composition
It is comprised of Opsin (a protein with 7 transmembrane helices) and Retinal (photoreactive chromophore)
Rhodopsin consists of?
- Opsin (membrane bound protein)
- Retinal (covalently-bound prosthetic group)
• Retinal is made from vitamin A
• Retinal is the light sensitive part
- exists in 2 forms: cis and trans forms
Describe the changes retinal undergoes from dark to light conditions?
• In the dark retinal is in the cis form.
• When it absorbs a photon of light it quickly switches to the trans form.
• This changes the shape of the opsin protein - a process called bleaching
Describe the changes retinal undergoes from light to dark conditions?
The reverse reaction (trans to cis)
- requires an enzyme reaction and is very slow (taking a few minutes)
- This process requires ATP, as rhodopsin has to be resynthesised
Describe the process of visual transduction?
> Bleaching of the rhodopsin in a rod cell
Alters the permeability of the membrane to Na+
nerve impulse
sensory neurone in the optic nerve
to the brain
Describe rhodopsin during visual transduction?
• Rhodopsin controls sodium channels
• Rhodopsin with cis retinal opens sodium channels (absence of light)
• Rhodopsin with trans retinal closes sodium channels (light)
Describe the chemical processes undergone by rod cells in the dark?
• In the dark the channel is open - Na+ flow in can cause rod cells to depolarise.
- Therefore in total darkness, the membrane of a rod cell is polarised
• Therefore rod cells release neurotransmitter in the dark
• However the synapse with bipolar cells is an inhibitory synapse i.e. the neurotransmitter stops impulse
Describe the chemical processes undergone by rod cells in the light?
As cis retinal is converted to trans retinal, the Na+ channels begin to close
less neurotransmitter is produced. If the threshold is reached, the bipolar cell will be depolarised
forms an impulse which is then passed to the ganglion cells and then to the brain
State the characteristics of Rods?
- Shape : Outer segment is rod shaped
- Number : 109 cells per eye
- distributed : throughout the retina, so used for peripheral vision.
- Sensitivity: Good sensitivity
- Type : Only 1 type > monochromatic vision
- Vision : Many rods connected to one bipolar cell > poor acuity = poor resolution
State the characteristics of cones?
- Shape : Outer segment is cone shaped
- Number : 106 cells per eye
- Distribution : found mainly in the fovea, so can only detect images in centre of retina.
- Sensitivity : Poor sensitivity
- Types : 3 types (R, G & B) - colour vision
- Vision : Each cone is connected to one bipolar cell > good acuity = good resolution
What are the 3 different cone cells?
Each have a different form of opsin (they have the same retinal)
• 3 forms of rhodopsin are sensitive to different parts of the spectrum
- 10% red cones
- 45% blue cones
- 45% green cones
What is the trichromatic theory of colour vision?
Coloured light will stimulate these 3 cone cells differently - by comparing the nerve impulses from the 3 kinds of cones the brain can detect any colour
- Red light > stimulates R cones
- Yellow light > stimulates R and G cones equally
- Cyan light > stimulates B and G cones equally
- White light > stimulates all 3 cones equally
How does colour vision work?
• When we look at something the image falls on the fovea and we see it in colour and sharp detail.
Objects in the periphery of our field of view are not seen in colour, or detail.
• The fovea has high density of cones.
• Each cone has a synapse with one bipolar cell and one ganglion -> each cone sends impulses to the brain about its own small area of the retina > high visual acuity
What is the pathway of light?
> Ganglion cells
bipolar cells
photoreceptor cells
What is the pathway of visual impulses?
> Photoreceptor cells
bipolar cells
ganglion cells
optic nerve
Describe the optic nerve?
• The ganglion cell axons in the retinal nerve fibre layer make a right- angled turn into the optic nerve at the optic
• disc, which has no photoreceptors and corresponds to the physiological blind spot.
• Behind the eyeball these axons become myelinated.
• Here the optic nerve is surrounded by cerebrospinal fluid in an anterior extension of the subarachnoid space and is protected by the same membranous layers as the brain.
Describe the nerve fibers of the optic nerve?
› contains around 1.2 million nerve fibers, which are axons of the retinal ganglion cells.
•1 mm in the globe.
• 25 mm in the orbit.
•9 mm in the optic canal.
• 16 mm in the cranial space
• Partial decussation occurs and about 53% of the fibers cross to form the optic tracts.
What are the 3 neurons of the visual pathway?
- Bipolar cell, lies within the retina.
- Ganglion cell, synapse in
lateral geniculate body - Third neuron terminates in
visual cortex
What are the cortex regions responsible for vision?
- Left occipital lobe receives visual images from nasal 1/2 of the right eye and temporal 1/2 of the left eye
- Right occipital lobe receives visual images from nasal 1/2 of the left eye and temporal 1/2 of the right eye
Describe the optics of the eye?
- Light must have a clearly pathway to be clearly focused on the sensory receptors of the retina
i.e. Clear cornea, anterior chamber, lens and vitreous cavity - The Refractive power of the eye is about 58 - 62 diopters.
