Organs - The Eye Flashcards
The eye can be split up in to three layers. These are…
What makes up these layers?
1) Fibrous - Sclera and Cornea
2) Vascular - Choroid, cilary body and Iris
3) Inner - Retinal pigment epithelial layer and inner neural layer
Fibrous layer
What is the function of the sclera and corneum?
Clinical relevance?
Sclera
1) Provides structure and support for deep layers.
2) Provides attachment to extraocular eye muscles.
Corneum
1) Light refraction
Clinical relevance of sclera
In jaundice, the sclera can become pigmented by accumulation of bilirubin pigment, such as in the case of Gilbert’s disease (hereditary condition where there is a build up of bilirubin in the blood).
Clinical relevance of corneum
Corneal transplants - sample can be obtained by anyone due to the lack of immune molecules in the vicinity.
Vascular layer
What are the functions of the chorid, ciliary body and iris?
Choroid
Extending from the cilary body is the choroid. It has two main functions:
1) Provides nourishment to outer layers of retina
2) Due to the pigment, it prevents scattering of the light that would otherwise affect vision.
Ciliary body
Comprise of two structures: the ciliaris muscle and ciliary processes. The ciliaris muscle attaches to the lens by the processes. The smooth muscle of the ciliaris is innervated by the autonomic nervous system to change the shape of the lens (accomodation).
Iris
A circular structure with an aperture in the centre, called the pupil. The diameter of the pupil is is altered by smooth muscle fibres within the iris - these are the circular (sphincter muscles) and the radial (dilator muscles)
Inner layer
What are the functions of the retinal pigement epithelium and the neural layer?
Macula? Fovea centralis? Optic disc?
Clinical relevance?
RPE
Formed by a single layer of cells. It is attached to the choroid. It has two functions:
1) Assists the choroid in absorbing light
2) Controls what’s leaving from the blood and into the retina.
Neural layer
Consists of photoreceptors (rods and cones) - detects light. Located posteriorly and laterally.
The centre of the retina is marked by the macula - this contains a structure called the fovea centralis that contains the majority of cone cells.
The optic disc is the are the optic nerve enters - it contain no photoreceptors.
Clinical relevance of vitreous humour and retina
Severe trauma can rupture the retina. This may lead to vitreous humour seeping in between the neural and RPE layers - this is known as retinal detachment and can quickly lead to blindness.
The Lens
Location?
Purpose?
Clinical relevance - cataracts?
Located between the vitreous humour and pupil.
The shape of the lens is altered by the ciliary body, altering its refractive power (accomodation)
Clinical relevance
in old this lens can become opaque due to an accumulation of crystaline proteins.
Other aetiology - smoking, diabetes, congenital, too much vitamin C.
Posterior Segment
What is found in the posterior segment?
What is its purpose?
Vitreous humour. It has two functions:
1) To act as medium to transmit light rays to the retina.
2) Maintain intraocular eye pressure.
The Anterior Segment
What makes up this segment?
Clinical relevance - Glaucoma?
Made up of two sections: anterior and posterior chambers. These chambers are filled with aqueous humour. The liquid provides nourishment to the corneum and maintains intraocular pressure.
Posterior chamber
Located between the lens and the iris.
Here the aqueous humour is produced by the ciliary body. It flows from the posterior chamber into the anterior chamber.
Anterior chamber
Located between the iris and corneum. The aquous humour then drains into the Canal of Schlemm at the base of the corneum.
Clinical relevance
When the Canal of Schlemm is blocked, this can lead to a build up of intraocular pressure that may lead to glaucoma. However, be aware that there can be normal pressure of the eye, yet people develop glaucoma - autoimmune causes.
Vasculature of the eye
Arterial supply?
Venous drainage?
Primarily the Ophthalmic artery (a branch of the internal carotid artery, immediately distal to the cavernous sinus).
This artery gives off branches to many components of the eye
Occlusion of this artery may result in blindness.
Venous drainage is carried out by the superior and inferior ophthalmic veins - these drain into the cavernous sinus.
Clinical relevance - Glaucoma
Classifications?
Refers to a group of diseases that result in damage to the optic nerve. There are two main classifications of the disease:
1) Open angle - outflow of the trabecular network is reduced. This causes a gradual loss in peripheral vision, until the end stages of the disease.
2) Closed angle - where the iris is forced against the trabecular network, preventing the drainage of the aqueous humour. It is an ophthalmic emergency, which can lead to blindness.
Clinical relevance - Papilloedema?
Refers to swelling of the optic disc, secondary to raised intracranial pressure. The optic disc is the area of the retina where the optic nerve enters and can be visualised by using an opthalmoscope.
Common causes include:
1) Intracerebral mass lesions
2) Cerebral haemorrhage
3) Meningitis
4) Hydrocephalus
In papilloedema, the cranium resists venous retun from the eye. This causes fluid to extravasate from blood vessels and collect in the retina, producing a swollen optic disc.
