Eye physiology

What does the accessory structures of the eye include?

Eyelids, eyelashes, eyebrows, lacrimal apparatus and extrinsic eye muscles 

What are the eyelids?

Sheets of voluntary muscle with the tarsal plate at the free edge

What does the tarsal plate contain in the eyelid?

Sebaceous glands that secrete the lipid layer of the tear film

What is the role of the sebaceous glands in the tarsal plates in the eyelid?

Delays evaporation of tears from the eyeball

What are the general properties of the eyelids?

Delays tear evaporation, upper eyelid more mobile than the lower, lined with a mucous membrane the conjunctiva 

What are the roles of the eyebrows?

Direct sweat and other moisture away from the eye

What is the primary function of the eyelashes?

Protect the eyeballs from sunlight, sweat and foreign objects

What type of gland does each eyelash have?

Sebaceous gland

What is the function of the sebaceous ciliary glands?

Release fluid to lubricate the eyeball

What is included in the lacrimal apparatus?

Lacrimal gland, lacrimal duct, nasolacrimal duct, lacrimal sac, lacrimal punctum and lacrimal canals

What is the role of the lacrimal apparatus?

Produce, distribute and drain tears

What is tear production stimulated by?

Parasympathetic fibres of the facial nerve in response to strong smells, irritants and emotion

Why do you get a runny nose when you cry?

Lacrimal overstimulation leads to the nasal cavity filling with fluid as the excess tears drain down the nasolacrimal duct 

How many extrinsic eye muscles are there?

6

What are the extrinsic eye muscles?

Superior rectus, inferior rectus, lateral rectus, medial rectus. superior oblique and inferior oblique

What is the diagram showing direction of movement controlled by each of six extrinsic muscles?

Medial rectus adducts (moves towards nose), lateral rectus abducts (away from nose), superior rectus elevates and inferior rectus moves eye down, superior oblique rotates top of eye towards nose and inferior oblique extorts away from nose

How many regions are there in the eyeball?

3 

What are the layers of the eyeball?

Fibrous layer, vascular layer and inner layer

What is the fibrous layer covered by?

Conjunctiva

What is the fibrous layer also known as?

Sclera 

What is the sclera made up of?

Collagen fibres and fibroblasts - give shape and rigidity to eyeball and its white colour 

What is the anterior sclera of the eye also known as?

Cornea

What does the cornea cover?

Iris 

What structure is present at the junction of the sclera and cornea?

Canal of Schlemm

What is the function of the canal of schlemm?

Drains aqueous humour from the anterior chamber of the eye

What is the uvea?

Highly vascular choroid layer lining the sclera 

What is the role of the uvea?

Provides nutrients to the retina and contains melanocytes

What do melanocytes do?

Produce melanin to absorb stray light that enters the eye and maintains sharpness on image of the retina

What does the choroid layer become in the anterior portion of the eyeball?

Ciliary body ending in ciliary processes 

What do the ciliary processes do?

Contain capillaries that secrete aqueous humour into the anterior chamber of the eye

What do suspensory ligaments do in the uvea?

Hold the lens in place

What do the ciliary muscles do in the uvea?

Change the shape of the lens and hence the focus of the eye 

What are the main structures in the uvea?

Melanocytes, choroid layer, ciliary body, ciliary processes, suspensory ligaments, ciliary muscle

What is the iris?

Coloured, doughnut shaped arrangement of circular and radial smooth muscle

Where is the iris attached to?

Ciliary processes and is suspended between cornea and lens 

What is the main function of the iris?

Regulates amount of light entering the eye by changing size of the pupil

What is a diagram showing the movement of the iris?

Where does the retina cover?

Posterior segment of the eye 

What are the properties of the retina?

Highly vascularised - appears red under examination

Where is the edge of the retina joined to?

Choroid is pigmented with melanin, absorbing any scattered light that enters the eye

How many layers of neurons are the retina made of?

3 neuron types

What are the types of neurons in the retina?

Photoreceptor, bipolar, ganglion

What are the 2 types of photoreceptor neurons?

Cones and rods 

What are the cones responsible for in the retina?

Colour vision, essential for bright light function, found at fovea centralis

What are the rods responsible for in the retina?

Found around edge of retina, essential for dark function

What are the ganglion neurons responsible for?

Pass signals from the retina to the optic nerve fibres 

Where does the optic nerve leave?

Back of eye at optic disc

Where do the central retinal artery and central retinal vein enter and leave?

Optic nerve via optic disc

What are the 2 cavities in the eye?

Anterior and posterior 

What does the anterior cavity of the eye consist of?

Anterior and posterior chambers and contains aqueous humour - front of the eye

What does the posterior cavity of the eye contain?

Vitreous humour - back of the eye

Where is the anterior cavity of the eye located?

Cornea and the iris

Where is the posterior cavity of the eye located?

Iris and the lens

What are both eye cavities filled with?

Aqueous humour secreted by the ciliary body 

How often is aqueous humour replaced and what is its role?

Every 90 minutes and maintains intraocular pressure between 16-25mmHg

What is the primary function of the vitreous humour?

Holds the retina flush against the choroid

What is vitreous humour made up of?

Water with collagen fibres and hyaluronic acid, some phagocytic cells

What do phagocytic cells in the vitreous humour do?

Removes debris e.g., dead retinal cells

Is vitreous humour renewed?

No - it is stagnant

What are floaters?

Harmless pieces of debris casting a shadow onto the cornea - happens when the patient looking at a single bright colour for a period of time