Flashcards in SDL: Eye and Ear Deck (38):
In life, what occupies the anterior chamber of the eye?
List the structures through which light entering the eye passes before reaching the retinal photoreceptors.
What and where is the conjunctiva? What is its function?
Runs from the junction of the cornea and the limbus covering the surface of the eye coming into contact with the air. Reflects into the eyelids.
Function - 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 is the function of the sclera?
Supports the eye. Provides insertion for the extra-ocular muscles.
What is the function of the iris?
Forms a diaphragm extending in front of the lens from the ciliary body so as to incompletely divide the anterior compartment into two chambers (anterior and posterior chamber). The highly pigmented iris acts as an adjustable diaphragm which regulates the amount of light reaching the retina.
What is the nerve supply to the circular muscle of the iris i.e. spincter pupillae?
It is controlled by parasympathetic fibers that originate from the Edinger-Westphal nucleus, travel along the oculomotor nerve (CN III), synapse in the ciliary ganglion, and then enter the eye via the short ciliary nerves.
What is the nerve supply to the radial muscle of the iris i.e. dilator pupillae muscle?
- Innervated by postganglionic sympathetic nerves arising from the superior cervical ganglion as the sympathetic root of ciliary ganglion.
- From there, they travel via the internal carotid artery through the carotid canal to foramen lacerum.
- They then enter the middle cranial fossa above foramen lacerum, travel through the cavernous sinus in the middle cranial fossa and then travel with the ophthalmic artery in the optic canal or on the ophthalmic nerve through the superior orbital fissure.
- From there, they travel with the nasociliary nerve and then the long ciliary nerve. They then pierce the sclera, travel between sclera and choroid to reach the iris dilator muscle.
What is the innervation of the ciliary muscle?
- Presynaptic parasympathetic signals that originate in the Edinger-Westphal nucleus are carried by cranial nerve III (the oculomotor nerve) and travel through the ciliary ganglion.
- postganglionic fibers are part of cranial nerve V
What effect does contraction of the ciliary muscle have on the lens?
When the ciliary muscle contracts, it pulls itself forward and moves the frontal region toward the axis of the eye. This releases the tension on the lens caused by the zonular fibers (fibers that hold or flatten the lens). This release of tension of the zonular fibers causes the lens to become more spherical, adapting to short range focus.
What is a cataract?
A build-up of protein in the lens that causes it to go cloudy.
Apart from controlling the shape of the lens, what else does the ciliary body do?
Synthesises aqueous humour.
Why is aqueous humour important?
Fills the anterior compartment of the eye and keeps it at a constant pressure. This helps maintain the shape which is important for the refractive properties of the eye.
Where is the aqueous humour reabsorbed into the bloodstream?
- Schlemm's canal, also known as canal of Schlemm or the scleral venous sinus, is a circular channel in the eye that collects aqueous humor from the anterior chamber and delivers it into the bloodstream via the anterior ciliary veins.
- Uveoscleral outflow: reabsorption of aqueous humor through the ciliary muscle
What is glaucoma?
Glaucoma is an umbrella term for eye conditions which damage the optic nerve, and which can lead to a loss of vision. The main cause of damage to the optic nerve is intraocular pressure , excessive fluid pressure within the eye, which can be due to various reasons including blockage of drainage ducts, and narrowing or closure of the angle between the iris and cornea.
Give two functions of the choroid layer of the eyeball?
- The choroid provides oxygen and nourishment to the outer layers of the retina.
- Along with the ciliary body and iris, the choroid forms the uveal tract.
Where is the retina located and what does it contain?
- Lines the most posterior compartment of the eye (area posterior to lens and ciliary body)
- Contains the photoreceptors, together with the first and second order neurons of the visual pathway
What is the retina made up of?
- An outer pigment cell layer - a single layer of cuboidal epithelial cells with melanin filled microvilli extending from their inner surface
- A multilayered neural retina
List three functional differences between rods and cones.
- Rods are much more sensitive to light and important for vision in dim conditions
- Cones can detect colour, rods only see in b&w
- Cones have a much higher visual acuity
At what point do the axons of the retinal ganglion cells converge on the inside of the eye?
Optic papilla (disc)
Why is the optic papilla a blind spot?
There are no photoreceptors there.
What is papilloedema? What is it a sign of?
Optic disc swelling that is caused by increased intracranial pressure. The swelling is usually bilateral and can occur over a period of hours to weeks.
How does papilloedema arise?
The subarachnoid space of the brain is continuous with the optic nerve sheath. Hence, as the cerebrospinal fluid (CSF) pressure increases, the pressure is transmitted to the optic nerve, and the optic nerve sheath acts as a tourniquet to impede axoplasmic transport. This leads to a buildup of material at the level of the lamina cribrosa, resulting in the characteristic swelling of the nerve head.
Swelling of the optic disc can occur due to other causes, Suggest two.
- Toxic/nutritional optic
- Adult optic neuritis
What is the macula lutea?
Yellow pigmented zone just lateral to the optic disc. Contains the fovea.
What is the fovea?
The fovea is the point on the retina at which the image of the object at the centre of the visual axis falls.
How is the fovea specialised for it's function?
The high spatial density of cones along with the absence of blood vessels at the fovea (ie covering it) accounts for the high visual acuity capability at the fovea. The fovea received blood from vessels in the choroid.
Where do the blood vessels that supply the retinal cells lie?
There are two circulations to the retina, both supplied by the ophthalmic artery.
- The outer and middle retinal layers, including the outer plexiform and outer nuclear layers, the photoreceptors, and the retinal pigment epithelium, are nourished by branches of the posterior ciliary arteries, which enter the back of the eye outside the optic nerve. These vessels also supply the choroidal layer, external to the retina.
- The inner retina is supplied by the central retinal artery, the branch of the ophthalmic artery that enters the optic nerve 4 mm posterior to the eye. The central retinal artery has 4 main branches within the retina.
What is a detached retina? Why is it important to repair a detatched retina as rapidly as possible?
When the retina detaches, it is lifted or pulled from its normal position. If not promptly treated, retinal detachment can cause permanent vision loss.
List the structures through which through which sound waves entering the external ear pass before reaching the auditory receptors.
What is the function of the tympanic membrane?
To transmit sound from the air to the ossicles inside the middle ear, and then to the oval window in the fluid-filled cochlea.
What is the function of the ossicles?
They serve to transmit sounds from the air to the fluid-filled cochlea.
What is the function of the oval window?
The oval window is the intersection of the middle ear with the inner ear and is directly contacted by the stapes.
What is the function of the round window?
The secondary tympanic membrane (or round window membrane) covers the round window, sealing off one of two openings into the inner ear. It separates the tympanic duct of the cochlea from the middle ear. It vibrates with opposite phase to vibrations entering the cochlea through the oval window as the fluid in the cochlea is displaced when pressed by the stapes at the oval window. This ensures that hair cells of the basilar membrane will be stimulated and that audition will occur.
What is the function of the eustachian tube?
Pressure equalisation and the drainage of mucous.
What is earwax made up of?
Shed skin cells, hair, and the secretions of the ceruminous and sebaceous glands of the outside ear canal. Major components of earwax are long chain fatty acids, both saturated and unsaturated, alcohols, squalene and cholesterol.
How does earwax travel along the ear canal?
Cells formed in the centre of the tympanic membrane migrate outwards to the walls of the ear canal, and move towards the entrance of the ear canal. Earwax is also carried outwards, taking with it any dirt, dust, and particulate matter that may have gathered in the canal. Jaw movement assists this process by dislodging debris attached to the walls of the ear canal, increasing the likelihood of its expulsion.
What is the function of the chorda tympani?
A branch of the facial nerve that originates from the taste buds in the front of the tongue, runs through the middle ear, and carries taste messages to the brain.