Ocular system Flashcards
Describe the anatomy of the eye?
The sclera is the thick, white connective tissue capsule around the eye and the cornea is a specialised region which is clear and dense.
The choroid is specialised into the iris, ciliary muscle and suspensory ligaments…
The action of the ciliary muscle and tension they enact on the suspensory ligaments determines the shape + therefore focusing power of the lens.
The retina contains photoreceptors, rods + cones.
Overall, eye is divided into 2 fluid-filled spaces for support.
At the front, between iris and cornea is the aqueous humour.
Between the lens and retina = viscous, vitreous humour.
What smooth muscle is there in the eye?
To regulate PUPIL = In the iris, there are two layers of SM.
Dilator (radial) and Sphincter (constrictor) SM = which regulates size of the pupil and therefore amount of light reaching the retina.
= Innervated by autonomic nerves
To regulate LENS = Ciliary SM tension applies to suspensory ligaments, to control the shape for accomodation of the lens.
What is the process of pupil dilation?
How does parasympathetic NS affect the lens?
Mydriasis = pupil dilation from radial (dilator) SM contraction.
= Sympathetic NS.
= NA acts on a1 adrenoceptors.
Sympathetic fibres from superior cervical ganglion.
Sympathetic NS = mydriasis = pupil dilation.
BUT
no effect on lens as ciliary muscle has no adrenergic receptors.
A1 agonism on radial SM causes pupillary dilation.
What is the process of pupil constriction?
Miosis =pupil constriction by contraction of Sphincter (constrictor) SM.
= ACh released acts on M3 muscarinic receptors.
= Parasympathetic NS.
= Cranial parasympathetic axons innervate the ciliary ganglion, from which postganglionic parasympathetic fibres innervate the sphincter SM.
How is Iris tone usually maintained?
What are consequences of overactivity?
Parasympathetic signalling is dominant over Sympathetic signalling.
= Inducing Sphincteric SM contraction - causing pupil dilation via ACh at M3 receptors.
= Miosis = pinpoint pupils.
= Mydriasis = dilated pupils.
How do a1 receptors and M3 receptors carry out SM contraction?
a1 receptors and M3 receptors are both Gq coupled…
= PIP2 -> IP3 + DAG
= Ca2+ release.
= Ca2+/Calmodulin binds MLCK.
= MLCK uses ATP to phosphorylate myosin cross-bridges.
= Which can bind to actin = SM contraction.
What is the pupillary light reflex?
Vs Pupillary dark response?
In bright light = miosis.
In low light = mydriasis.
Consensual response = response in one eye should stimulate the other eye = Important for diagnostics.
In order to protect photoreceptors from bright lights…
Pupils usually dilate when its dark - relaxation of iris sphincter (constrictor) and contraction of iris radial/dilator SM
What is Angel’s trumpet?
A flow which contained scopolamine, hysocyamine, and atropine….
Causes the eye it touches to lose pupillary light reflex and accomodation.
What happens to the pupils and lens when muscarinic agonists/antagonists are applied?
Muscarinic agonist = M3 receptor activation = Contraction of sphincter pupillae.
= Miosis = pupils constrict.
M3 activation causes ciliary muscle contraction = lens contracts = Near sight.
Muscarinic antagonist = block contraction of sphincter pupillae = mydriasis = pupils dilate.
Ciliary muscle relaxation = lens relaxes = far sightedness
What are the class names of muscarinic antagonists?
Mydriatics = block constrictor/sphincter muscle = pupillary dilation.
Cycloplegics = paralyse ciliary muscle to block accomodation.
What are the uses of muscarinic antagonists?
Tropicamide = short-lasting anti-muscarinic used during ocular examination, to cause mydriasis for better visualisation of the retina…
Lazy eye/amblyopia = Use atropine on the good eye, to blur vision in good eye + force use of slow eye.
What is glaucoma?
What diseases can cause glaucoma?
What are the symptoms of glaucoma?
Optic nerve damaged, tends to be due to increased intraocular pressure : depending on volume of aqueous humour.
Diabetes, hypertension, age.
Intra-ocular pressure above 21mmHg, optic nerve damage and loss of peripheral vision.
What is the role of the aqueous humour?
The aqueous humour is produced by ciliary body epithelium to provide nutrients to cornea, lens, etc. and maintain intraocular pressure.
The cornea and lens are avascular!!
(AH is anterior/in front of lens and iris, between the cornea).
How does aqueous humour circulate?
AH produced by the ciliary body, flowing from posterior chamber through the iris into the anterior chamber in front of iris…
AH drains through (90%) trabecular meshwork, through Canal of Schlemm into veins…. with some via uveoscleral outflow.
What is open-angle glaucoma?
There is obstruction/blockage of drainage through Trabecular meshwork and Canal of Schlemm, reducing AH drainage into veins.
So the drainage of anterior chamber is reduced and intraocular pressure rises, causing optic nerve damage.
