Eye Flashcards
Three main pathways by which structures can enter and leave the orbit
Optic canal
Superior orbital fissure
Inferior orbital fissure
Optic canal transmits
Optic nerve and ophthalmic artery
Superior orbital fissure transmits
Oculomotor nerve (III), Trochlear nerve (IV), Abducens nerve (VI), Ophthalmic (V1) nerve. Superior ophthalmic vein
Inferior orbital fissure transmits
Zygomatic branch of the maxillary V2 nerve, inferior ophthalmic vein and sympathetics
Seven extraocular muscles
The levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, lateral rectus, inferior oblique and superior oblique.
Levator palpebrae superioris
Raising the superior eyelid. Oculomotor nerve (CN III).
The superior tarsal muscle (located within the LPS) is innervated by the sympathetic nervous system.
Superior rectus
Elevation of eyeball, oculomotor nerve
Inferior rectus
Depression of eyeball, oculomotor nerve
Medial rectus
Medial movement (adduction), oculomotor nerve
Lateral rectus
Lateral movement (abduction), abducens nerve
Superior oblique
Depresses, lateral abduction, medial rotation, Trochlear nerve
Inferior oblique
Elevates, medial adduction, lateral rotation, oculomotor nerve
Lacrimal gland is
A compound tubuloacinar gland, comprised of lobules – which are formed by multiple acini. The acini contain serous cells and produce a watery serous secretion (lacrimal fluid).
Lacrimation reflex
When something irritates the conjunctiva or cornea of the eye, the lacrimal reflex causes nerve impulses to pass along the ophthalmic branch of the fifth cranial nerve (trigeminal V1) and reach the midbrain.
The efferent limb of this reflex arc is parasympathetic facial (VII) via the pterygopalatine ganglion.
These nerve fibres stimulate the lacrimal glands of the orbit, causing the outpouring of tears.
Blind reflex
The nasociliary branch of the ophthalmic branch (V1) of the trigeminal nerve (CN V) senses the stimulus on the cornea on (afferent fiber).
The temporal and zygomatic branches of the facial nerve (CN VII) initiate the motor response (efferent fiber).
The center (nucleus) is located in the pons of the brainstem.
Fibrous layer of the eyeball consists of
Sclera (majority of the fibrous layer) and the cornea (refracts light entering the eye)
Cornea histology
External stratified epithelial layer,
Thin anterior limiting lamina (Bowman’s membrane), which is free of fibroblasts.
Then there is a thick layer of parallel running collagen fibrils, interspersed with fibroblasts, which forms the stroma of the cornea.
The inner surface of the cornea is lined by another thin membrane, the posterior limiting lamina (Descemet’s membrane)
Layer of endothelial cells.
The vascular layer consists of
The choroid, ciliary body and iris
Choroid
layer of connective tissue and blood vessels. It provides nourishment to the outer layers of the retina.
Ciliary body
Comprised of two parts, the ciliary muscle and ciliary processes.
Ciliary muscle - smooth muscles fibres. These are attached to the lens of the eye by the ciliary processes.
The ciliary body controls the shape of the lens, and contributes to the formation of aqueous humor
Iris
Circular structure, with an aperture in the centre (the pupil).
Diameter of pupil controlled by smooth muscle fibres within the iris, which are innervated by the autonomic nervous system. It is situated between the lens and the cornea.
Inner Layer
The inner layer of the eye is formed by the retina; its light detecting component.
Retina two components
Pigmented outer layer
Neural inner layer
Pigmented outer retina
Single layer of cells
Attached to the choroid, and supports the choroid in absorbing light (preventing scattering of light within the eyeball).
Continues around the whole inner surface of the eye.
Neural (inner) retina
Consists of photoreceptors, the light detecting cells of the retina.
Ophthalmoscopy shows
The centre of the retina is marked by an area known as the macula. It is yellowish in colour, and highly pigmented.
The macula contains a depression called the fovea centralis, which has a high concentration of light detecting cells.
The area that the optic nerve enters the retina is known as the optic disc – it contains no light detecting cells.
Layers of retina, cell order
(inner to outer)
SCLERA
CHOROID
PIGMENTED EPITHELIUM
RECEPTOR LAYER (RODS AND CONES)
OUTER NUCLEAR LAYER
OUTER PLEXIFORM LAYER
INNER NUCLEAR LAYER
INNER PLEXIFORM LAYER
GANGLION CELL LAYER
NERVE FIBRE LAYER
Lens
The lens of the eye is located anteriorly, between the vitreous humor and the pupil.
Anterior and Posterior Chambers:
There are two fluid filled areas in the eye – known as the anterior and posterior chambers.
The anterior chamber is located between the cornea and the iris. and the posterior chamber between the iris and ciliary processes.
The chambers are filled with aqueous humor – a clear plasma-like fluid that nourishes and protects the eye.
Aqueous humour drainage
The aqueous humor is produced constantly, and drains via the trabecular meshwork, an area of tissue at the base of the cornea, near the anterior chamber.
Schlemm’s canal is a circular lymphatic-like vessel in the eye that collects aqueous humor from the anterior chamber and delivers it into blood vessels via aqueous veins - it is covered by the trabecular meshwork.
What happens if aqueous humour drainage is obstructed?
Glaucoma can result.
Lamina cribrosa
Nerve fibers forming the optic nerve exit the eye posteriorly through a hole in the sclera that is occupied by a mesh-like structure called the lamina cribrosa.
Multilayered network of collagen fibers that insert into the scleral canal wall.
Lens accommodation
When the ciliary muscle contracts, it pulls itself forward. This releases the tension on the lens caused by the zonular fibers.
This release of tension of the zonular fibers causes the lens to become more spherical, adapting to short range focus.
Conversely, relaxation of the ciliary muscle causes the zonular fibers to become taut, flattening the lens, increasing the focal distance, increasing long range focus.
What do the ciliary muscles do for short range focus?
Contract
What do the ciliary muscles do for long range focus
Relax
Sphincter pupili role and innervation
Functions to constrict the pupil in bright light (pupillary light reflex) or during accommodation.
It is controlled by parasympathetic fibers of the muscarinic acetylcholine receptor (M3) 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.
Dilator pupili role and innervation
The iris dilator muscle acts to dilate the pupil.
Innervated by the sympathetic system, which acts by releasing noradrenaline, which acts on α1-receptors.
Alpha muscarinic are used to dilate the pupil
Sphincter pupili shape
Circular muscles (contract and make pupil smaller)
Dilator pupili shape
Radial muscles ( contract and make pupil larger)
How does ANS affect pupil size?
Parasympathetic (constriction) Sympathetic (dilation)
Arterial eyeball supply
The eyeball receives arterial blood primarily via the ophthalmic artery.
This is a branch of the internal carotid artery, arising immediately distal to the cavernous sinus.
Venous drainage of eyeball
Venous drainage of the eyeball is carried out by the superior and inferior ophthalmic veins.
These drain into the cavernous sinus, a dural venous sinus in close proximity to the eye.
Main fluid surrounding retina
Virtuous humour
What makes optic nerve head damage and headache a common side effect of brain stem lesions
Papilledema
