WEEK 1: Anatomy of the eye Flashcards
State the 2 main functions of the extraocular muscles.
*Responsible for eye movement – Recti and oblique muscles.
*Responsible for superior eyelid movement – Levator palpebrae superioris.
Outline all the 7 extraocular muscles.
There are seven extraocular muscles – the levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, lateral rectus, inferior oblique and superior oblique.
Describe the Levator Palpebrae Superioris under the following:
Attachments
Actions
Innervation.
Levator Palpebrae Superioris
The levator palpebrae superioris (LPS) is the only muscle involved in raising the superior eyelid. A small portion of this muscle contains a collection of smooth muscle fibres – known as the superior tarsal muscle. In contrast to the LPS, the superior tarsal muscle is innervated by the sympathetic nervous system.
Attachments: Originates from the lesser wing of the sphenoid bone, immediately above the optic foramen. It attaches to the superior tarsal plate of the upper eyelid (a thick plate of connective tissue).
Actions: Elevates the upper eyelid.
Innervation: The levator palpebrae superioris is innervated by the oculomotor nerve (CN III). The superior tarsal muscle (located within the LPS) is innervated by the sympathetic nervous system.
There are four recti muscles.
State their attachments, actions and innervation.
These muscles characteristically originate from the common tendinous ring. This is a ring of fibrous tissue, which surrounds the optic canal at the back of the orbit. From their origin, the muscles pass anteriorly to attach to the sclera of the eyeball.
The name recti is derived from the latin for ‘straight’ – this represents the fact that the recti muscles have a direct path from origin to attachment. This is in contrast with the oblique eye muscles, which have an angular approach to the eyeball.
Superior Rectus
Attachments: Originates from the superior part of the common tendinous ring, and attaches to the superior and anterior aspect of the sclera.
Actions: Main movement is elevation. Also contributes to adduction and medial rotation of the eyeball.
Innervation: Oculomotor nerve (CN III).
Inferior Rectus
Attachments: Originates from the inferior part of the common tendinous ring, and attaches to the inferior and anterior aspect of the sclera.
Actions: Main movement is depression. Also contributes to adduction and lateral rotation of the eyeball.
Innervation: Oculomotor nerve (CN III).
Medial Rectus
Attachments: Originates from the medial part of the common tendinous ring, and attaches to the anteromedial aspect of the sclera.
Actions: Adducts the eyeball.
Innervation: Oculomotor nerve (CN III).
Lateral Rectus
Attachments: Originates from the lateral part of the common tendinous ring, and attaches to the anterolateral aspect of the sclera.
Actions: Abducts the eyeball.
Innervation: Abducens nerve (CN VI).
There are two oblique muscles. State their attachments, actions and innervations.
There are two oblique muscles – the superior and inferior obliques. Unlike the recti group of muscles, they do not originate from the common tendinous ring.
From their origin, the oblique muscles take an angular approach to the eyeball (in contrast to the straight approach of the recti muscles). They attach to the posterior surface of the sclera.
Superior Oblique
*Attachments: Originates from the body of the sphenoid bone. Its tendon passes through a trochlea and then attaches to the sclera of the eye, posterior to the superior rectus.
*Actions: Depresses, abducts and medially rotates the eyeball.
*Innervation: Trochlear nerve (CN IV).
Inferior Oblique
*Attachments: Originates from the anterior aspect of the orbital floor. Attaches to the sclera of the eye, posterior to the lateral rectus
*Actions: Elevates, abducts and laterally rotates the eyeball.
Innervation: Oculomotor nerve (CN III).
The extraocular muscles are innervated by three cranial nerves.
Damage to one of the cranial nerves will cause paralysis of its respective muscles.
This will alter the resting gaze of the affected eye. Thus, a lesion of each cranial nerve has its own characteristic appearance.
Describe the presentation of the palsies.
Oculomotor nerve (CN III) – A lesion of the oculomotor nerve affects most of the extraocular muscles. The affected eye is displaced laterally by the lateral rectus and inferiorly by the superior oblique. The eye adopts a position known as ‘down and out’.
Trochlear nerve (CN IV) – A lesion of CN IV will paralyse the superior oblique muscle. There is no obvious affect of the resting orientation of the eyeball. However, the patient will complain of diplopia (double vision), and may develop a head tilt away from the site of the lesion.
Abducens nerve (CN VI) – A lesion of CN VI will paralyse the lateral rectus muscle. The affected eye will be adducted by the resting tone of the medial rectus.
Horner’s syndrome refers to a triad of symptoms produced by damage to the sympathetic trunk in the neck.
Describe the 3 hallmarks of Horner’s syndrome.
*Partial ptosis (drooping of the upper eyelid) – Due to denervation of the superior tarsal muscle.
*Miosis (pupillary constriction) – Due to denervation of the dilator pupillae muscle.
*Anhidrosis (absence of sweating) on the ipsilateral side of the face – Due to denervation of the sweat glands.
*Horner’s syndrome can represent serious pathology, such as a tumor of the apex of the lung (Pancoast tumor), aortic aneurysm or thyroid carcinoma.
The eyeball is a bilateral and spherical organ, which houses the structures responsible for vision. It lies in a bony cavity within the facial skeleton – known as________.
Anatomically, the eyeball can be divided into three parts. State them.
The eyeball is a bilateral and spherical organ, which houses the structures responsible for vision. It lies in a bony cavity within the facial skeleton – known as the bony orbit.
Anatomically, the eyeball can be divided into three parts – the fibrous, vascular and inner layers.
State 2 parts that make up the fibrous layer and their functions.
The fibrous layer of the eye is the outermost layer. It consists of the sclera and cornea, which are continuous with each other.
Their main functions are to provide shape to the eye and support the deeper structures.
The sclera comprises the majority of the fibrous layer (approximately 85%). It provides attachment to the extraocular muscles – these muscles are responsible for the movement of the eye. It is visible as the white part of the eye.
The cornea is transparent and positioned centrally at the front of the eye. Light entering the eye is refracted by the cornea.
The vascular layer of the eye lies underneath the fibrous layer. Describe the 3 parts that make it up.
It 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. The ciliary muscle consists of a collection of 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). The diameter of the pupil is altered by smooth muscle fibres within the iris, which are innervated by the autonomic nervous system. It is situated between the lens and the cornea.
The inner layer of the eye is made by the retina. Describe the different parts of the retina.
The inner layer of the eye is formed by the retina; its light detecting component. The retina is composed of two layers:
*Pigmented (outer) layer – formed by a single layer of cells. It is attached to the choroid and supports the choroid in absorbing light (preventing scattering of light within the eyeball). It continues around the whole inner surface of the eye.
*Neural (inner) layer – consists of photoreceptors, the light detecting cells of the retina. It is located posteriorly and laterally in the eye.
Anteriorly, the pigmented layer continues but the neural layer does not – this is part is known as the non-visual retina. Posteriorly and laterally, both layers of the retina are present. This is the optic part of the retina.
The optic part of the retina can be viewed during ophthalmoscopy.
The center of the retina is marked by an area known as the __________. It is yellowish in colour, and highly pigmented.
The macula contains a depression called the ________, which has a high concentration of light detecting cells. It is the area responsible for high acuity vision.
The area that the optic nerve enters the retina is known as the ________– it contains no light detecting cells.
The center 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. It is the area responsible for high acuity vision.
The area that the optic nerve enters the retina is known as the optic disc – it contains no light detecting cells.
Structures of the Eyeball.
Vitreous Body
It is marked by a narrow canal which runs from the optic disc to the lens called________.This is a fetal remnant.
- Describe the vitreous humor.
- State the functions of the vitreous humor.
The vitreous body is a transparent gel which fills the posterior segment of the eyeball (the area posterior to the lens).
It is marked by a narrow canal which runs from the optic disc to the lens – the hyaloid canal. This is a fetal remnant.
The vitreous body has three main functions:
*Contributes to the magnifying power of the eye
*Supports the lens
*Holds the layers of the retina in place
Structures of the Eyeball.
The lens.
- Describe the location of the lens.
- What is its function?
In old age, the lens can become opaque – a condition known as________.
The lens of the eye is located anteriorly, between the vitreous humor and the pupil.
The shape of the lens is altered by the ciliary body, altering its refractive power. In old age, the lens can become opaque – a condition known as a cataract.
There are two fluid filled areas in the eye – known as the anterior and posterior chambers. Describe their location.
The chambers are filled with a clear plasma-like fluid that nourishes and protects the eye.
Name the fluid.
Where does it drain?
If the drainage of is obstructed, a condition known as _____________can result.
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. 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.
If the drainage of aqueous humor is obstructed, a condition known as glaucoma can result.
