Anatomy of the Eye and Orbit Flashcards
List key superficial features of eye and orbit
There are several key superficial features of the eye. They are:
- pupil :the circular opening in the centre of the iris, through which light passes into the lens of the eye ^[Concise medical dictionary Oxf.]
- upper eyelid
- lower eyelid
- sclera
- iris
- limbus: where sclera meets iris
- lacrimal lake (where tears pool)
- cornea (not shown in diagram below)
- lateral and medial commissure (i.e. the corners of the eye, where the eyes meet)
Describe the components of the eyelids
tarsal plates
levator palpebae superioris
superior tarsal muscle
orbicularis oculi
detailed descriptions in next questions
Describe the tarsal plates
Tarsal plates or the tarsus are cartilaginous plates, found within both the upper and lower eyelid. The upper tarsal plate is larger. The tarsal plates have Meibomian/tarsal glands on their inner aspect, and secrete a lipid-rich substance onto he margins of the lids. This substance is known as meibum and it helps to trap tears, preventing them from evaporating ^[Kenhub].
The superior tarsal muscle (with Levator palpebrae superioris) attaches to the tarsal plate and helps to raise the upper eyelid*
Describe the levator palpebrae superioris
The levator palpebrae superioris is the dilator of the orbit ^[not to be confused with pupil dilation]. It consists of dense connective tissue that attaches anteriorly to the superior tarsal plate. It runs above the superior rectus muscle and inserts in the lesser wing of the sphenoid bone.
It is under somatic control, and is innervated by CNIII ^[somatic motor innervation]
The action of the levator palpebrae superioris is to elevate the upper eyelid, in other words, to open the eye.
Describe the superior tarsal muscle
The superior tarsal muscle is a smooth muscle ^[it is thus under autonomic control] attached to the levator palpebrae superioris and the tarsal plate. It also works to elevate the upper eyelid, in other words, to open the eye.
The superior tarsal muscle opens eyes even more, accentuates this action.. Note that superior tarsal muscle receives sympathetic innervation i.e. NOT cranial nerve innervation from the carotid plexus whose fibres join CNIII, when it passes through cavernous sinus*
Together, the superior tarsal muscle, levator palpebrae superioris and tarsal plates work to open your eyes
Describe the orbicularis oculi
The orbicularis oculi is the sphincter of the orbit. It is innervated by CNVII (temporal - orbital and zygomatic branches - palpebral) and consists of two parts:
- the orbital part
- fibres attach to the medial palpebral ligament and encircle the lateral margin of the orbit
- the palpebral part -
-fibres attach to the medial palpebral ligament, and cross the portion of the eyelid anterior to the tarsal plates (upper and lower). They intersect laterally to form the lateral palpebral ligament. The palpebral part of orbicularis oculi is the blinking muscle, it wipes tears across the surface of the cornea and keeps it clean and ‘germ-free’.
The tears contain immunoglobulins that help combat infections ^[see [[Immunology Lecture 1#Components of innate immune system (and adaptive)]]]. The tears drain through the **palpebral punctae** to reach the opening in the lacrimal bone, into the naso-lacrimal duct, which terminates anteriorly and inferiorly into the nasal cavity (inferior meatus, see also [[Anatomy Lecture 8]])
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The action of the orbicularis oculi muscle is to draw the eyelids together (using the palpebral part), and draw skin inwards (forming characteristic crow's feet). Note: orbicularis oculi also helps open eyes.
Note the corrugator supercilli ^[L. wrinkler of the eyebrows] - action: furrowing of eyebrows, “frowning muscle”, the principal muscle in the expression of suffering. ^[wikipedia, could be rubbish] ^[side note: innervated by CNVII]
Clinical significance of corrugator supercilii: severed or paralysed with botox - prevention of migraines ^[unclear if common practice], aesthetic choices
Describe the conjunctiva and its function
The conjunctiva is a delicate mucous membrane. It covers the front of the eye, and completely lines the inner surface of the eyelids.
The palpebral conjunctiva is the epithelial layer that lines the inner eyelids. It is continuous around the fornix (a small opening for lacrimal duct). It is poorly vascularised.
The bulbar conjunctiva is the part that covers the visible portion of the sclera. In contrast to the palpebral conjunctiva, it has a rich microvascular supply that is susceptible to irritation (source of ‘red eyes’).
The bulbar conjunctiva contributes secretions to the tear film.
Break down the key components of the eyeball
- fibrous tunic
The fibrous tunic is the most external layer of the eyeball. It covers the entirety of the globe. It is composed of two regions of connective tissue- cornea - a transparent layer, anteriorly continuous with sclera. It occupies the anterior one-sixth of the tunic/eyeball. ^[note it has epithelia, stratified squamous, basement membrane, dense regular connective tissue (collagen fibres), and endothelium –wiki, [[Histology Lecture 3]]] It is densely innervated, by sensory fibres of V1 (ophthalmic branch of trigeminal nerve). High density of pain receptors.
- sclera - which itself comprises the posterior 5/6ths of the tunic. It is a white, opaque ^[due to presence of dense irregular type I fibres, see [[Histology Lecture 3]]] region (i.e. the whites of the eyes). Its function is to provide shape to the eyeball and serve as an anchor for eye muscles. ^[similar to cornea, sclera is composed of connective tissue, and endothelium]
Note that there can be physiological or pathological variation in the pigmentation of the sclera. Yellowing could be due to aging, or jaundice. Bluing could be due to osteogenesis imperfecta (collagen defect, see [[Histology Lecture 3]])
- vascular tunic
The vascular tunic is composed of capillary networks or highly vascularised tissue. It has three components, moving posteroanterioirly, they are:- choroid: this supplies blood to the retina. It is a richly vascular layer found between the sclera and the retina. It contains fenestrated capillaries (see [[Physiology Lecture 7]]) and blood vessels ^[thus is susceptible to bleeding].
- ciliary body: which forms a ring around the lens
- iris
- nervous tunic/layer (also known as the retina)
The nervous tunic or layer is the innermost layer of the eyeball. It occupies the posterior two thirds.
The singular component of the nervous tunic is the retina. The retina devleops from the diencephalon (see [[Anatomy 3- The brain]], and is thus considered to be part of the central nervous system. It contains three layers of nerve cells, and is the site of initiation of vision.
Describe the components of the anterior segment of the eyeball
Anterior segment of eyeball
Is comprised of several structures:
- ciliary body
- lens
- iris and pupil (Central opening in iris)
- cornea
The segment contain aqueous humour (produced by ciliary body, see [[Anatomy Lecture 9]]) and is divided into two parts, the posterior chamber, and the anterior chamber, by the iris.
The posterior chamber sits between the lens and the iris.
The anterior chamber sits between the iris and the cornea.
**Note: DO NOT confuse posterior and anterior segments of the eye with anterior and posterior chambers
Describe the primary functions of the ciliary body
The two primary functions of the ciliary body are
Production of aqueous humour.
The aqueous humour is continuously secreted by an epithelium that overlies the ciliary processes, on the posterior aspect of the ciliary body.
Aqueous humour flows out through the canal of Schlemm, which collects it from teh anterior chamebr, which drains into the conjunctival/episclearl blood vessels.
Glaucoma is an issue of blockage in this system.
The second function of the ciliary body is to ‘do’ accommodation.
The ciliary body is richly vascular and contains muscles that control lens shape (see Anatomy Lecture 9 > Ciliary muscles), thus they affect accommodation.
The ciliary body provides attachment for the lens capsule (zonular fibre, suspensory ligaments).
Describe the lens and the ciliary muscles
The lens is contained within the lens capsule.
It is attached by zonular fibres to the ciliary body, and is required for close vision.
It has a pliable structure that hardens and yellows with age [15].
During distance vision, the lens capsule holds the lens tight, causing it to flatten.
During near vision, it engages in accommodation.
Muscles in the ciliary body attach to ‘scleral spur’
When muscle contracts ciliary body is drawn anteriorly, closer to spur. This reduces tension in the zonular fibres, allowing the lens to bulge/relax.
A more rounded lens bends light more effectively in order to focus on near objects.
Describe the iris
Iris is is an annular, highly vascular structure, which forms the coloured part of the eye. It consists of radially and circularly arranged muscle fibres that dilate and contract the pupil respectively i.e. dilator and sphincter pupillae muscles, in order to regulate the mount of light that enters the eye (see Anatomy Lecture 10).
List the muscles within the orbit
Levator palpebrae superioris - moves the eyelid
Extraocular muscles: 4 rectus (superior, inferior, lateral and medial), 2 oblique (superior and inferior) - moves the eye
All muscles are innervated by oculmotor nerve, emanating from oculomor nucleus in midbrain, and exiting cranium via superior orbital fissure.
EXCEPT trochlear nerve from trochlear neuron in caudal midbrain, exits also via superior orbital fissure. Controls superior oblique muscles
AND abducens nerve, exits from pontomedullary junction, from abducens nucleus in pons.
Exits also via superior orbital fissure.
Innervates lateral rectus
Describe the actions of the eye muscles
SR:
- action: elevation and adduction og eye ball
- test: elevation and abduction
IR:
- action: depression and addiction
- test: depression and abduction
MR and LR:
- moves medially and laterally respectively i.e. addiction and abduction
IO
- elevation and abduction
- test: elevation and adduction
SO:
- depression and abduction
- test: depression and adduction
why: listing, 3d orientation, three axes of rotation in single plane…
..start from a primary mosition, move to second position by rotating on axis, perpendicular to plane
opp: i.e. isolate function of single muscle, eg rule out LR and IR for ST movement. Follow with H test
extorsion and intorsion - rotating upper part of pupil medially or laterally
The axis of each orbit is directed slightly laterally from back to front, but each eyeball is directed anteriorly (Fig. 8.97). Therefore the pull of some muscles has multiple effects on the movement of the eyeball, whereas that of others has a single effect.
Compare and contrast sclera and cornea
Both found within fibrous tunic.
Cornea is ant, 1/6
Sclera is post, 5/6
Cornea is transparent, scleara is white and opaque due to presence of dense irregular type I collagen fibres.
Cprnea is also collgen, dense, stratified squamous epithelium and endothelium
Cornea is densely innervate with a high density of pain rececptors
