Anatomy / Phys Flashcards
What are the layers of the eye?
Fibrous, vascular + nervous.
What is the fibrous layer of the eye?
Sclera (85%) + cornea: provides shape and supports deeper structures
- Sclera: attachment for extraocular muscles (+ penetrated by optic nerve)
- Cornea: continuous with cornea anteriorly, refracts light
What is the vascular layer of the eye?
Choroid, ciliary body + iris
- Choroid: connective tissue + blood vessels (supplies outer layers of retina).
- Ciliary body (circular): encircles eyeball, comprised of ciliary smooth muscle + ciliary processes. Also contributes to formation of aqueous humour.
- Ciliary muscle: 3 layers: longitudinal, circular + radial
- Ciliary process: attaches lens to ciliary body via zonicular fibres (which collectively form the suspensory ligament of the lens) > ciliary body ∴ determines shape of lens
- Contraction of the ciliary muscle > reduced ciliary body diameter > laxing of zonicular fibres > lens more rounded > accommodation (near vision)
- Relaxed ciliary muscle > increased body diameter > tension on zonicular fibres > lens pulled flat (long distance vision)
3. Iris: eye colour, controls size of pupil. Circular fibres: sphincter pupillae muscle; parasympathetic supply > pupil contriction. Radial fibres: dilator pupillae muscle; sympathetic > pupil dilation.
What is the nervous layer of the eye?
Retina: neural & pigmented layer
- Neural layer = photoreceptors (light detecting cells). Located posteriorly & laterally.
- Pigmented layer (non-visual retina) = support for neural layer, attached to choroid. Located around whole inner surface of eye (continues anteriorly, whereas neural layer does not).
*Optic part of the retina = the area where both layers are present (i.e. posteriorly and laterally): can be viewed with ophthalmoscope.
Outline the light pathway
- Cornea (refracting power ~43 dioptres: ¾ of total eye refracting power)
- Lens (variable refracting power ~15 dioptres: ¼ total eye refracting power)
- Vitreous (gelatinous mass with no refracting power)
- Retina (10 layers)
Outline the visual pathway
Retina receives images that are inverted.
Retina > optic nerve. Info received by the nasal retinas cross at the optic chiasm. This means the left visual field (left temporal and right nasal) travels in the left optic tract and the right visual field (right temporal and left nasal) travels in the right optic tract.
Optic tract > lateral geniculate body > geniculo-calcarine tract > primary visual area (striate cortex).
*The cortex receives the encoded images of the contralateral visual fields of both eyes.
Outline the anatomy of the eyelids
Palpebrae: protect globe + distribute tears
Anterior > posterior: skin, orbicularis oris, tarsal plate, Meibomian glands, conjunctiva
- Tarsal plate provides shape
- Elevated by levator muscle (oculomotor / III) + Muller’s muscle (sympathetic nerves)
Sensory: V1/ophthalmic (upper lid) + V2/maxillary (lower lid).
Motor: oculomotor opens lid (levator muscle) + facial closes lid (orbicularis oculi).
Define: chalazion, sty, epiphera
If meibonium glands blocked (left image) / inflamed / infected would affect tear film.
If just blocked = Chalazion.
If uncomfortable + infected = sty.
Epihera = blocked punctum. If lack of tears / dry eyes may be deliberately blocked to prevent drainage of small amount of tears
Bones of the orbit?
Fissures?
frontal, ethmoid, lacrimal, palatine, sphenoid, zygomatic + maxillary.
Superior orbital fissure is found between the greater + lesser wings of the sphenoid bone and transmits: oculomotor (superior & inferior divisions), trochlear, ophthalmic/V1 (lacrimal, frontal and nasociliary branches) + abducens. Also carries the ophthalmic vein (superior + inferior divisions) + sympathetic fibres (from the cavernous plexus).
Inferior orbital fissure formed by the sphenoid + maxilla, and separates the medial wall + floor of orbit. Transmits zygomatic branch of maxillary (V2) + ascending branches of the pterygopalatine ganglion. Also carries inferior division of the ophthalmic vein, and the infraorbital vessels (which enter the infraorbital canal + exit through infraorbital foramen).
Infraorbital foramen = maxilla (transmits infraorbital artery, vein + nerve).
Outline actions of muscles on the eye
Eyelid opening = Levator palpebrae superioris + Muller’s muscle
Eyelid closing = Orbicularis Oculi (facial nerve)
- When acting alone, superior oblique causes intorsion (internal rotation) and inferior oblique causes extorsion (external rotation). These are the primary actions.
Because orbit centre line is >20° angle from the centre line, action of superior + inferior rectus would cause the eye to rotate around its long axis (retina > pupil), therefore, inferior rectus + superior oblique contract together to prevent extortion of the eye when looking down, and superior rectus + inferior oblique contract together to prevent intorsion of the eye when looking up. This keeps vision horizontally level regardless of eye position in the orbit.
Superior muscles = intorters
inferior muscles = extorters
Vertical recti = adductors
Obliques = abductors.
Outline the lacrimal system
Lubricates eyeball + protects cornea. Tears secreted via facial nerve (parasympathetic supply) - lacrimal gland > excretory duct to upper fornix
Drainage route:
- Puncta (sup + inf)
- Canaliculi (sup + inf)
- Lacrimal sac
- Nasolacrimal duct
- Nasopharyngeal cavity
Describe tear film
- Basal flow 1.2µl/min (imbalance in any constituents will cause symptoms. Grittiness / irritation = ‘dry eye’.)
1. Lipid: from meibomium glands (slows tear evaporation / prevents spillover)
- Aqueous: from lacrimal + accessory lacrimal glands of Krause and Wolfring (provides lubrication + corneal oxygenation; mostly water with glucose, lysosyzme, lactoferrin, IgA & ions: Na⁺, K⁺, Mg²⁺, Ca²⁺)
o 90% of tear volume - Mucin: from conjunctival goblet cells (adheres tears to conjunctiva, lines of microvilli + microplicae of cornea / conjunctival epithelium improves tear film viscocity
Two main structures immediately posterior to orbit?
Cavernous sinuses + pituitary gland: found immediately posterior to the orbit:
What is the conjunctiva?
Thin, transparent mucous membrane lining the inner eyelids (bulbar conjunctiva) + sclera (palpebral conjunctiva).
Continues beneath eyelids (fornices): if painful red eye, ensure you evert the lids to look under eyelids and pull bottom eyelid down > examination of upper + lower FORNICES (important in burn / chemical injuries – act as pocket away from surface, chemicals can get stuck)
2 layers:
• Stratified non-keratinised epithelium
• Lamina propria (connective tissue blood vessels, nerves, glands / goblet cells, mast cells, macrophages + lymphocytes).
What is the sclera / episclera?
Tough white shell starting from the limbus, extending all around the eye.
Episclera: vascularised cover to sclera (supplies nutrients to sclera), connected by Tenon’s capsule. Supplied by dense plexus of anterior ciliary arteries.
Sclera: outer coating of eye: white, opaque, dense, irregular collagen fibrous tissue. Terminates at corneoscleral limbus, nutritional supply from choroid + episclera.
