the human eye has a total optical power of about +60 diopters
70% to 80% of total refraction (+42 to +43 diopters) occurs at the cornea however, its focusing power is fixed and cannot change shape
this is because it is at the sharp boundary between air (n=1.0) and the aqueous humor (n = 1.38)
20% to 30% of total refraction occur through the lens which can change shape via the ciliary muscles to produce accommodation
presbyopia is primarily caused by the natural aging of your eyes - with age, the crystalline lens gradually loses its flexibility and becomes more rigid, while the surrounding eye muscles lose their efficiency. This combination prevents the lens from changing shape to properly focus on near objects.
the aqueous and vitreus humor compartments add minimal direct refractive power as their refractive indices (n) are very close to water (n = 1.33)
field of view:
it has a total horizontal field of view (FOV) of about 200 to 220 degrees and a vertical FOV of around 135 degrees
the overlapping field where both eyes see the same area (binocular vision) is about 120 degrees horizontally, providing depth perception and stereopsis
central vision field of view is about 3 to 5 degrees of high-acuity focus directly in front, expanding to roughly 15 to 20 degrees for clear central detail, and up to 30 degrees for the broader central zone
Physiology
extra-ocular muscles
there are six extra-ocular muscles which allow movements of the eye for gaze in various directions
oculomotor nerve (CN III) supplies:
superior rectus - moves the eye upward and inward
inferior rectus - moves the eye downward and inward
this is the most common muscle that is caught in an inferior orbital floor “blow-out” fracture resulting in inability to look fully upwards, diplopia and pain on looking upwards and traction on the trapped muscle may cause a oculocardiac reflex resulting in nausea, vomiting or bradycardia
medial rectus - turns the eye inward toward the nose
can be trapped in medial orbital wall “blow-out” fractures
inferior oblique - turns the eye upward and outward
trochlear nerve (CN IV) supplies:
the superior oblique - turns the eye downward and outward
trochleitis is inflammation of the fibrous pulley (trochlea) located in the upper, inner corner of the eye socket where the superior oblique tendon passes - looking up and inward stretches this tendon and provokes sharp or aching pain and there is likely to be tenderness of the trochlea
abducens nerve (CN VI) supplies:
the lateral rectus - turns the eye outward away from the nose
eye protection systems
in particular, these protect the vulnerable cornea and help to prevent it drying out
bony orbit
eyelid
provides protection to the cornea by blinking as well as providing more uniform hydration of the cornea
blinking is partly due to:
the facial nerve (CN VII) acts as the motor (efferent) pathway that voluntarily triggers the muscle to close the eyelid
central UMN MN facial nerve palsy eg. stroke, tumour, MS, etc.
characteristically presents with forehead sparing (the patient can still wrinkle their forehead) because the upper face receives dual-hemisphere brain input
Müller's muscle (a small, sympathetically innervated smooth muscle in the upper eyelid), paralysis such as in Horner's syndrome results in a mild (1 to 2 mm) drooping of the lid - ie. a partial ptosis
eye lashes
act as the eye's first line of defense
hair thickness and maximum length are determined entirely by genetics - cutting them will not change this!
shield your eyeballs from dust, debris, and sweat
are highly sensitive and act as tiny sensors that trigger a protective blink reflex when touched
limiting airflow over the eyeball, lashes reduce tear evaporation and their curved shape also helps channel away sweat and rainwater to maintain clear vision
individual new eye lashes grow over a 6-8 week period (although can take up to 16 weeks) and eventually fall out after 3-4 months
NB. plucking eyelashes may damage the follicle and may delay re-growth by up to 3-4 months
NB. trimming lashes removes the natural tapered end, which may cause blunt ends to poke the eyelid or lead to minor abrasions
corneal blink reflex
the trigeminal nerve (CN V) acts as the sensory (afferent) pathway that detects touch on the cornea, and the facial nerve (CN VII) acts as the motor (efferent) pathway that triggers the muscle to close the eyelid.
lacrimal gland
located in the upper, outer part above the eyelid but below the eyebrow
tear production
tear duct apparatus
drainage of tears into the nose via an upper and lower punctum which are located at medial ends of each eyelid and drain into the lacrimal sac via the inferior and superior lacrimal canals, and thence to the lacrimal duct
optic system
cornea
provides most of the refraction of light entering the eye
blocks most UV light under 300nm
anterior chamber
filled with aqueous humor (AH) which nourishes the eye and keeps it inflated
AH is produced by the ciliary body
most of the aqueous humor flows out of the eye through the drainage angle, in front of the iris.
when the ciliary muscle (see below) contracts, it pulls on the scleral spur, mechanically widening the spaces in the trabecular meshwork which increases the outflow of aqueous humor into Schlemm's canal, which helps lower eye pressure and prevents ocular hypertension.
the circadian clock regulates IOP, disruptions to a person's internal clock can lead to increased risk of glaucoma
norepinephrine increases the levels of a molecule called RHOB in the eye's drainage system (the trabecular meshwork) resulting in reduced drainage and higher IOP at night, at least in mice1)
normal intraocular pressure (IOP) typically ranges between 10 mmHg and 21 mmHg, with an average of about 15 to 16 mmHg.
IOP > 21mmHg is regarded as ocular hypertension and is a risk of glaucoma developing
low IOP may be a sign of penetrating eye injury
mydriatics (pupil-dilating drops) can increase the risk of acute angle-closure glaucoma in people with narrow eye drainage angles (those at risk include hyperopia (farsightedness), where eyes are often smaller, and older adults, as the natural lens grows thicker with age and pushes the iris forward)
when the pupil widens, the iris bunches up, which can block the fluid drainage angle of the eye.
this traps fluid inside, causing a fast and painful rise in eye pressure
pupillary muscles adjust pupillary size mainly to offset brightness issues but also as a signalling system as dilated eyes tend to make one more attractive to potential mates
Sphincter Pupillae (Pupillary Constrictor)
involuntary muscle controlled by the parasympathetic nervous system which constricts the pupil
constricting the pupil not only reduces the amount of light coming onto the retina but it also increases “depth of field” allowing a greater range of subject distances to be in focus - hence doctors use the “pin-hole” test to mimic this which helps assess visual acuity partly “correcting” any refractive errors of the eye
Dilator Pupillae (Pupillary Dilator)
involuntary muscle controlled by the sympathetic nervous system which dilates the pupil
adjust focusing of the lens to provide accommodation
for near vision:
muscle contracts, moving the ciliary body inward, this releases tension on the suspensory ligaments (zonules of Zinn) attached to the lens which becomes thicker and more spherical, increasing its power to focus on close objects
for far vision:
muscle relaxes, which pulls the zonules tight which flattens the lens and decreases its refractive power, allowing focus on distant subjects
these also have an important role in maintaining intra-ocular pressures
lens
provides further refraction and focusing of the light onto the retina
blocks most UV light 300-400nm
posterior chamber
filled with vitreous humor which is a clear, jelly-like substance and makes up about 80% of the eye's volume
it stops the eye from collapsing and keeps it round, cushions the inside of the eye from trauma and helps keep the light-sensitive retina flat and secure
helps manage oxygen levels inside the eye to keep tissues healthy
retinal vision system
the main visual sensory cells are rods (mainly monochromatic but more sensitive to light than cones) and cones (3 main types which detect colour)
cones are most dense at the macula which is the central vision component of the retina
the fovea is a small pit located in the macula that provides the sharpest visual acuity
the “blind spot” of the retina is optic disc where the optic nerve leaves the retina and there are no rods or cones to detect light in this region
may be compressed due to pituitary tumours, meningiomas, aneurysm, etc, classically causing a bitemporal hemianopia, a loss of vision in the outer (temporal) halves of the left and right visual fields due to the fact that the nasal fibres from each retina cross to the other side at the chiasm and this part of the retina “sees” the outer visual field
processing centres
the lateral geniculate nucleus of the thalamus
receives the vast majority of optic tract fibers; acts as a major sensory relay station that sends visual signals via optic radiations to the primary visual cortex for conscious sight
the superior colliculus in the midbrain tectum
coordinates rapid head and eye movements to direct gaze toward visual stimuli
pretectal area of the midbrain at the midbrain-diencelphalon junction
mediates the pupillary light reflex by processing ambient light intensity
the suprachiasmatic nucleus - part of the hypothalamus
uses direct non-image-forming light signals to synchronize circadian rhythms (the body's internal day-night clock)
primary visual cortex
located in the occipital cortex
a stroke may cause:
homonymous hemianopia: loss of the entire contralateral half of vision
quadrantanopia: loss of one-quarter (upper or lower) of the contralateral visual field
NB. macular sparing is common in occipital strokes due to rich blood supply in that area
NB. cortical blindness with total perceptual vision loss may occur from bilateral occipital damage
fluid and waste management
ciliary body
produces aqueous humor
aqueous humor (AH)
a clear fluid and helps maintain intraocular pressure (IOP) and nourishes avascular ocular tissues
produced by the ciliary body
AH exits the eye through two primary routes:
conventional outflow pathways
drains AH via the trabecular meshwork and Schlemm's canal and into episcleral veins
unconventional outflow pathways
involves drainage through the iris root, ciliary body, anterior choroid, and sclera, and orbit
includes connective tissue-filled spaces between ciliary muscle sheaths, extending through the supraciliary and suprachoroidal spaces to the macula and optic nerve head
via this pathway, AH and stromal proteins can exit the eye trans-sclerally, via the choroidal vasculature, and through recently identified ciliary body lymphatics which 2)
intra-ocular lymphatic pathways
drain to ipsilateral cervical lymph nodes
“uveolymphatic” pathway
discovered in 2009
lymphatic-related drainage pathway at the front of the eye
the posterior ocular lymphatic outflow, or POLO pathway