The Eye. Accessory structures or Adnexa 4 layers: 1. Skin – thinnest in the body 2. muscle – orbicularis oculi and levator palpebrae superioris 3. Connective tissue – tarsal plate contains tarsal or Meibomian glands - Chalazion. 4. Conjunctiva – mucous membrane
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Accessory structures or Adnexa
1. Skin – thinnest in the body
2. muscle – orbicularis oculi and levator palpebrae superioris
3. Connective tissue – tarsal plate contains tarsal or Meibomian glands - Chalazion
4. Conjunctiva – mucous membrane
sebaceous ciliary glands at base of hair follicles – hordeolum or stye
Lacrimal apparatus – forming and draining tears.
Strabismus – turned eye
Phoria – weakness of eye muscles
Eyeball – 3 layers or tunics:
1. Fibrous tunic (outer tunic)
Cornea – clear - avascular
Scleara – white –means “hard”
2. Vascular tunic : Uvea
Choroid – blood vessels and pigment
Ciliary body :
ciliary processes make aqueous humor
ciliary muscle - accomodation
Focusing the eye to see close objects
Lens is thin when stretched by suspensory ligaments (low power)
When round ciliary muscle contracts, tension is released from lens and it becomes thicker (higher power lens).
Lens – avascular, clear, elastic
contains proteins called crystallins
becomes opaque = cataract
Iris – pigmented, divides anterior and posterior chamber
Aqueous humor – drains into scleral venous sinus (Schlemm’s canal)
intraocular pressure - glaucoma
3. Nervous Tunic – Retina
several layers “inside out”
Rods and Cones (photoreceptors)
Light is focused by cornea and lens on
the Fovea centralis (“central pit”)which is in the
center of the Macula lutea (“yellow spot”)
The third refractive component is the
length of the eyeball.
Emmetropia – good vision 20/20
Myopia – nearsightedness
Hyperopia – farsightedness
Astigmatism – light does not focus to a single point on the retina
Presbyopia – “old sight” – loss of ability to accommodate or see up close
In the dark, Na+ channels are held open by a nucleotide called cyclic GMP (guanosine monophosphate)
Inflow of sodium (“dark current”) triggers the continual release of neurotransmitter .
This neurotransmitter is inhibitory- it prevents bipolar cells from firing by hyperpolarizing them.
When light strikes the retina, retinal (from vitamin A) which is bent, straightens out, and no longer fits into the opsin.
The two separate – this is called bleaching.
The opsin becomes an active enzyme, that activates other enzymes that break down cyclic GMP.
Without cyclic GMP, the Na+ channels close.
The receptor hyperpolarizes, stopping the release of inhibitory neurotransmitter.
This decrease in inhibition allows the bipolar cells to fire, and information is sent to the visual cortex.
Differentiation of color is assisted by horizontal cells.
In darkness, retinal isomerase converts trans-retinal back to cis-retinal, which binds with opsin forming a functional photopigment.
Uses three different photopigments :
blue, green and red
Wavelength of pigments may be shifted, causing
Red – green color blindness most common
Sex-linked trait carried on X chromosome
(Males only have one gene for color vision)
Can see in dim lightRequire more light
See in black and white See in color
Respond to movementGive best acuity
Found more in periphery More central, esp.
in fovea centralis
Use both eyes to perceive depth – “depth perception”
Nasal fibers cross at the optic chiasm
Temporal fibers do not cross over
Each visual cortex (R &L) receives information from both eyes so it can compare what each eye sees.