Eyes and Vision

• vision is our dominant sense: 70% of sensory receptors are in the eyes
• anterior one-sixth of the eye's surface is visible, rest enclosed by cushion of fat and bony orbit walls
• only small portion of eye tissues are actually involved in photoreception
• eyebrows shade the eyes and catch sweat
• eyelids or palpebrae protect the eyes from dust, dirt, and debris
• lacrimal caruncle contains sebaceous and sweat glands that produce eye boogers
• tarsal plates which are connective tissue sheaths internally support the eyelids; they also anchor orbicularis oculi and levator palpebrae superioris muscles
• orbicularis muscle closes the eye and levator palpebrae superioris elevates the eyelid
• blinking is reflexive, happening every 3-7 seconds to keep the eye moist
• follicles of eyelashes are richly innervated by hair follicle receptors to trigger reflexive blinking for protection
• tarsal glands produce an oily secretion to lubricate the eyelid and eye to prevent the lids from sticking together
• modified sweat glands between lash follicles are ciliary glands
• conjunctiva is a transparent mucous membrane that lines the eyelids as palpebral conjunctiva and reflects over anterior surface to form bulbar conjunctiva; major function is lubrication
• bulbar conjunctiva covers the sclera except where it is covered by the cornea; very thin so vessels are visible
• conjunctival sac is where contact lenses and medication are applied
• lacrimal fluid has mucus, antibodies, and lysozomes to clean and protect eyes as its being lubricated
• two oblique muscles move the eye in the vertical plane when the eye is already turned medially by rectus muscles
• the superior oblique muscle moves the eye downward and laterally (inferiorly)
• the inferior oblique muscle moves the eye upward and laterally (superiorly)
• lateral pull of oblique muscle is needed to cancel medial pull of superior and inferior recti
• lateral rectus

  • lateral movement
  • controlled by abducens nerve (6)
• medial rectus

  • medial movement
  • controlled by oculomotor (3)
• superior rectus

  • elevated eye, turns medially
  • controlled by oculomotor (3)
• inferior rectus

  • depresses eye, turns medially
  • controlled by oculomotor (3)
• inferior oblique

  • elevates eye, turns medially
  • controlled by oculomotor
• superior oblique

  • depresses eye, turns laterally
  • controlled by trochlear (4)
• extrinsic eye muscles most precise and rapid skeletal muscles due to high axon-to-muscle-fiber ratio
• eyeball internal cavity filled with fluid called humors to maintain shape
• fibrous layer

  • outermost layer
  • dense avascular connective tissue
  • sclera and cornea
• posterior portion of sclera forms bulk of fibrous layer; anterior portion is white of eye that protects and shapes eye while anchoring extrinsic eye muscles
• anterior sixth of fibrous layer is modified to be transparent cornea that lets light enter eye
• external sheet of the cornea protects from abrasion while deep corneal endothelium lines inner surface of cornea to maintain clarity via sodium pumps
• cornea well supplied with nerve endings, most are pain receptors
• cornea is highly regenerative and avascular so transplants have little risk of rejection
• vascular layer

  • middle coat
  • pigmented
  • choroid, ciliary body, and iris
• choroid is highly vascularized membrane forming posterior 5/6th of vascular layer; blood vessels nourish all eye layers and it brown pigment, melanocytes, absorbs light
• ciliary body

  • ciliary muscles: control lens shape
  • ciliary processes: secrete fluid filling anterior eye cavity
  • ciliary zonule: suspensory ligament that holds lens upright
• iris is visible colored part and most anterior portion of vascular layer, lying between cornea and lens; round central opening is pupil which allows light to enter
• sphincter pupillae constricts the pupil
• dilator pupillae dilates the pupil