LENS Flashcards
LENS
- transparent biconvex structure
- attached to ciliary body by zonular fibres (suspensory ligament)
LENS LOCATION
- behind iris + pupil
- in front of vitreous body
TOTAL EYE DIOPTRIC POWER
58D
LENS DIOPTRIC POWER
15D (can change)
ACCOMMODATION
15-16D at birth
8D at 40
1-2D at 60
ANTERIOR + POSTERIOR POLES
CENTRE POINTS ON ANT. + POST. SURFACES
AXIS OF LENS
LINE JOINING POLES
EQUATOR
MARGINAL CIRCUMFERENCE
STRUCTURE OF LENS
- capsule
- epithelial layer
- stroma
LENS CAPSULE
- elastic basal membrane
- envelopes the lens
- elastic nature allows accommodation
LENS EPITHELIAL LAYER
- cuboidal cells below capsule on ant. surface
- at equator cells elongate + form columnar cells - transform into lens fibres
WHERE IS MITOTIC ACTIVITY GREATEST IN LENS?
AT EQUATOR
STROMA LENS
- made up of lens fibres
- makes up bulk of lens
- fibres form from epithelial cells in germinal zone (at equator)
WHAT KEEPS LENS FIBRES IN PLACE?
BALL + SOCKET JOINTS
LENS FIBRES FROM SAME LAYER JOIN AT?
SUTURES
LENS ENERGY
70% - glucose metabolised by anaerobic pathways
30% - aerobic metabolism of lens epithelium
ADVANTAGE OF LENS USING MOSTLY ANAEROBIC METABOLISM
not susceptible to oxygen starvation - must have glucose
OSMOTIC CONTROL OF WATER LEVELS IN LENS ACHIEVED BY WHAT?
SODIUM PUMP
only at anterior surface of lens
WHAT HAPPENS TO CENTRAL NUCLEUS AS LENS AGES?
- becomes sclerosed (stiff) + yellow
- refractive index goes up
- results in reduced visual acuity
- increased absorption of blue (short wavelength) light
LENS CAPSULE THICKEST AT WHAT POINT
ANTERIOR + POSTERIOR SURFACES CLOSE TO THE EQUATOR
thinnest at posterior pole
