Lens - structure and insertion, accommodation
The lens is avascular, transparent elastic tissue that focuses light on the retina, developed from thickening ectoderm – Lens placode.
It is highly elastic, a feature that is lost with age as lens tissue hardens.
Structure[edit | edit source]
Consist of 3 components:
- Lens capsule: Think of it as the Len's protective shield.Basement membrane which surrounds the entire lens, it is comprised of collagen
- Lens epithelium: Simple cuboidal epithelium, located anteriorly
- Lens fibres: Develop from stem cells, eventually lose their nuclei and their organelles and become filled with specialised proteins known as crystalline which are adapted to refract light.The fibers are densely packed together, forming a perfectly transparent tissue highly specialized for light refraction.
The lens is held in place by the ciliary zonule, which inserts on both the lens capsule and on the ciliary body.
Ciliary muscles and their function:[edit | edit source]
Thanks to ciliary muscle, the shape of the Lens changes. Contraction of the ciliary muscle results in the change of the shape of the ciliary body which through its connection to the lens via the suspensory ligament of the lens (also known as the zonules, or zonular fibres) causes the lens to become more rounded, therefore, better adapted for shorter range vision, known as visual accommodation
Accommodation reflex[edit | edit source]
When the eye needs to adapt from looking at a distant object, onto looking on a near object. The accommodation reflexes activated - results with contraction of the medial rectus muscle, constriction of the pupil, and with thickening of the lens (to increase refractive power)
The thickening of the lens occurs by parasympathetic innervation of the ciliary muscle.
Reflex pathway:[edit | edit source]
- retina
- optic nerve
- optic chiasma
- optic tract
- lateral geniculate body
- visual cortex
- frontal eye field
- fibers rise from frontal eye field:
- Travel to the oculomotor nucleus and innervating the medial rectus muscle.
- Run to the Edinger Westphal nucleus, causing constriction of pupil and thickening of the lens.
Watch out for Glaucoma![edit | edit source]
If drainage of the aqueous humour is obstructed, then glaucoma can occur.
reference:[edit | edit source]
Salvador notes ( final notes )
