26 - Risk factors for cataract

1. Increasing age - oxidative damage to membranes and proteins


2. sunlight ( especially the UV-A, UV-B components ) - hydration


3. severe diarrhoeal dehydration - denaturation of proteins


4. vitamins A, C, E deficiency - opacification of lens fibres with fibrous metaplasia


5. diabetes - opacification of lens epithelium


6. smoking - accumulation of pigmented molecules
                    ( 3-hydroxykynurenine and chromophores which lead to yellowing )


7. corticosteroids - disturbance in osmotic balance


8. genetic - failure of ion pumps ( increased intralenticular calcium and sodium with decrease in potassium )

25 - Sherrington law of reciprocal innervation

During the initiation of an eye movement, increased innervation to an extraocular muscle is accompanied by simultaneous inhibition ( a reciprocal decrease in innervation ) of the direct antagonist of the contracting muscle of the same eye.

                             

If the left medial rectus muscle receives innervational flow to initiate adduction of the left eye, there is simultaneous decreased and inhibitory flow to the left lateral rectus muscle to make it relax and enable the eyeball to move medially.

24 - Embryological derivatives of Eye and its parts

1. Derivatives of Neural ectoderm : smooth muscle of the iris, optic vesicle and cup, iris epithelium, ciliary epithelium, part of the vitreous, retina, retinal pigment epithelium, fibres of the optic nerve.

2. Derivatives of surface ectoderm :  conjunctival epithelium, corneal epithelium, lacrimal glands, tarsal glands, lens.

3. Derivatives of mesoderm : extraocular muscles, corneal stroma, sclera, iris, vascular endothelium of eye and orbit, choroid and part of the vitreous .

4. Derivatives of neural crest* : corneal stroma, keratocytes and endothelium, sclera, trabecular meshwork endothelium, iris stroma, ciliary muscles, choroidal stroma, part of the vitreous, uveal and conjunctival melanocytes, meningeal sheaths of the optic nerve, ciliary ganglion, schwann cells of the ciliary nerves, orbital bones, orbital connective tissue, connective tissue sheath and muscular layer of the ocular and orbital blood vessels . 

( *during the folding of the neural tube, a ridge of cells comprising the neural crest develops from the tips of the converging edges and migrates to the dorsolateral aspect of the tube. neural crest cells from this region subsequently migrate and give rise to various structures within the eye and the orbit )