Vision*

Question 1

How does a sharp image form on the retina - explanation of refraction? closer vs further?

Answer 1

Light waves from an object bend at the cornea, and bend further at the lens to form a clear image on the retina
This bending of light waves is called refraction
Closer objects needs more bending power to focus on an object, this makes the lens thicker and more powerful

Question 2

Accommodation - definition? 3 different steps of accomodation? Step 1 changes of the eye? Step 2 changes of the eye?

Answer 2

The ability of our eyes to change between distant and close objects
Lens thickens
Pupil constricts
Eyes converge

Lens thickens:
- ciliary body contraction (suspensory ligs become lax) (para III) causes lens to become thicker and more spherical
- thicker lens is more powerful and so can focus on closer objects
Pupil constricts:
- Closer object needs sharper focus, to do this the pupil must constrict achieved by the sphincter pupillae (para III)
Eyes converge:
- must turn in to look at the object (converge) via the medial rectus muscle (III), thicker than lateral recti muscles

Question 3

Refractive errors - short-sightedness? long-sightedness? and astigmatism?
retina relation?

Answer 3

Short: myopia; image forms in front of the retina
Long: hyperopia; image forms behind the retina
Astigmatism: non-spherical curvature of cornea/lens

Question 4

Pupillary reaction to light - illumination changes? impulse travel (nerves)? pupillary reflex arch? efferent limb process?

Answer 4

Increased illumination = para = pupils constrict
Decreased illumination = symph = pupils dilate
Impulses travel from the retina to the optic nerve to the optic chiasma and to the optic tract
Fibres for the pupillary reflex do not go to the lateral geniculate body, they go to the midbrain (III nuc)
Part of the III nuc is in the edinger-westphal nuc (EWN) for para and so the reflex go to EWN of both sides
Efferent limb: fro EWN to pregang para via III nuc to orbit, para fibres to ciliary gang, post fibres to constrictor pupillae = constrict

Question 5

Phototransduction - definition? rods and cone colours? Rhodopsin? Phototransduction cascade? vitamin A?

Answer 5

Conversion of light energy to an electrochemical response by the photoreceptors and the rods and cones act the optic nerve neurons
Rod: black and white (rhodopsin)
Cone: colour vision (iodopsin)
11-cis retinal a chromophore in the opsin formed from vit A, on light 11-cis retinal isomerises to all-trans retinal, can’t fit into opsin, rhodopsin splits, bleaching visual purple
Cascade:
- Photon hits rhodopsin and forms opsin
- Transducin-GTP activates PDE
- PDE catalyses cGMP to GMP
- GMP closes Na channels
- hyperpolarisation of the photoreceptor cell
- hyperpol transmitted by Ca influx to synapse with bipolar cell
- glutamate release stopped at synapse
- stim bipolar cell and the ganglion cell
Vit A forms AT-Rol forming 11 cis-Rol then to 11 cis-retinal for rhodopsin regeneration