g.neurophysiology of Vision and the eye

Physiology of vision

1. VISION Dr. S hwetha

2. Anatomy of the eye Sclera- outer protective layer Cornea- modification of the sclera anteriorly to be transparent – for light to enter Choroid- layer inside the sclera ; contains many blood vessels for nourishment of the eye. Retina- neural tissue with receptor cells lining the posterior 2/3 of choroid.

3. Lens- crystalline, held by a circular lens suspensory ligament ( zonule ) Ciliary body- thickened anterior part of the choroid where the zonule attached to. Ciliary body has circular and longitudinal muscle fibres that attach near the corneo -scleral junction.

4. Anatomy of the eye Iris- opaque structure before the lens; coloured portion of the eye Iris contains contains circular muscle fibres that constrict and radial fibres that dilate the PUPIL . Vitreous humour – clear gelatinous material that fills the space between the lens and retina Aqueous humor- a clear liquid that nourishes the cornea and lens, produced in the ciliary body . Moves by diffusion and active transport from plasma. It flows through the pupil and fills the anterior chamber of the eye .

5. C anal of Schlemm ,- a network of trabeculae that reabsorbs A. H . a venous channel at the junction between the iris and the cornea (anterior chamber angle).

6. RETINA 10 layers and contains the rods and cones, which are the visual receptors. Four types of neurons: bipolar cells, ganglion cells, horizontal cells, and amacrine cells The rods and cones , which are next to the choroid , synapse with bipolar cells, and the bipolar cells synapse with ganglion cells.

7. R etina 12 different types of bipolar cells occur, based on morphology and function. The axons of the ganglion cells converge and leave the eye as the optic nerve. Horizontal cells connect receptor cells to the other receptor cells in the outer plexiform layer.

8. Retina Amacrine cells connect ganglion cells to one another in the inner plexiform . 29 types of amacrine cells. Gap junctions also connect retinal neurons to one another, and the permeability of these gap junctions is regulated.

9. light rays must pass through the ganglion cell and bipolar cell layers to reach the rods and cones. The pigment epithelium absorbs light rays, preventing the reflection of rays back through the retina, otherwise blurring of the visual images. optic disk – region where The optic nerve leaves the eye and the retinal blood vessels enter it. There are no visual receptors over the disk, and consequently this spot is blind (the blind spot ). Near the posterior pole of the eye is a yellowish pigmented spot, the macula lutea / fovea

10. F ovea centralis - R od-free portion of the retina present in humans and other primates. C ones are densely packed , and each synapses to a single bipolar cell, which, in turn, synapses on a single ganglion cell, providing a direct pathway to the brain. There are very few overlying cells and no blood vessels. point where visual acuity is greatest .

11. When attention is attracted to or fixed on an object, the eyes are normally moved so that light rays coming from the object fall on the fovea. The retinal vessels supply the bipolar and ganglion cells. receptors are nourished mainly by the capillary plexus in the choroid

12. Receptors – Rods and cons Each rod and cone is divided into an outer segment, an inner segment includes a nuclear region, and a synaptic zone . The outer segments are modified cilia and are made up of regular stacks of flattened saccules or discs . These saccules and disks contain the photosensitive compounds that react to light, initiating action potentials in the visual pathways.

13. Rods and cons inner segments are rich in mitochondria . The rods are named for the thin, rodlike appearance of their outer segments. Cones generally have thick inner segments and conical outer segments, although their morphology varies from place to place in the retina.

14. Rods and cons Rods- extremely sensitive , can be triggered by a single photon . [ At very low light levels, visual experience is based solely on the rod signal ; why colors cannot be seen at low light levels Cones require significantly brighter light (i.e . a larger numbers of photons) in order to produce a signal. In humans, there are three different types of cone cell, distinguished by their pattern of response to different wavelengths of light. The three types of cone cell respond (roughly) to light of short, medium, and long wavelengths. The different responses of the three types of cone cells are determined by the likelihoods that their respective photoreceptor proteins will absorb photons of different wavelengths .

15. I ONIC BASIS OF PHOTORECEPTOR POTENTIALS Na + channels in the outer segments of the rods and cones are open in the dark, so current flows from the inner to the outer segment . Current also flows to the synaptic ending of the photoreceptor. The Na+–K+ pump in the inner segment maintains ionic equilibrium.

16. Release of synaptic transmitter is steady in the dark- inhibitory in nature When light strikes the outer segment leads to close some of the Na+ channels, and the result is a hyperpolarizing receptor potential. The hyperpolarization reduces the release of synaptic transmitter , and this generates a signal in the bipolar cells that ultimately leads to action potentials in ganglion cells. The action potentials are transmitted to the brain.

17. Rods and cons The photosensitive pigment in the rods is called rhodopsin (visual purple) Rhodopsin has a peak sensitivity to light at a wavelength of 505 nm. It is one of the many receptors coupled to G proteins . cGMP is the second messenger system used . Enzyme cGMP phosphodiesterase converts cGMP to 5- cGMP , cGMP directly acts on Na channels to keep it open When it is converted to 5- cGMP- Na channels close .