March 26, 2013 Lecture
-The function of the eye is to convert photons of light into action potentials (nerve impulses). -The inner layer is a nervous tunic (made out of nerve cells)=retina. -Refraction: Bends light.
-The purpose of the cornea and the lens is to take an object and focus every little detail of the object on the retina on the back of the lens. Upside down and backwards. -When light goes through medias, it bends.
-Lens are suspended by ligaments behind the pupil. Suspensory ligaments. -Aqueous humor, vitreous, intraocular pressure. Liquid that creates internal pressure much like air in a basketball. Excess amount of intraocular pressure=glaucoma. -The natural thermodynamically stable configuration or shape of a lens is more round like a marble (SPHERICAL). -Why is an eye ball flat? The ligaments. The intraocular pressure stretches the lens flat? Flat is good for distance vision. Round is good for close vision. -Ciliary muscle. When it contracts it will squeeze the eyeball smaller so it loosens the ligaments. Contract the muscle will make the lens round. -The eye muscles are relaxing when viewing far away.
Photon: unit of light.
-Want to take the photons of light and convert them to nerve impulses.
-Converting a photon of light into nerve impulse: transduction.
Two kinds of photoreceptors: receive photons of light.
Rods: Can only see in very very dim light. It is too bright in room for them to function. They bleach out outside in sunlight. They are excellent in dim light like at the dead of night.
Cones: Unlike rods. They can see in detail and color.
Photoreceptors cells synapse with a bi-polar cell which synapses with a ganglion cell. The axons of ganglion cells emerge from the eye all packed together to make up the optic nerve. The optic nerve will carry nerve impulses to the brain. Light hits the photoreceptor.
Direct pathway from photoreceptor to ganglion cells.
In the darkness, rod/cone are releasing neurotransmitters. Turns bi-polar cell off? When a photon of light hits the rod, the light turns rod off and no longer releases inhibition. The bi polar cell fires and activates ganglion cells by firing neurotransmitters. -------------------------------------------------
Guanalate Cyclase: takes GTP and guanalate cyclase used to convert GTP to cGMP and phosphate (P – P). Takes GTP and creates cGMP+(P-P). cGMP opens sodium channels by attaching. Sodium in the darkness comes in and go up and creates a graded potential. Depolarizing the cell. Releases inhibitory transmitter. There is a pump inside that pumps sodium out of the cell. The sodium goes out and comes back in.
Makes a current called a dark sodium current. To stop the dark current which stops of release of inhibition. Needs to stop cGMP. Phosphodiesterase stops cGMP by breaking it down to 5prime GMP. Shape changes when cGMP breaks down and does not remain attached to the channel. Channel stops and sodium stops coming in. No longer depolarizing the cell so it does not release inhibitory neurotransmitter. Neurotransmitter is released into bi-polar cell. In the dark the channel is open.
-Where is PDE? In the discs there is a protein in the membrane called opsin attached to a vitamin (derivative of a vitamin) called retinene. -There are two forms of retinene called 11-cis retinene and there is kind called all-trans retinene. Two types/two shapes. -At rest, it is normally 11-cis form. On the photodisc, opsin (protein) and retinene made from vitamin A (carotene) g-proteins. Enzyme called PDE. In the dark the enzyme is inactivated. PDE does the mozy over thing when light shines. 11-Cis is converted to its other form when light hits the opsin protein. When it converts g-protein mozy over and activates the enzyme, PDE. The job of PDE is to break down cGMP to close the channel and stops the dark sodium current. Stop depolarization. Stops inhibitory...
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