(From Dr. Glasser’s Lecture, 4 July 2000, by Brian Buschman)
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In the retina the rods and cones are located deep into the retina and the signal is passed more superficially as it goes from one cell level to another. All photoreceptors contain 11-cisretinol-opsin with a different bound protein differentiating the different photopigments. In the rods the photopigment is rhodopsin and in the cones there are three different pigments that correspond to blue, green ad red light.
Leaving the retina at the optic dist is the optic nerve (CN II) which goes to the optic chiasm and then the optic tract. The optic tract bypasses the thalamus by crossing in the brachium of the superior colliculi. In the case of the pupilary light reflex the signal is then passed to the pretectum and then to the Edinger-Westphal nucleus of CN III. From the Edinger-Westphal nucleus parasympathetic fibers leave to cause miosis. This system functions bilaterally so if you shine a light in a person’s right eye their left pupil should contract as well. Miosis on the same side is called a direct response and on the opposite side is a consensual response.
If you shine a light in the right eye and you get direct but not consensual miosis the optic tract is still intact but you probably have a legion in the contralateral nerve III. I expect an exam question of this nature on mini III.
Accommodation is the focusing of the eye from far vision to near vision. Accommodation involves three things:
1) The lens becomes more spherical.
2) The pupils contract.
3) Your eyes converge.
The pathway come from the signal entering via CN II to the Lateral Geniculate Nucleus (LGN) of the thalamus and then projects in order to areas 17, 18 and then 19 (the normal pathway for vision). From area 19 the signal is passed to the superior colliculi and on to the Edinger-Westphal nucleus where it goes out on CN III to cause the accommodation. It is not quite bilateral but it’s not quite unilateral either.
Pupilary constriction is common to both accommodation and the pupilary light reflex. If the pupil is unresponsive to light but is responsive to accommodation the legion must be in the pretectum as it’s the only component of the pupilary light reflex not common with the accommodation pathway. This is called an Argile Robertson Pupil and is associated with tertiary syphilis (neuro-syphilis).
The eye has two functional lenses, the cornea and the lens. We pay attention to the lens as the machinery that does the actual focusing because the cornea’s function is static while that of the lens is dynamic. As we look further away the lens becomes more flattened and as we look closer it becomes more rounded. The lens has no blood supply so it’s center is the first part to go bad. It causes presbyopia which is when the near-point gradually migrates further away. As we know this is a natural part of aging.
Myopia (near sightedness) is caused by the lens focusing the light falls short of the retina. It can be corrected by diverging lenses. Hyperopia (far sightedness) is caused by light being focused behind the retina and can be corrected with converging lenses.
When light hits the photoreceptor is triggers a response that is passed on to the ganglion cells. In the part of the retina with the rods one photon is all it takes to activate a rod and many rods project onto a single ganglion cell. This shows that vision with rods is more sensitive in low light conditions but we have less visual acuity. It is the opposite with bright vision. Cones require many photons to activate them but only a few photons project onto any one ganglion cell.
Scotopic vision is dark vision associated with the rod function and photopic vision is bright vision mediated by the cones. It’s important to know that scotopic vision involves less visual acuity and photopic more.
The macula is a region of the retina that contains little dimples called a fovea. Foveas are dimples in the retina that allow you to pack in more cones for better acuity. One difference is that in the fovea all of the other structures (bipolar and ganglion cells) that the light normally has to pass through to get to the photoreceptor are pushed aside in this situation.
The macula is a region that is located near the optic disk so cones (and hence photopic vision) are located in the center of the retina. The rods (and scotopic) vision are located near the periphery.
There us only one light dependent chemical reaction that takes place in the eye which involves rotating the 11’ bond of the photopigment from cis to trans. The all trans retinol stimulated phosphodiesterase to break down cGMP. cGMP is required to keep Na+ channels open so this closure causes hyperpolarization. Under normal circumstances photoreceptors fire action potentials while at rest and when stimulated they slow down.
In the retina there are two types of transmission of information:
1) Vertical transmission (online) transmits info directly down the line from a photoreceptor to a bipolar cell to the ganglion cell.
2) Horizontal transmission (offline) transmits info laterally from a photoreceptor to a horizontal cell. The horizontal cell then transmits the information laterally to another bipolar cell which passes it on to a ganglion cell not in the original line.
A retinol ganglion cell has two receptor areas, a center and a surround. Under dark conditions a ganglion cell will fire APs as some set rate. There are two types of ganglion cells. The first is a “center on” ganglion cell where a spot of light that falls in the center area will cause the firing rate to increase. If it falls in the surround it will cause the firing rate to decrease. If it completely fills the center without touching the surround it will cause it to fire at the maximum rate and vise versa. The second type is a “center off” ganglion cell which is the exact opposite of the “center on.” If you have equal areas of the center and surround being illuminated then there will be no change and it will appear as if you are in the dark. This is why in dim hazy light it can be very had to see. The center is associated with vertical transmission and the surround with horizontal.
Since the effect of light on the photoreceptors is hyperpolarization cones and rods release more NT in the dark then in the light. There are two types of bipolar cells, those which have EPSPs as a result of receiving neurotransmission and those that have IPSPs. In the EPSP ones light causes hyperpolarization and in the IPSP ones light causes depolarization. Regardless of the type both of them are excitatory on the ganglion cells but if you want to know more you will just have to wait for tomorrow’s lecture.
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