* Become aware of the main proteins involved in the signal transduction pathway that lead to changes in membrane potential, calcium flux, and ultimately changes in amount of neurotransmitter released by photoreceptive cells in response to light
* **Iris**– colored portion of the eye, contains muscles that adjust the pupil size under neural control. Open during dim light, closed during bright light
* **Ciliary body**– ring of tissue that encircles the lens and includes both a muscle component and a vascular component
* **Choroid**– composed of a rich capillary bed that serves as the main blood supply for the photoreceptors and contains melanin containing cells
Accommodation to focusing on near objects involves the contraction of the ciliary muscle, which reduces tension of the Zonule fibers and the lens is allowed to increase its curvature
Lens held in place by zonule fibers (connective tissue bands). Two opposing forces-- tension of lens tends to keep it rounded up (into a sphere if removed) and tension of zonule fibers which tend to flatten it.
* Efferent pathway controlling the iris and ciliary muscle are via the Edinger-Westphal nucleus --> ciliary ganglion (parasympathetic, cranial nerve III, oculomotor nerve) --> ciliary muscle
<figure><figcaption class="big">Reduced accommodation with age</figcaption><img src="figs/Neuroscience5e-Box-11A-0_125b2a4.jpg" height="300px"><figcaption>Neuroscience 5e Box 11A</figcaption></figure>
diopter is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (that is, 1/metres)
>Crystallins are water-soluble proteins that compose over 90% of the protein within the lens
>The three main crystallin types found in the human eye are α-, β-, and γ-crystallins.
>The refractive index of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens.[10] This index gradient enhances the optical power of the lens
>Crystallins tend to form soluble, high-molecular weight aggregates that pack tightly in lens fibers
>lens capsule is a smooth, transparent basement membrane that completely surrounds the lens. The capsule is elastic and is composed of collagen. It is synthesized by the lens epithelium and its main components are Type IV collagen and sulfated glycosaminoglycans (GAGs)
>cells of the lens epithelium also serve as the progenitors for new lens fibers. It constantly lays down fibers in the embryo, fetus, infant, and adult, and continues to lay down fibers for lifelong growth
>lens fibers form the bulk of the lens. They are long, thin, transparent cells, firmly packed, with diameters typically 4–7 micrometres and lengths of up to 12 mm long
>In many aquatic vertebrates, the lens is considerably thicker, almost spherical, to increase the refraction
>among terrestrial animals, however, the lens of primates such as humans is unusually flat
* Surrounded on one side by pigmented epithelium which contains melanin that helps reduce backscattering of light. Also plays a key role in maintenance of photoreceptors
>Because the rods and cones are at the back of the retina, the incoming light has to go through the other two layers in order to stimulate them. We do not fully understand why the retina develops in this curious backward fashion.
>One possible reason is the location behind the receptors of a row of cells containing a black pigment, melanin (also found in skin)
number of rods and cones vary across the retina. In the center where vision is best (fovea) there are only cones. This area is about 0.5mm in diameter.
125 million rods and cones in each eye. But only 1 million ganglion cells. How is visual information then preserved. Think of two paths: the direct path and an indirect path involving lateral interactions mediated by horizontal cells between receptors and bipolars and amacrine cells between bipolars and ganglion cells.
>The total area occupied by the receptors in the back layer that feed one ganglion cell in the front layer, directly and indirectly, is only about one millimeter
high degree of convergence, together with more direct path in and near fovea (one cone—>one bipolar—>one ganglion cell) can explain the 125:1 ratio of receptors to optic nerve fibers without having really bad vision.
>researchers at Technion–Israel Institute of Technology in Haifa built a computer model of a human retina and then compared how light behaves in the model with the way it behaves in the retinas of guinea pigs.
>The comparison showed that when light travels through cell layers before reaching the rods and cones (photoreceptors), it's actually being sorted into red, green, and blue light
>What's doing the sorting? Tiny structures known as Muller glia cells, according to the researchers.
>"We should also remember that several animal classes do not have a 'backward-pointing' eye, and also have Muller cells,"
>study was presented at a meeting of the American Physical Society on March 5, 2015 in San Antonio, Texas.
* Photoreceptors do not exhibit action potentials– light causes a graded change in membrane potential that changes the rate at which neurotransmitter is released
* In the dark the number of voltage-gated Ca²⁺ channels open at the synaptic terminal is relatively high, and therefore the rate of neurotransmitter release is high. In the light the number of open voltage-gated Ca²⁺ channels is reduced and rate of neurotransmitter release is reduced
In the dark channels open due to cGMP binding. Na⁺ and Ca²⁺ rushes in and cell is depolarized
<figure><img src="figs/Neuroscience5e-Fig-11.08-0_5a9e700.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 11.8</figcaption></figure>
Note:
cyclic nucleotide gated cation channels in the outer membrane segment of the photoreceptors
the nucleotide cyclic guanosine monophosphate
*these cGMP gated channels are permeable to both Na+ and Ca2+ actually*
Balanced by K+ selective channels in the inner segment of the photoreceptor cell
Light transduction results in a **decrease in cGMP levels** thus closing the cGMP cation channels. K+ efflux becomes dominant and hyperpolarization ensues. Then less Ca2+ dependent transmitter release at synapse with bipolar cells
* A photon of light is absorbed by photopigment (retinal or retinaldehyde, an aldehyde of Vitamin A) that is coupled to a protein in the outer segment called opsin. Absorption causes a change in conformation of retinal (photon absorbtion breaks a carbon double bond and switching from cis to trans configuration) that in turn changes the conformation of opsin
* The opsin then can activate the trimeric G-protein **transducin**
* Transducin in turn activates a cGMP phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP to GMP. Channel opening probability decreases, cell gets hyperpolarized
* One photon of light can activate 800 transducin molecules. This leads to about 800 phosphodiesterases activated. Each phosphodiesterase cleaves 300 or so cGMPs/second. This can result in the closing of about 200 ion channels (2% of total). 10<sup>6</sup> – 10<sup>7</sup> Na⁺ ions per second are prevented from entering the cell for a period of ~1 second
Tremendous amplification. Single photon hitting rhodopsin is estimated to activate 800 transducin molecules, about 8% of transducin molecules on disk surface. Each transducin molecule activates a single phosphodiesterase molecule and ea PDE can catalyze the breakdown of 6 cGMP molecules. Results in closure of 200 ion channels or ~2% of n channels in ea rod open in dark, resulting in net change in membrane potential of 1 mV.
* Rhodopsin kinase/arrestin– activated rhodopsin is phosphorylated by rhodopsin kinase, permitting the protein arrestin to bind to rhodopsin. **Prevents further activation of transducin**, thus ending the phototransduction cascade
Retinoid cycle one important part of light adapation (other being horizontal cell-photoreceptor interactions). Rate of retinal regeneration sufficient even under bright illumination.
## Light adaptation– or how do we adjust to different light intensities?
* There is a million times more photons in a bright sunny day than at starlight and yet we can detect difference in light intensity under both conditions.
* Because at low levels of light more channels close per photon than at higher levels of light. Therefore, as light levels increase it takes more photons to close the same number of channels.
* This is due to the changes in the intracellular Ca²⁺ levels. Ca²⁺ can come in through Na⁺ channels. When they close (in the light), Ca²⁺ levels decrease. This does a number of things to make it harder to close more channels with each new photon. 1. Ca²⁺ normally inhibits guanylyl cyclase, lower Ca²⁺ in light leads to more cGMP. Therefore more PDE activation is needed to reduce cGMP levels and close more channels. 2. Ca²⁺ also inhibits rhodopsin kinase. Lower Ca²⁺ levels activates more kinase. With more kinase the activated opsin becomes inactivated. Leads to less PDE activation per photon, less channels closed per photon.
* This prevents us from saturating our photoreceptors and thus allows us to see changes in illumination over a wider range of light intensities.
* Rods and cones– have an outer segment comprised of membranous disks that contain photopigment and an inner segment that contains the cell nucleus and synaptic terminals
* The absorption of light by photopigment in outer segment initiates a signal transduction cascade that changes the membrane potential of the cell, and therefore the amount of neurotransmitter released plus or minus light energy
* Photoreceptors synapse with bipolar cells and horizontal cells in the outer plexiform layer
Why the cone shape? Shape of cone preferentially accepts light directed straight into the eye through the pupil instead of off axis. Known as the Stiles–Crawford effect.
<div><figcaption class="big">EM section through a kangaroo rat rod cell</figcaption><img src="figs/image7_2fed646.png" height="100px"><figcaption></figcaption></div>
* Rods produce a reliable response to a single photon of light, it takes over a 100 photons to produce a comparable response in a cone
* Cones adapt better than do rods– about 200 ms for a cone, 800 ms for a rod
* Rods synapse onto specific bipolar cells (rod bipolars) that synapse onto amacrine cells which contact both cone bipolars and ganglion cells. Cones go bipolar to RGC directly
* Rods exhibit convergence– many rods synapse onto a single bipolar cell, many bipolars onto a single amacrine cell
<div><figcaption class="big">outward currents after light flashes</figcaption><img src="figs/Neuroscience5e-Fig-11.12-1R_9eda22a.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 11.12, Baylor J Physiol 1984, 1987</figcaption></div>
<div><figcaption class="big">convergence in rod pathway</figcaption><img src="figs/Neuroscience5e-Fig-11.12-2R_72996d5.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 11.12</figcaption></div>
Figure show electrical recordings (suction electrodes) of the current flowing across the photoreceptor membranes of primate (*Macaca fasciculari* / cynomolgus monkeys/crab-eating macaque) rods and cones for ligh flashes of successive higher intensity.
Cone response over in about 200 ms (with an overshoot of inward current), whereas the rod response can continue for more than 600 ms. Both rods and cones adapt to operate over a range of luminance values, but the adaptation mechanisms of cones are more effective.
[Ca2+] in outer segment plays key role in light adaptation (light induced modulation of photoreceptor sensitivity). More light leads to less [Ca2+] leading to more guanylate cyclase activity and more cGMP production and higher [cGMP]. Less [Ca2+] also leads to incr activity of rhodopsin kinase and more arrestin binding to rhodopsin so that rhodopsin is inactivated quicker. This is the basis of the enhanced cone light adaptation and the briefer cone response and the outward current overshoot compared with rods.
>The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the darkest light signal that the eye can sense are a factor of roughly 1,000,000,000 apart.
> in any given moment of time, the eye can only sense a contrast ratio of one thousand.
>the eye adapts its definition of what is black.
> takes approximately 20–30 minutes to fully adapt from bright sunlight to complete darkness and become ten thousand to one million times more sensitive than at full daylight
>takes approximately five minutes for the eye to adapt to bright sunlight from darkness
>Dark adaptation is far quicker and deeper in young people than the elderly
* Bipolar cells– cell bodies in the inner nuclear layer. Gets info from photoreceptors in outer plexiform layer and transmits it to ganglion cells and amacrine cells in inner plexiform layer. Rods and cones use specific types of bipolars
* Ganglion cells– cell bodies in ganglion cell layer. Output neurons of the retina. Receives info from bipolar and amacrine cells and sends it out through the optic nerve
* Horizontal cells– cell bodies in inner nuclear layer. Makes multiple contacts with photoreceptors and bipolar cells. Largely responsible for luminance contrast
* Amacrine cells– cell bodies in inner nuclear layer. Makes contact in the inner plexiform layer with bipolar cells and ganglion cells. Several distinct subclasses. Coordinate ganglion cell activity. e.g. motion
* RGCs are the cell that sends action potentials to the brain
* Much of the information in vision has to do with changes in light intensity. Example black and white movies
* In order to understand how the brain makes sense of the differences in light intensity that the eye sees, it is important to know what makes RGCs fire
Now because RGCs are the output neurons of the eye, there has been a long interest in understanding the response properties of these cells, i.e. what their receptive fields look like.
* Measured the action potentials from specific RGCs after shining light on the retina
* Determined that RGCs have receptive fields. Found that a receptive field can be divided into center and a surround
* Ganglion cells come in two types- ON-center/OFF surround and OFF-center/ON surround, in roughly equal proportions
* ON center RGCs fire more when light that hits the center is brighter than that of the surround and fire less when it is darker in the center than in the surround. OFF center fire less when it is brighter in center and more when it is darker in the center
* Acts like having separate luminance channels. **Changes in intensity** (increases or decreases), are conveyed by action potentials. **RGCs are not photodetectors but are detecting the contrast** between areas
<figure><img src="figs/Neuroscience5e-Fig-11.19-1R_3af4560.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 11.19, 6e Fig. 11.20</figcaption></figure>
* For an ON- center/OFF-surround RGC, a point of light that fills the entire center but not in the surround will give maximal stimulation (increased action potentials). i.e. brighter in center than in surround
* A point of light in surround but not in the center will hyperpolarize the RGC (reduce baseline spike rate)
* Light that crosses into both will be in the middle depending on the relative amounts
* Both center and surround illuminated is basically the same as being in the dark (background levels)
* RGCs fire depending on contrast, not by absolute light intensity
* There are two types of bipolar cells– ON center and OFF center. OFF center uses AMPA receptors (ionotropic) that cause the cell to depolarize in response to glutamate released by photoreceptors. ON center use metabotropic glutamate receptors that lead to the closing of Na⁺ channels and hyperpolarize the cell
* Light hits cone causes hyperpolarization of cone, leads to less release of glutamate
* Two bipolar cells synapse with cone, an on-center and off center bipolar cell
* **On center bipolars are normally inhibited by glutamate**, less glutamate, less inhibition, more release of neurotransmitter onto RGCs which increases of on-center RGC firing
* **Off center bipolars are normally activated by glutamate**, become hyperpolarized, decrease transmitter release, which leads to a decrease in firing rate of Off-center RGCs
* Light hitting surround cones hyperpolarizes causing less glutamate to be released onto horizontal cell dendrites
* Horizontal cells hyperpolarize because of less glutamate (have AMPA receptors) and decrease their rate of transmitter release (GABA) onto the synaptic terminals of the nearby photoreceptors
* Horizontal cells normally inhibit cones (use GABA), thus now cones are less inhibited (depolarized), and release more glutamate than without surround
* This leads to a depolarization of off-center RGCs, causing them to increase their firing rate
* And hyperpolarizes on-center RGCs, causing them to decrease their firing rate
* Light falls on photopigment, that is transformed to action potentials that ganglion cells convey to the brain
* Phototransduction occurs in rods and cones that have different properties that meet the conflicting demands of sensitivity and acuity
* RGCs have a center-surround arrangement of receptive fields that makes them good at contrast detection and relatively insensitive to background illumination
