Lecture 11 Notes: The Retina and Receptive Fields PDF

The Retina (continued) and Receptive Fields I. Rods versus Cones A. Rods contain rhodopsin and are more light sensitive than cones (a single photon of light may be detectable by rods but not by cones). 1. One reason for this is that the process of signal amplification is greater in rods. 2. Cones ha...

The Retina (continued) and Receptive Fields I. Rods versus Cones A. Rods contain rhodopsin and are more light sensitive than cones (a single photon of light may be detectable by rods but not by cones). 1. One reason for this is that the process of signal amplification is greater in rods. 2. Cones have coneopsin instead of rhodopsin. a. There are three different types of coneopsin: red, green, and blue. Meaning that they are most sensitive to light in the red, or green or blue wavelengths. b. The difference between the three forms of coneopsin is small. The difference is a small change in the amino acid sequence allowing for maximal sensitivity to different wavelengths of light. As an interesting tidbit, there are 2 versions of the red coneopsin gene in men and these produce red opsins that vary by a single amino acid. This difference results in the two red coneopsins being maximally sensitive to slightly different wavelengths of red light. Thus, men with different versions of this opsin do not perceive the same thing when they see a red object. This answers the philosophical question about whether different people see objects, colors etc. in the same way. They don't. 3. Another reason that rods are more sensitive to light is that the outer segment of rods is larger; therefore they have a larger surface area to absorb light. 4. Rods also have more photopigment densely packed into the membrane of the optic disks, so they absorb more light. B. In bright light the photopigment in rods (but not cones) is saturated (bleached). Therefore rods aren't functional in bright light, while cones are. 1. This means that we have two parallel visual systems, one for bright light and one for very dim light. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 1 2. At night colors appear to be muted. But the same spectral frequencies exist in bright and dim light. The reason we don’t perceive colors in dim light is that cones don't work in dim light, so we don’t perceive color because we’re using a part of the visual system that is color blind. II. Receptive fields of cells A. How do RGCs code information that results in perception of the visual stimulus? B. Below is a diagram exemplifying receptive fields of RGCs: Photoreceptors RGC Receptive Fields of RGC in different locations 1. A neuron’s receptive field is the location in the environment (or the surface of the body) from which an appropriate stimulus will change that cell's activity. The term Receptive Field applies to cells in the visual system other that RGCs and also applies to neurons in other sensory modalities (e.g., touch, auditory). 2. Different RGCs have receptive fields in different locations in the visual field/retina. 3. RGCs in the fovea have exquisitely tiny receptive fields and the size of the receptive fields increases in RGCs with distance from the fovea. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 2 4. Light anywhere within the cell’s receptive field will change the cell's activity. C. Some features of receptive fields of RGCs: 1. The receptive fields of RGCs are circular. 2. The receptive fields of RGCs vary in size. 3. Receptive fields of adjoining RGCs may overlap. D. What are the mechanisms that account for the differences in size of receptive fields? 1. RGCs on the edges of the retina (so their receptive fields are in the periphery of the visual field) collect information from a greater number of photoreceptors than do RGCs closer to the fovea. Therefore, their receptive fields are larger. 2. Convergence is a situation where many neurons converge onto a few neurons. (There are about 120-130 million photoreceptors and only 1 million RGCs; therefore there is a lot of convergence of photoreceptors onto RGCs). This is shown in the diagram below: Photoreceptors RGC Convergence of synaptic input to RGC = large receptive field. Mostly in periphery of retina. No convergence = small receptive field Mostly in fovea. 3. Divergence is few (or one unit) projecting onto many units. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 3 Divergence: One or a few units projecting to many units. 4. In the fovea there is less convergence of photoreceptors onto RGC (via bipolar cells) than in the periphery of the retina and this is a mechanism that explains the differences in size of the receptive fields of RGCs. E. Convergence also helps to explain light sensitivity. There is less convergence in the cone system, therefore RGCs receiving input from cones are not as sensitive to light. F. Receptive fields overlap and the consequence of this is that light from one point in the visual field will affect many different RGCs. This is depicted in the diagram below: © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 4 These photoreceptors project to both RGCs F. Receptive fields of retinal cells are modeled as concentric circles. 1. There are two types of concentric fields, on center and off center (for both BPs and RGCs). 2. These two RGCs are defined by their response to light in the center of their receptive fields. On center RGCs are turned on (generate AP) by light and Off Center RGCs are turned off by light in their receptive field centers. 3. Off center RGCs generate more action potential generation in the dark than do on center RGCs. 4. These concentric fields have antagonistic centers and surrounds because light falling in the center of their receptive fields has the opposite effect of light in their receptive field surrounds. 5. Below is a diagram showing an on and off center RGC: © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 5 ON Center RGC because it generates AP with light onto receptive field center. OFF Center RGC because it stops generating AP when light is shined onto its receptive field center. G. Mechanisms for on center versus off center receptive field properties. 1. It is because of a difference in receptor type expressed by BP cells. The receptor expressed by the On Center BP cell produces an IPSP in response to glutamate and the Off Center BP cell produces an EPSP in response to glutamate. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 6 Right Side: Responses of Off Center Bipolar and RGC to light shining onto receptive field center. Left Side: Responses of On Center Bipolar and RGC to light shining onto receptive field center. Glutamate is excitatory here: In light, < glutamate from PR cell produces hyperpolarization in BP cell. Glutamate inhibitory here: In light, < glutamate from PR cell produces depolarization in BP cell. Glutamate excitatory here: In light, > glutamate from BP cell depolarizes RGC. #3 #1 #2 Glutamate is excitatory here: In light, < glutamate from BP cell hyperpolarizes RGC. 2. When is a neurotransmitter excitatory? Answer: When its presence produces depolarization. Refer to numbers in diagram above: #1: Glutamate, which is a transmitter released by the bipolar cells, is excitatory here because an increase in neurotransmitter release results in a depolarization. #2: Glutamate is excitatory here because a decrease in neurotransmitter release results in a hyperpolarization. #3: Glutamate in inhibitory here because a decrease in neurotransmitter release results in a depolarization. (A useful rule of thumb: If the direction of polarization (“synaptic sign”) is the same in two neurons where one is being driven by the other, then the neurotransmitter © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 7 between them is excitatory. In contrast, if the direction of polarization is opposite, the neurotransmitter is inhibitory. Also, remember the general rule that neurotransmitter release by a presynaptic terminal is increased by depolarization and decreased by hyperpolarization). 3. So, the same neurotransmitter (glutamate) is released onto both ON and OFF center BP cells but there are different receptors for that neurotransmitter the BP cells (glutamate inhibits the receptor expressed by the ON center BP cell and glutamate excites the receptor on the OFF center BP cell) and this accounts for the difference between on and off center receptive fields. 4. Why do we have both ON and OFF center systems? The on center system is most sensitive to increases in illumination and off center system is most sensitive to decreases in illumination. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 8