Neuroscience - Problem Drill 13: The Eye and Visual Processing

Transcription

1 Neuroscience - Problem Drill 13: The Eye and Visual Processing Question No. 1 of 10 needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed. 1. Which of the following statements is TRUE regarding light? Question #01 (A) Radiation emitted at a high frequency has short wavelengths and the lowest energy content. (B) Radio waves are good examples of low frequency radiation. (C) Most of the electromagnetic spectrum is detectable by our visual system. (D) Reflection is the bending of light rays that occur as they travel from one transparent medium to another. (E) Something that appears blue has absorbed blue wavelengths. Radiation emitted at a high frequency has short wavelengths and has the highest energy content. B. Correct! Radiation emitted at lower frequencies (long wavelengths; greater than 1 mm) has less energy such as radar and radio waves. Only a very small part of the electromagnetic spectrum is detectable by our visual system (400 to 700 nm range). Refraction is the bending of light rays that occur as they travel from one transparent medium to another. Some compounds absorb energy only in a limited range of wavelengths. For example, something that appears blue has absorbed all wavelengths except for those in the blue wavelength range. The energy content of electromagnetic radiation is proportional to its frequency. Radiation emitted at a high frequency (short wavelengths; less than 1 nm) has the highest energy content as shown by gamma and x-rays. Conversely, radiation emitted at lower frequencies (long wavelengths; greater than 1 mm) has less energy such as radar and radio waves. Only a very small part of the electromagnetic spectrum is detectable by our visual system (400 to 700 nm range). Each visible colour has its own characteristic wavelength and it is the mix of these visible wavelengths together that appears to our eyes as white light. Reflection is the bouncing of light rays off a surface. The angle the light ray hits the surface will equal the angle of its reflection. This is seen by mirror-like surfaces such as bodies of water. Most of what we see is light that has been reflected off objects in our environment. Refraction is the bending of light rays that occur as they travel from one transparent medium to another. This can be seen in the stem of a flower placed in a glass of water. The stem appears to shift to one side when it is in the water compared to its position in the air. This occurs because the speed of light differs in the two media; light passes through air more rapidly than through water. The greater the difference between the speed of light in the two media, the greater the angle of refraction. The transparent media in the eye bend light rays to form images on the retina. Absorption is the transfer of light energy to a particle or surface. The light from the sun will feel hot on our skin. Surfaces that appear black absorb all visible wavelengths. Some compounds absorb energy only in a limited range of wavelengths. For example, something that appears blue has absorbed all wavelengths except for those in the blue wavelength range. These wavelengths would then be reflected from the surface and we would only see these non-absorbed wavelengths. Light-sensitive photoreceptor cells in the retina contain pigments and use the energy absorbed from light to generate changes in membrane potential. The correct answer is (B).

2 Question No. 2 of 10 Instructions: (1) Read the problem and answer choices carefully, (2) Work the problems on paper as 2. When considering the retina, the yellow spot consists of what two structures? Question #02 (A) The blood vessels and Fovea. (B) The blood vessels and Macula. (C) The blood vessels and Optic Nerve fibers. (D) The Optic Nerve fibers and the Fovea. (E) The Macula and Fovea. While the Fovea is part of the yellow spot in the retina, the blood vessels are not. While the Macula is part of the yellow spot in the retina, the blood vessels are not. The blood vessels are found throughout the retina and the Optic Nerve fibers exit the retina through the Optic Disk. While the Fovea is part of the yellow spot in the retina, the Optic Nerve fibers exit the retina through the Optic Disk. E. Correct! The Macula and the Fovea are structures found in the yellow spot of the retina. Another view of the eye is obtained using an ophthalmoscope. This is a device which allows us to peer into the eye through the pupil to the retina. The most obvious feature of the retina is the blood vessels on its surface. These retinal vessels originate from a pale circular region called the optic disk which is also where the optic nerve fibers exit the retina. There are no photoreceptors located in the optic disk and the large blood vessels cast shadows on this spot so there is no sensation of light in this location. Therefore the optic disk is also known as the blind spot. However, our perception of the visual world appears seamless and we are not aware of any holes in our field of vision. This is because our brain fills in our perception of this area. In the middle of the each retina is a darker-colored region with a yellowish hue. This yellow spot consists of the macula and the fovea. It is the macula that is the part of the retina for central (as apposed to peripheral) vision. Besides its color, the macula also is distinguished by its lack of large blood vessels. The blood vessels tend to arc from the optic disk to the macula. The fovea is found close by the macula. The fovea is seen as a dark spot about 2mm in diameter and is a great anatomical reference point. The fovea is the actual center of the retina and is the location where the retina is at its thinnest. The correct answer is (E).

3 Question No. 3 of 10 Instructions: (1) Read the problem and answer choices carefully, (2) Work the problems on paper as 3. Which of the following statements is FALSE regarding the Pupillary Light Reflex? Question #03 (A) It involves connections between the retina and neurons in the brainstem. (B) It is a consensual reflex. (C) Shining a light in one eye affects the pupil of only that eye. (D) Allows for the continuous adjusting for different ambient light levels. (E) Changing the size of the pupil results in an increased depth of focus. It is true that this reflex involves connections between the retina and neurons in the brainstem. It is true that this reflex is a consensual reflex. C. Correct! Shining a light in one eye will affect the pupil in both eyes equally. It is true that this reflex allows for the continuous adjusting for different ambient light levels. It is true that changing the size of the pupil results in an increased depth of focus. In addition to the cornea and the lens, the pupil also contributes to the optical functioning of the eye by continuously adjusting for different ambient light levels. This is referred to as the Pupillary Light Reflex. This is a reflex action that involves connections between the retina and neurons in the brain stem that control the muscles that constrict the pupils. What is interesting about this reflex is that it is consensual; meaning that shining the light in only one eye makes both pupils constrict. Changing the size of the pupil results in an increased depth of focus. The correct answer is (C).

4 Question No. 4 of Which of the following statements is TRUE regarding the basic system for the retinal information processing? Question #04 (A) The basic system of retinal information processing involves only a direct pathway. (B) All the cell types within the retina are light-sensitive. (C) Horizontal cells receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells, and other horizontal cells. (D) The direct pathway for visual information to exit the eye is from the photoreceptors to bipolar cells to ganglion cells. (E) The ganglion cells and amacrine cells are the only sources of output from the retina. The basic system of retinal information processing involves a direct pathway plus two additional cell types that influence the retinal processing. Of all the cell types in the retina, the photoreceptor is the only cell type that is light-sensitive. It is the Amacrine cells that receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells, and other amacrine cells. D. Correct! The direct pathway for visual information to exit the eye is from the photoreceptors to bipolar cells to ganglion cells. The ganglion cells are the only source of output from the retina. No other retinal cell type projects an axon through the optic nerve. The basic system of retinal information processing involves a direct pathway plus two additional cell types that influence the retinal processing. The direct pathway for visual information to exit the eye is from the photoreceptors to bipolar cells to ganglion cells. The ganglion cells fire action potentials in response to light and these signals travel down the optic nerve to the rest of the brain. However, along this pathway are two cell types that influence this processing. Horizontal cells receive input from the photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors. Amacrine cells receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells, and other amacrine cells. There are two main points to remember: First, of all these cell types mentioned in the retina, the photoreceptor is the only cell type that is light-sensitive. All other cell types are influenced by light only through direct or indirect synaptic interactions with the photoreceptors. Second, the ganglion cells are the only source of output from the retina. No other retinal cell type projects an axon through the optic nerve. The correct answer is (D).

5 Question No. 5 of Which of the following statements is TRUE regarding the laminar organization of the retina? Question #05 (A) The ganglion cell layer is the layer closest to the front of the eye and contains the cell bodies of the ganglion cells. (B) The inner nuclear layer contains the cell bodies of the photoreceptor cells. (C) Between the ganglion cell layer and the inner nuclear layer is the outer plexiform layer. (D) Between the outer and inner nuclear layers is the inner plexiform layer, where the photoreceptors make synaptic contact with the bipolar and horizontal cells. (E) The outer plexiform layer contains the synaptic contacts between bipolar cells, amacrine cells, and ganglion cells. A. Correct! It is true that the ganglion cell layer is the layer closest to the front of the eye and contains the cell bodies of the ganglion cells. It is the outer nuclear layer that contains the cell bodies of the photoreceptor cells. Between the ganglion cell layer and the inner nuclear layer is the inner plexiform layer. Between the outer and inner nuclear layers is the outer plexiform layer, where the photoreceptors make synaptic contact with the bipolar and horizontal cells. It is the inner plexiform layer which contains the synaptic contacts between bipolar cells, amacrine cells, and ganglion cells. The term laminar organization refers to the fact that the cells in the retina are arranged in layers. The first striking characteristic is this arrangement appears to be inside out with the cells that are light-sensitive, the photoreceptors, being located at the back of the retina with all the other cells in front of it. The light must travel from the vitreous humor through the ganglion cells and bipolar cells before it reaches the photoreceptors. However, the cells in front of the photoreceptors are relatively transparent and therefore image distortion is minimal as light passes through them. Note that pigmented epithelium lies below the photoreceptors. These cells play a critical role in the maintenance of the photoreceptors and photo-pigments. They also absorb any light that passes entirely through the retina thus minimizing the reflection of light within the eye that might blur the image. The ganglion cell layer is the layer closest to the front of the eye and contains the cell bodies of the ganglion cells. The inner nuclear layer which contains the cell bodies of the bipolar cells, the horizontal cells and the amacrine cells. The outer nuclear layer contains the cell bodies of the photoreceptor cells. The layer of photoreceptor outer segments contains the light-sensitive elements of the retina. The outer segments are embedded in the pigmented epithelium. Between the ganglion cell layer and the inner nuclear layer is the inner plexiform layer which contains the synaptic contacts between bipolar cells, amacrine cells, and ganglion cells. Between the outer and inner nuclear layers is the outer plexiform layer, where the photoreceptors make synaptic contact with the bipolar and horizontal cells. The correct answer is (A).

6 Question No. 6 of Which of the following is TRUE regarding phototransduction? Question #06 (A) In complete darkness, there is no influx of sodium in photoreceptors. (B) Phototransduction in rods uses G-protein coupled receptors. (C) In the photoreceptor, light stimulation of the photopigment which causes a membrane ion channel to open. (D) The movement of positive charge efflux, which occurs in the dark, is referred to as the dark current. (E) Photoreceptors depolarize in response to light. In complete darkness, photoreceptors experience a continual influx of sodium ions through special channels in the outer segment membrane. B. Correct! Phototransduction in rods uses a similar mechanism as used in the transduction of chemical signals into electrical signal that occurs during synaptic transmission; the use of G-protein coupled receptors. In the photoreceptor, light stimulation of the photopigment which causes a membrane ion channel to close. The movement of positive charge influx, which occurs in the dark, is referred to as the dark current. Photoreceptors hyperpolarize in response to light. Phototransduction in rods uses a similar mechanism as used in the transduction of chemical signals into electrical signal that occurs during synaptic transmission; the use of G-protein coupled receptors. The mechanism is similar in the photoreceptor, but with an opposite effect. In the photoreceptor, light stimulation of the photopigment activates G-proteins, which in turn activate an effector enzyme that changes the cytoplasmic concentration of a second messenger molecule. This change causes a membrane ion channel to close, and the membrane potential is thereby altered. In complete darkness, photoreceptors experience a continual influx of sodium ions through special channels in the outer segment membrane, therefore the membrane potential of the rod outer segment is about -30mV. This movement of positive charge influx, which occurs in the dark, is referred to as the dark current. These sodium ions are stimulated to open by a second messenger, cyclic GMP which is continually produced in the photoreceptors. When light enters the photoreceptor, the production of cyclic GMP is decreased and the sodium ions close. With the sodium ions closed, the membrane becomes more negative. Thus, photoreceptors hyperpolarize in response to light. The hyperpolarizing response of light is initiated by the absorption of electromagnetic radiation by the photopigment in the membrane of the stacked disks in the rod outer segments. In rods, this pigment is called rhodosin. The correct answer is (B).

7 Question No. 7 of Which of the following statements is TRUE regarding bipolar cells? Question #07 (A) In the OFF bipolar cells, glutamate-gated cation channels mediate a classical hyperpolarization from the influx of sodium. (B) When the photoreceptor is in the dark and thus is releasing a lot of glutamate, it is the ON bipolar cells that respond by depolarizing. (C) The receptive field center is due to the bipolar cells receiving indirect synaptic input from surrounding photoreceptors via horizontal cells. (D) The response of a bipolar cell s membrane potential to light in the receptive field center is opposite to that of light in the surround. (E) The receptive field of bipolar cells is defined as the area of the retina that when stimulated by darkness, changes the cell s membrane potential. In the OFF bipolar cells, glutamate-gated cation channels mediate a classical depolarization EPSP from the influx of sodium. When the photoreceptor is in the dark and thus is releasing a lot of glutamate, it is the OFF bipolar cells that respond by depolarizing. The receptive field center is due to the bipolar cells receive direct synaptic input from a cluster of photoreceptors. D. Correct! It is true that the response of a bipolar cell s membrane potential to light in the receptive field center is opposite to that of light in the surround. The receptive field of any cell in the visual system, including bipolar cells, is defined as the area of the retina that when stimulated by light, changes the cell s membrane potential. Bipolar cells can be categorized into two classes, based on their responses to the glutamate released by the photoreceptors. These two categories are OFF bipolar cells and ON bipolar cells. In the OFF bipolar cells, glutamate-gated cation channels mediate a classical depolarization EPSP from the influx of sodium. When the photoreceptor is in the dark and thus is releasing a lot of glutamate, it is the OFF bipolar cells that respond by depolarizing. The ON bipolar cells have G-protein coupled receptors and respond to glutamate by hyperpolarizing. Thus, under dark conditions, the ON bipolar cells are hyperpolarized, but in light conditions, they do not. Notice that the names OFF and ON refer to whether these cells depolarize in response to light off (more glutamate) or to light on (less glutamate). Bipolar cells receive direct synaptic input from a cluster of photoreceptors. This represents the receptive field center. They also receive indirect input from surrounding photoreceptors via horizontal cells. The receptive field of any cell in the visual system, including bipolar cells, is defined as the area of the retina that when stimulated by light, changes the cell s membrane potential. The receptive field of a bipolar cell has two parts: a circular area of retina providing direct photoreceptor input, called the receptive field center; and a surrounding area of retina providing input via horizontal cells, called the receptive field surround. The response of a bipolar cell s membrane potential to light in the receptive field center is opposite to that of light in the surround. Thus, these cells are said to have a center-surround receptive field and this organization is passed on from the bipolar cells to the ganglion cells via synapses in the inner plexiform layer. The amacrine cells in the inner plexiform layer also add to this complexity. The correct answer is (D).

8 Question No. 8 of Which of the following statements are true regarding M-type ganglion cells? Question #08 (A) Compared to the P-type cells, the M-type ganglion cells have smaller receptive fields. (B) Compared to the P-type cells, the M-type ganglion cells conduct action potentials more slowly in the optic nerve. (C) Compared to the P-type cells, the M-type ganglion cells are less sensitive to low-contrast Stimuli. (D) M-type cells respond to stimulation of their receptive field centers with a sustained discharge as long as the stimulus is on. (E) M-type ganglion cells constitute abut 5% of the ganglion cell population. Compared to the P-type cells, the M-type ganglion cells have larger receptive fields. Compared to the P-type cells, the M-type ganglion cells conduct action potentials more rapidly in the optic nerve. Compared to the P-type cells, the M-type ganglion cells are more sensitive to lowcontrast Stimuli. M-type cells respond to stimulation of their receptive field centers with a transient burst of action potentials, whereas the P-type cells respond with a sustained discharge as long as the stimulus is on. E. Correct! It is true that M-type ganglion cells constitute abut 5% of the ganglion cell population. Most ganglion cells in the mammalian retina have a center-surround receptive field with either an ON or an OFF center. They can be further categorized based on their appearance, connectivity, and electrophysiological properties. Two major types of ganglion cells are distinguished using these criteria: large M-type ganglion cells and smaller P-type ganglion cells. P-type ganglion cells constitute abut 90% of the ganglion cell population. M-type cells constitute about 5% and the remaining 5% is made up of a variety of nonm-nonp ganglion cell types that are less well characterized. Compared to the P-type cells, the M-type ganglion cells have larger receptive fields, they conduct action potentials more rapidly in the optic nerve, and they are more sensitive to low-contrast stimuli. Further to this, M-type cells respond to stimulation of their receptive field centers with a transient burst of action potentials, whereas the P-type cells respond with a sustained discharge as long as the stimulus is on. A further distinction between ganglion cells is that some P cells and some nonmnonp cells are sensitive to differences in the wavelength of light. The majority of these color-sensitive neurons are called color-opponent cells, reflecting the fact that the response to one wavelength in the receptive field center is canceled by showing another wavelength in the receptive field surround. There are two types of opponency: red verses green and blue verses yellow. The correct answer is (E).

9 Question No. 9 of Which of the following statements is TRUE regarding the Retinofugal projection? Question #09 (A) The optic nerves exit the left and right eyes at the optic chiasm. (B) The retinofugal projection includes the optic nerve, the optic chiasm, and the optic tract. (C) A total decussation of the retinofugal projection occurs at the optic chiasm. (D) The optic chiasm lies at the back of the brain. (E) The axons of the retinougal projections form the optic tracts, which run just under the pia along the medial surfaces of the diencephalon. The optic nerves exit the left and right eyes at the optic disks. B. Correct! The retinofugal projection includes the optic nerve, the optic chiasm, and the optic tract. Because only the axons originating in the nasal retinas cross, we say that a partial decussation of the retinofugal projection occurs at the optic chiasm. The optic chiasm lies at the base of the brain, just anterior to where the pituitary gland dangles down. The axons of the retinougal projections form the optic tracts, which run just under the pia along the lateral surfaces of the diencephalon. The neural pathway that leaves the eye and travels to the brain stem is often referred to as the retinofugal projection and includes the optic nerve, the optic chiasm, and the optic tract. The optic nerves exit the left and right eyes at the optic disks, travel through the fatty tissue behind the eyes in their bony orbits, then pass through holes in the floor of the skull. The optic nerves from both eyes combine to form the optic chiasm which lies at the base of the brain, just anterior to where the pituitary gland dangles down. At the optic chiasm, the axons originating in the nasal retinas cross from one side to the other; in other words, they decussate. Because only the axons originating in the nasal retinas cross, we say that a partial decussation of the retinofugal projection occurs at the optic chiasm. This partial decussation allows information from the left visual field to travel through the right pathway. Following the partial decussation at the optic chiasm, the axons of the retinougal projections form the optic tracts, which run just under the pia along the lateral surfaces of the diencephalon. The correct answer is (B).

10 Question No. 10 of All of the following statements is TRUE regarding Blob receptive fields? Question #10 (A) Most blob cells are wavelength sensitive. (B) Blob receptive fields do not contain the great majority of color-sensitive neurons. (C) Blob receptive fields display binocularity. (D) Blob receptive fields display orientation selectivity. (E) Blob receptive fields display direction selectivity. A. Correct! It is true that most blob cells are wavelength sensitive. The most important feature to remember about these cells is that they contain the great majority of color-sensitive neurons outside layer IVc. Most blob cells are wavelength sensitive and are monocular. Most blob cells lack orientation sensitivity. Most blob cells lack direction sensitivity. Blob Receptive Fields: Neurons in the interblob regions have some, if not all of the properties we have been discussing, such as binocularity, orientation selectivity, and direction selectivity. They are both simple and complex cells and generally are not wavelength sensitive. However, most blob cells are wavelength sensitive and are monocular and they lack orientation or direction sensitivity. These blob cells receive input directly from the koniocellular layers of the LGN and magnocelular and parvocellular input from layer IVc. The receptive fields of most blob cells are circular. The most important feature to remember about these cells is that they contain the great majority of color-sensitive neurons outside layer IVc and therefore appear to be specialized for the analysis of object color. The correct answer is (A).