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Chapter 2

chapter 2

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Matthias Niemeier

CHAPTER 2 THE FIRST STEPS IN VISION: FROM LIGHT TO NEURAL SIGNALS  Light  form of electromagnetic radiation (energy produced by vibrations of electrically charged material)  2 ways to conceptualize light: o Wave o Stream of photons, tiny particles that each consist of one quantum of energy.  Gamma rays  short wavelengths  Radio and television waves  long wavelengths  Visible light  400-700 nanometers  As light reaches the atmosphere from space, some of the starlight’s photons are absorbed, while somelight is scattered by different particles. Most photons make it through the atmosphere and hit the surface of an object.  If the ray of starlight strikes a light-colored surface, most light is reflected. Light hitting dark surfaces is absorbed.  Light that is neither reflected nor absorbed is transmitted through the surface. o Example, light travelling from air into a glass window, some light rays will be refracted as they are transmitted.  Refraction also occurs when light passes from air into water or into the eyeball. EYES THAT SEE LIGHT  Single celled organisms such as amoebas respond to light and change their direction of motion to avoid bright light when it’s detected.  An eye can form an image of the outside world; it can use light to recognize objects.  The first tissue that light encounters is the cornea. o Cornea provides a window to the world because it’s transparent. o It’s made of a highly ordered arrangement of fibers and it contains no blood vessels or blood, which absorb light  thus most light is transmitted and not absorbed or reflected o Has a rich supply of transparent sensory nerve endings, which force the eyes to close and produce tears if the cornea is scratched, preserving its transparency  Aqueous humor, a fluid derived from blood, fills the space immediately behind the cornea, supplying oxygen and nutrients to, and removing waste from, the cornea and the crystalline lens. o Crystalline lens enables the changing of focus  Like the cornea, the lens has no blood supply, so that it can be completely transparent. o The shape of the lens is controlled by the ciliary muscle.  To get to the lens, the light must pass through the pupil, which is simply a hole in a muscular structure called the iris. o The iris gives the eye its distinctive color. o The pupil controls the amount of light that reaches the retina, via the pupillary light reflex.  When level of light increases or decreases, the iris automatically expands and contracts to allow more or less light into the eye.  When you emerge from a dark room into bright light (e.g., coming out of a movie theater), not only does your pupil constrict, but there's a good chance that you will sneeze.  Sneezing in response to being exposed to a bright light: o the "photic sneeze reflex”  not yet understood  After passing through the lens, light enters the vitreous chamber (space between lens and th retina), where it’s refracted for the 4 and final time by the vitreous humor. o This chamber comprises 80% of the internal volume of the eye.  After passing through the vitreous chamber, the light is brought into focus at the retina o Retina contains rods and cones which receive an image from the lens and send it to the brain through the optic nerve o Only about half the light that arrives at the cornea reaches the retina. o Role of retina is to detect light and tell the brain about aspects of light that’re related to objects in the world (the retina is where seeing begins) FOCUSING LIGHT ONTO THE RETINA  Cornea, aqueous and vitreous humors help refract light, but the refractive power of these 3 structures is fixed so they can’t bring close objects into focus. This job is performed by the lens  The lens can alter the refractive power by changing its shape, a process called accommodation.  Accommodation (change in focus) is done through contraction of the ciliary muscle. The lens is attached to the ciliary muscle through tiny fibers (zonules of Zinn). o When ciliary muscle is relaxed  zonules stretch and lens is relatively flat  eye will focus on distant objects in this state o When ciliary muscle contracts  tension on zonules is reduced enables lens to bulge  eye will focus on close objects. o Fatter lens  more power  Ability to accommodate declines with age, starting from about 8 years old, and we lose about 1 diopter of accommodation every 6 years up to age 30 (and more after age 30).  Presbyopia: “old sight”. The loss of near vision because of insufficient accommodation. o why does this occur?  lens becomes sclerotic (harder) and the capsule encircling the lens (enables it to change shape) loses elasticity.  Lens is normally transparent because the crystallins (proteins that make up lens) are packed together and are regular. Anything interfering with regularity of crystallins will result in loss of transparency (areas that are opaque or “opacities”). o Opacities of the lens are called cataracts.  Congenital cataracts (present at birth) are rare, most are discovered after age 50.  Cataracts interfere with vision because they absorb and scatter more light than the normal lens does.  To focus a distant star on the retina, the refractive power of the 4 optical components of the eye must be perfectly matched to the length of the eyeball, known as emmetropia.  Refractive errors occur when the eyeball is too long or too short relative to the power of the optical components  If the eyeball is too long for the optics, the image of our star will be focused in front of the retina, and the star will thus be seen as a blur rather than a spot of light. This condition is called myopia (or "nearsightedness"). o Myopia can be corrected with negative (minus) lenses,which diverge the rays of starlight before they enter the eye  If the eyeball is too short for the optics, the image of our star will be focused behind the retina-a condition called hyperopia (or "farsightedness"). o Hyperopia can be corrected with positive (plus) lenses,which converge the rays of starlight before they enter the eye o Most newborns are hyperopic because the optical components of their eyes are relatively well developed at birth compared to the length of their eyeballs.  The most powerful refracting surface in the eye is the cornea, contributing 2/3of the eye’s focusing power.  When the cornea is not spherical, the result is astigmatism. A vertical line may be focused in front of the retina while horizontal lines may be focused at the back of it (or vice versa). One or more of the lines may appear to be out of focus, while other lines appear sharp. THE RETINA  Process of seeing begins at the retina, where light energy is transduced into neural energy that can be interpreted by the brain.  Eye doctors use an ophthalmoscope to look at the back surface of the patients’ eyes, which is called fundus.  The optic disc is where the arteries and veins that feed the retina enter the eye, and where the axons of ganglion cells leave the eye via the optic nerve  This portion of the retina contains no photoreceptors, and is blind (blind spot).  Fundus is the only place in the body where one can see the arteries and veins directly. They spread out across the retina but stop short of the fovea.  A photomicrograph reveals that the retina is a layered sheet of clear neurons, with another layer of darker cells (the pigment epithelium) lying behind the final layer.  Transduction of light energy into neural energy beginsin the backmost layer of the retina, made of cells called photoreceptors.  When photoreceptors sense light, they can stimulate neurons in intermediate layers (bipolar cells, horizontal cells, amacrine cells). These neurons connect with the frontmost layer of the retina made of ganglion cells whose axons pass through the optic nerve to the brain. RETINAL INFORMATION PROCESSING  Retina contains 5 major classes of neurons: o Photoreceptors, horizontal cells, bipolar cells, amacrine cells, ganglion cells. LIGHT TRANSDUCTION BY ROD AND CONE PHOTORECEPTORS  Retina contains about 100 million photoreceptors  to capture light, initiate act of seeing by producing chemical signals.  2 types of photoreceptors: rods, cones.  Human retinas have both rods and cones so they’re considered to be duplex retinas. o Rats and owls have mostly rod retinas, certain lizards have mostly cone retinas.  Both rods and cones contain an outer segment (adjacent to pigment epithelium), an inner segment, and a synaptic terminal.  Molecules called visual pigments are made in the inner segment (filled with mitochondria) and stored in the outer segment where they’re incorporated into the membrane.  Visual pigments consist of a protein (opsin) ,which determines which wavelengths of light they absorb, and a chromophore, which captures light photons  Each photoreceptor has only one of the 4 types of visual pigments found in the human retina  The pigment rhodopsin is found in the rods, concentrated in the stack of membranous discs in the outer segment.  Each cone has one of the other 3 pigments, which respond to long, medium, and short wavelengths respectively. rd  Evidence suggests that there may be a 3 type of photoreceptor that “lives” among the ganglion cells, and is involved in adjusting our biological rhythms to match the day and night of the external world.  These photoreceptors are sensitive to the ambient light level and contain the photopigment melanopsin, and they send their signals to the suprachiasmatic nucleus (SCN), home of the brain’s circadian clock, which regulates 24 hour patterns of behavior and physiology.  When a photon goes into the outer segment of a rod and is absorbed by a molecule of rhodopsin, it transfers its energy to the chromophore portion of the visual pigment molecule.  This process of photoactivation initiates a biochemical cascade of events eventually resulting in the closing of channels in the cell membrane that normally allowions to flow into the rod outer segment  Closing these channels alters the balance of electrical current between the inside and outside of the rod outer segment, making the inside of the cell more negatively charged. This process is hyperpolarization.  Hyperpolarization  closes calcium channels at synaptic terminal  thus reducing the concentration of free calcium inside cells  Lowering of calcium concentration  reduces concentration of neurotransmitter (glutamate) molecules at the synaptic terminals  this signals to the bipolar cell that the rod has captured a photon.  Unlike most other neurons, photoreceptors don’t respond in an all-or-nothing fashion. They pass their information on to bipolar cells via graded potentials instead of action potentials.  Humans have many more rods (90 million) than cones (4-5 million) and these 2 types of cells have different distribution on the retina.  Rods are absent from the center of the fovea. Their density increases to a peak at about 20 degrees and declines again.  Cones are most concentrated in the center of the fovea. Their density drops off with retinal eccentricity (distance from the fovea).  Cones are smaller and more tightly packed in the foveal center.  If we look directly at an object whose image is smallerthan 1 degree, the image will land on a region of the retina that has only cones.  We use the fovea to identify objects, read, and to inspect fine detail.  We use the periphery to detect and localize stimuli that we aren’t looking
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