Week 6

Process of refraction: •When a light enters a medium of ? density it is slowed down •The direction of the ray changes if it strikes the surface of a new medium at any angle other than P •When light rays pass from air into a medium of different density, such as glass or water, they B or R •When a light strikes the curved surface of any object of greater density, the direction of refraction depends on the angle of the C

greater perpendicular bend refract curvature

Pathways of vision and pupillary reflex: •In the first step of the visual pathway light from the environment enters the eye •Before it strikes the retina, the light is modified in two ways: 1. The amount of light that reaches photoreceptors is modulated by changes in the size of the P 2. The light is focused by changes in the shape of the L

pupil lens

The eye and vision: •It focuses light on a light-sensitive surface ( the R) using a lens and a opening (the P) whose size can be adjusted to change the amount of entering light •Vision is a process through which light reflected from objects in our environment is translated into a MI

retina pupil mental image

Photoreceptors: Rods and Cones: Rods outnumber cones by ??:? except in the fovea where there are only cones --Rods and cones have the same basic structure. 1.The outermost layer which lies closest to the eyes exterior and detects light stimulus 2. The inner most layer which lies in the middle of the photoreceptors and contains metabolic machinery of the cell ( cell N, M) 3.ST which lies closest to the eyes interior facing bipolar cells. The outermost segment which is rod shaped in rods and cone shaped in cones consists of stacked, flattened discs containing light sensitive photopigment molecules. Each retina has ? million photoreceptors and a ? photopigment molecules Photopigment molecules undergo chemical alterations when activated by light. They are transducers that convert light energy into a change in membrane potential that leads to an action potential which transmit this information to the brain for visual processing Rods have one type of visual pigment rhodopsin. R consist of O: a protein that is an integral part of the disc M and R a derivative of vitamin ? that is bound within the interior of the opsin molecule. It is the light absorbing part of the photopigment. Cones have ? different pigments closely related to rhodopsin

20:1 nucleus mitochondria synaptic terminal 150 billion rhodopsin opsion membrane retinal A 3

Vision defects: Hyperopia= far-sightedness, occurs when the focal point falls ? the retina Hyperopia occurs when the shape of the lens is too F or the eyeball itself is too ?. In either cause light doesn't bend soon enough so the focal point falls behind the retina so the image is out of focus. A C lens helps to bend the light before reaching the eye so that the focal point lands on the retina. Myopia= near-sightedness, occurs when the focal point falls ? the retina Myopia occurs if the lens is too R or if the eyeball is too ?. In this cause the light rays converge too soon and the focal point falls in front of the retina (contains photoreceptors). A C lens will spread the light rays so they converge later and the focal point lands on the retina

behind flat short convex in front of round long concave

Light entering the eye controlled by the iris: •Not all light passing through the C reach light sensitive photoreceptors because of the presence of the I •Iris is a T, P smooth muscle that forms a visible ring like structure within the aqueous --Circular : run in a R like fashion within the iris --Radial: fibers project outward from the PM Iris is a muscle that controls the amount of light entering the eye. This is controlled by the A system. In dim light the sympathetic division will cause pupil D ( in fight or flight you need to see all around you) In bright light the parasympathetic division will C the pupil

cornea iris thin, pigmented ring pupillary margin autonomic dilation constrict

Eye's Refractive Structure: •Refractive ability of the lens can be adjusted by changing its C as needed for near or far vision •Rays from light sources more than ?? feet away are considered parallel by the time they reach the eye The closer the object to the eye the ? the lens needs to be in order to allow the focal point to land on the retina of the eye that contains photoreceptors

curvature 20 stronger

The Retina photoreceptors: •Backing the photosensitive portion of the human retina is a D pigment epithelium layer •Photoreceptors are the neurons that convert light into electrical signals --The main types are rods and cones --Light entering the eye must pass through several layers of neurons before striking the photoreceptors Rods and cones are on bottom layer with their photosensitive tips against the P layer. Light passes through several layers before reaching photoreceptors (rods and cones). Once light has reached the photoreceptors they convert the energy into an action potential that moves forward sending the signal to the O nerve. Light hits the rods and cones and then moves forward to B cells which generate a GP and then to the G cells which send an action potential to the optic nerve

dark pigment optic bipolar graded potential ganglion

What happens to photoreceptor activity in the light? •Exposure to light ? cGMP concentration •R changes shape when it absorbs light and this activates photo pigment •Rod and cone cells contain a G-protein called T •Activated photo pigment activates transducin which activates enzyme PD •The enzyme degrades cGMP decreasing its concentration which causes sodium channels to ? •This stops depolarizing of sodium leak and the membrane hyperpolarizes. •Hyperpolarization spreads to the synaptic terminal •This closes calcium channels reducing neurotransmitter release from synaptic terminal Neurotransmitter is G

decreases retinal transducin phosphodiesterase close glutamate

Phototransduction: •The process of converting light stimulus into electrical signals is the same for all photoreceptors •The mechanism is contrary to the usual means by which receptors respond to adequate stimulus --Photoreceptors HP when light is absorbed.

hyperpolarize

Steps in the vision process: 1.Light enters the eye, and the L focuses the light on the R 2.Photoreceptors of the retina transduce light energy into an ES 3.Neural pathways from retina to brain process electrical signals into VI

lens retina electrical signal visual images

Rods: •Rods function well in ? light and are used in ? vision •Rods are most abundant in the P • Rods have ? sharpness but ? sensitivity •There is much C in rods •Rods can bring about action potentials in response to ? amounts of light they are much ? sensitive than cones •? ability to distinguish between two nearby points

low night periphery low high convergence small more poor

Accommodation: •The process in which the eye adjusts the shape of the lens to keep objects in focus is known as accommodation •The closest distance at which it can focus an object is known as the NP of accommodation •Ciliary muscles a ring of smooth muscle surrounds the lens and is attached to it by the inelastic ligaments called Z •If no tension is placed on the lens by the ligaments assumes it natural R shape •Tension on the ligaments is controlled by ciliary muscle. •When the ciliary muscle is relaxed the ring is more O and the lens is pulled into a F shape •This shape is required for ? vision •When this circular muscle contracts, the muscle ring gets ?, releasing tension on the ligaments •The lens becomes R •This is needed for ? vision

near point zonules rounded open flatter distant smaller rounded near

Fovea and Macula: •One exception to this organization pattern is the fovea •This area has no neurons or blood vessels that would block light reception •When you look at an object the lens focuses the object image on the fovea. This is where vision is the S •Only ? are located in the fovea making the fovea the point of D vision •Area surrounding the fovea is the macula which also has a high concentration of ? as well

no sharpest cones distinct cones

Eye refracts entering light to focus the image on the retina: •Light waves diverge (radiate ?) in all directions from every point of a light source •The forward movement of a light wave in a particular direction is known as a LR •Divergent light rays reaching the eye must be bent ? to be focused back into a point (FP) on the light sensitive retina --Provides accurate image of the light source The C (which is not adjustable) bends the light first, then the light will be bent by the L ( which can be adjustable based on the distance of the object)

outward light ray inward focal point cornea lens

•The major function of the eye is to focus light rays from the environment on the R and C, the photoreceptor cells of the retina •Photoreceptors transform the light energy into electrical signals for transmission to the CNS •Photo transduction takes place when light hits the retina •The retina develops from the same embryonic tissue as the B (C) -Neurons in the retina are organized into L

rods cones brain cortex layers

Cones: •Cones are responsible for S vision and C vision during the ? •Cones are most abundant in the F •Cones have ? sensitivity to light, but ? sharpness so they provide ? resolution •Little convergence •Cones provide highly D vision at the expense of sensitivity.

sharp color daytime fovea low high high detailed

What happens to photoreceptor activity in the dark? •The plasma membrane of a photoreceptor contains chemically gated ? channels •These channels respond to an internal second messenger CG •Binding of cGMP to sodium channels keeps them ? •In absence of light cGMP is high, which means the sodium channels of the photoreceptors are open in absence of stimulation ( aka in the D) •Passive inward leak of sodium depolarizes the photoreceptor. •The depolarization spreads to the synaptic terminal which keeps voltage gated ? channels open •Calcium entry triggers the release of neurotransmitter from the synaptic terminal Neurotransmitter is G

sodium cGMP open dark calcium glutamate

Anatomy of the eye: The eye itself is a hollow sphere divided into ? compartments (chambers) separated by a ? After passing through the opening of the pupil, light strikes the lens, which has two C surfaces. The C and lens together bend incoming light rays so that they focus on the retina. The lens is adjustable based on the D of the object. The cornea is not. Retina contains the photoreceptors that send information about the image to the brain Lateral to the OD is a small dark spot, the F. The fovea and a narrow ring of tissue surrounding it, the M are the regions of the retina with the most A vision

two lens convex cornea distance optic disk fovea macula acute

Optics of the eye: Parallel light rays striking a concave lens, are refracted into a ? beam Parallel rays striking a convex lens bend ? and focus to a point. Convex lenses converge light rays. When parallel light rays pass through a convex lens the single point where the rays converge is called the FP The distance from the center of a lens to its focal point is known as the FL of the lens. For any given lens the focal length is F. For the focal length to change the S of the lens must change Convergence is essential for bringing an image to focal point, which is why our lens shape is convex.

wider inward focal point focal length fixed shape