Neuroscience Chapter 9

How many photoreceptors are there?

-100 million

The lens contributes how many diopters to light refraction in the eye? Does the lens contribute more to refracting light from images that are closer or farther away?

-12 diopters -It is involved in forming crisp images of objects closer than 9 m away (closer images require greater refractive power to bring into focus)

The cornea contributes how many diopters to light refraction in the eye? What is the focal distance?

-42 diopters depending on cornea curvature -About 2.4 cm (0.024 m)

Amplification

-A functional consequence of using a biochemical cascade for transduction -A photopigment molecule activates many g-proteins and a PDE enzyme breaks down many cGMP molecules -Gives the visual system incredible light sensitivity -This allows for rods to sense a single photon

Conjunctiva

-A membrane that folds back from the inside of the eyelids -The red part on the underside of your eyelid -Attaches to the sclera

Optic disk

-A pale, circular region where retinal blood vessels originate from -Where optic nerve fibers exit the retina -No photoreceptors here -"Blind spot"

Pupil

-A part of the eye -An opening that allows light to enter the eye and reach the retina -Appears dark because of the light-absorbing pigments in the retina -Surrounded by the iris

Cornea

-A part of the eye -Covers the pupil and iris -A glossy, transparent surface -Continuous with the sclera -The site of most of the refractive power of the eyes because it is made of water and is the first part of the eye that light hits

Iris

-A part of the eye -Surrounds the pupil -Pigmentation provides eye color -Contains two muscles that can vary the size of the pupil (one makes it smaller and one makes it larger)

Sclera

-A part of the eye -The white of the eye -Forms the tough wall of the eyeball -Has three pairs of extraocular muscles -Continuous with the cornea

Rhodopsin

-A photopigment in the membrane of the stacked disks in the rod outer segment that absorb electromagnetic radiation and cause less cGMP release, causing hyperpolarization -A receptor protein (opsin) with a pre-bound chemical agonist (retinal) -Absorption of light causes a change in the conformation of retinal so that it activates the opsin (called bleaching)

Thalamus

-A structure in the middle of the brain -Located between the cerebral cortex and the midbrain -Works to correlate several important processes, including consciousness, sleep, and sensory interpretation

ON bipolar cell

-A type of bipolar cell -Receives informations from photo transmitters -Depolarize (turn on) when light is shined onto a cone -G-protein-coupled glutamate receptors -Respond to glutamate by hyper polarizing

OFF bipolar cell

-A type of bipolar cell -Receives informations from photo transmitters -Hyperpolarize (turn off) when light is shined onto a cone -Ionotropic glutamate receptors -Releases less neurotransmitters

Ganglion cell

-A type of neuron located near the inner surface of the retina -The final output neurons of the vertebrate retina -Collect visual information in their dendrites from bipolar cells and amacrine cells and transmit it to the brain through out their axon to the brain -Fire action potentials in response to light that propagate along the optic nerve and into the rest of the brain

Diopter

-A unit of measurement that shows the reciprocal of focal distance in meters

Retinitis pigmentosa

-A vision disorder -A hereditary disease -Characterized by black pigmentation of the retina -Results in loss of peripheral and night vision

Detached retina

-A vision disorder -A retina that has become separated from the underlying choroid tissue at the back of the eye -Results in a loss of vision in the affected area

Myopia-nearsightedness

-A vision disorder -Close objects can be seen clearly but distant ones can not -Concave lenses

Cataract

-A vision disorder -Clouding of the lens -Results in blurred vision

Hyperopia-farsightedness

-A vision disorder -Distant objects can be seen clearly but close ones can not -Convex lenses

Glaucoma

-A vision disorder -Increased intraocular pressure -Results in gradual loss of sight

Macular degeneration

-A vision disorder -Leading cause of severe vision loss in people over 60 -The macula begins to deteriorate -Results in loss of central vision

Exotropia

-A vision disorder -Problems with the extraocular muscles -One or both eyes turn inward (cross-eyed) -Input from one eye suppressed

Esotropia

-A vision disorder -Problems with the extraocular muscles -One or both eyes turn outward (wall-eyed) -Input from one eye suppressed

Electromagnetic radiation

-A wave of energy

Photopigments

-Absorb light and trigger changes in photoreceptor membrane potentials

Bleaching

-Absorption of light by rhodopsin causes a change in the conformation of retinal so that it activates the opsin -Changes the wavelengths absorbed by rhodopsin (pigment goes from purple to yellow) -Stimulates transducin

Bipolar cell

-Act directly or indirectly to transmit signals from the photoreceptors to the ganglion cells -Receive input from one photoreceptor in fovea -Thousands are in the in peripheral retina -Sends output to horizontal cells

Eye

-Acts like a camera and forms clear images of the world onto the retina -Adjusts to differences in illumination and focuses on objects of interest -Can track moving objects and keep its transparent surfaces clean (tears and blinking)

Accommodation

-Additional focussing power provided by changing the shape of the lens -The colliery muscle contracts and swells in size, making the area in the ring smaller and decreasing tension in suspension ligaments -The lens becomes rounder and thicker, increasing curvature and thereby increasing refractive power -Relaxing ciliary muscles flattens the lens -This ability gets worse with age

Cerebral Cortex

-Also known as the cerebrum -The largest portion of the brain -Located right under the forehead -The most highly developed part of the human brain -Responsible for thinking, perceiving, producing and understanding language

Phosphodiesterase (PDE)

-An effector enzyme activated by transducin -Breaks down the cGMP that is normally present in the cytoplasm of the rod (in the dark)

Every photoreceptor has what for regions?

-An outer segment -An inner segment -A cell body -A synaptic terminal

Retina

-At the back of the eye -Part of the brain -Each eye has two that overlap (one for low light levels and one for high light levels) -Contains photoreceptors for converting light into neural activity -Specialized to detect DIFFERENCES in the intensity of light falling on different parts of it -The first step of image processing

Vision

-Based on the light bouncing off of objects around us, we somehow make sense of the complex world -An extremely complex process -A half of the human cerebral cortex is designated to this process -Throughout evolution it has provided new ways to communicate and has caused the rise of art

What are the three different colors of cones

-Blue (430 nm), long wavelength -Green (530 nm), medium wavelength -Red (560 nm), short wavelength

Why is there a debate over what color dashboard lights should be?

-Blue or green lights are more likely to be seen by rods at night but red lights activate cones instead of rods and don't detract from your night vision

Axons of retinal neurons

-Bundled into optic nerves

Optic nerve fibers

-Carry sensory information relating to vision from the retina of the eye to the brain -Connect to the thalamus

Laminar organization

-Cells are organized in layers

Outer segment of a photoreceptor

-Contains a stack of membranous disks with light-sensitive photopigments in their membranes that trigger changes in photoreceptor membrane potentials

Inner nuclear layer

-Contains the cell bodies of bipolar cells, horizontal cells, and amacrine cells

Outer nuclear layer

-Contains the cell bodies of photoreceptors

Ganglion cell layer

-Contains the cell bodies of the ganglion cells

Layer of photoreceptor outer segments

-Contains the light-sensitive elements of the retina -Embedded in the pigmented epithelium

Inner plexiform layer

-Contains the synaptic contacts between bipolar cells, amacrine cells, and ganglion cells

In the dark, the membrane potential of the rod outer segment is ____.

-Depolarized

Dowling and Werblin

-Discovered in early 1970s how ganglion cell responses are build from the interactions of horizontal and bipolar cells

Visual angle

-Distance across the retina can be described in terms of degrees

Optic nerve

-Distributes visual information in the form of action potentials to several brain structures -Sends signals that regulate biological rhythms dealing with the daily light-dark cycle -Sends signals that control eye-position and optics -Carries axons from the retina -Exits the back of the eye -Passes through the orbit -Reaches the base of the brain near the pituitary gland

Mesopic

-During intermediate light levels -Cones and rods do equal work

Photopic

-During the day -Cones do the bulk of the work

Scotopic

-During the night -Rods do the bulk of the work

Light

-Electromagnetic energy in the form of waves that is visible to our eyes -Consists of wavelength between 400-700 nm -A mix of wavelengths appears white, no wavelengths appears black, and one wavelength appears as a color -Hot colors = less energy and cool colors = more energy

Ciliary muscle

-Forms a ring inside the eye that the zonule fibers attach to

What are the layers of the retina from innermost to outermost?

-Ganglion cell layer -Inner plexiform layer -Inner nuclear layer -Outer plexiform layer -Outer nuclear layer -Layer of photoreceptor outer segments

Dark adaptation

-Getting used to the dark -Sensitivity to light increases a million fold during this period -Pupils dilate -Adjustment of retinal circuitry -Unbleached rhodopsin is regenerated

Ionotropic glutamate receptors

-Glutamate-gated channels that mediate a classical depolarizing excitatory post-synaptic potential from the influx of Na+

Organization of the retina

-Has laminar organization -Organization is inside out (light has to go through ganglion and bipolar cells to get to photoreceptors) -Has a pigmented epithelium

Central retina

-Has much greater spacial sensitivity (the fovea at the center of the retina is specialized to maximize visual acuity) -Blind at scotopic light levels (unable to perceive color differences at night)

Duplex retina

-Having two complementary systems in one eye (rods and cones) -Some animals have only rods and cones and therefore do not have this

Under what conditions will an OFF-center ganglion cell increase its firing rate?

-If a small dark spot covers the receptive field center

How does phototransduction occur in rods?

-In many ways this is similar to synaptic transmission -Receptor binds to G-protein-coupled neurotransmitter receptor and G-proteins are activated in the membrane -G-proteins activate various effector enzymes that DECREASE second messenger molecules which DECREASES Na+ conductance

Kuffler, Hartline, and Barlow

-In the 1950s they discovered which aspects of visual image were encoded as ganglion cell output -Studied horseshoe crabs, frogs, cats, and monkeys -Only retinal ganglion cells fire action potentials

Saturated

-Increasing the light level causes no additional hyperpolarization -Happens in bright light with rods (500 nm light) -Because of this, day vision depends on cones that require more light to be bleached

Cyclic guanosine monophosphate (cGMP)

-Intracellular second messenger enzymes that gate the sodium channels in rods -Produced in the photoreceptor by the enzyme guanylyl cyclase -Light reduces this and causes sodium channels to close and membrane potential to become more negative

What will an OFF-center ganglion cell do if a dark spot covers its receptive field center?

-It will fire more action potentials

M cells have _______________ dendritic fields/ P cells have __________________ dendritic fields.

-Large -Small

Pigmented epithelium

-Lies below photoreceptors -Absorbs light that passes entirely through the retina -Minimizes the scattering of light that would blur the image

Aqueous humor

-Lies between the cornea and the lens -This fluid nourishes the cornea because the cornea lacks blood vessels

Vitreous humor

-Lies between the lens and the retina -Viscous and jelly-like -Keeps the eyeball spherical

Zonule fibers

-Ligaments that attach the lens to the ciliary muscle

Receptive field

-Light applied to a very small part of the retina changes the firing rate of the neuron -Light anywhere but here has no effect on firing rate -Change in shape to the stimulus that makes the neuron most active

Extraocular muscle

-Move the eyeball in the orbit -Not visible -They lie behind the conjunctiva

Ophthalmoscope

-Offers another view of the eye -A device that enables one to peer from the pupil to the retina -Can see the blood vessels on the surface of the retina

Refraction

-One of the three main optic interactions -How images are formed in the eye -The bending of light rays that can occur when they travel from one transparent medium to another -Things look bent when they are underwater because light travels faster through air than it does through water -The transparent media in the eye bends light rays to form images on the retina

Reflection

-One of the three main optic interactions -The bouncing of light rays off of a surface -The manner at which it bounces off depends on the angle at which it hits the surface -Most light that we see has been bounced off of objects in the environment

Absorption

-One of the three main optic interactions -The transfer of light energy to a particle or surface -Black surfaces have had all visible light energy transferred to them -Why your pupil is black

Horizontal cell

-One of the two types of cells that modify the direct visual pathway -Receive input from photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors

Amacrine cell

-One of the two types of cells that modify the direct visual pathway -Receive input from photoreceptors and project neurites laterally to influence surrounding bipolar cells, ganglion cells, and other amacrine cells

Rod photoreceptor

-One of the two types of photoreceptors in the retina -Have a long, cylindrical outer segment containing many disks -Have a higher photopigment concentration -1000 times more sensitive to light than cones -92 million -Responsible for our peripheral vision -None are in the fovea

Cone photoreceptor

-One of the two types of photoreceptors in the retina -Have a shorter, tapering outer segment with few membranous disks -5 million -Responsible for the ability to see color (three different pigments) -Most are in the fovea

What can bipolar cells connect to?

-Photoreceptors -Horizontal cells

Phototransduction in Rods

-Photoreceptors covert (transduce) light energy to membrane potential changes -20X more rods than cones -Membrane potential is -30 mV (caused by the dark current, steady influx of Na+)

What is the most direct pathway for visual information to exit the eye?

-Photoreceptors to bipolar cells to ganglion cells

Peripheral retina

-Poor at discriminating colors (more rods and few cones) and details -More sensitive to low light levels (rods better with low light levels because they have more receptors cells and photoreceptors per ganglion cell)

Transducin

-Stimulated by bleaching -A G-protein in the disk membrane -Activates phosphodiesterase (PDE)

Phototransduction in cones

-Takes more light to saturate cones -Relatively similar to rods but cones have three opsins that give photopigments different spectral sensitivities

Visual acuity

-The ability of the eye to distinguish two points near each other -Depends on the spacing of the photoreceptors in the retina and the precision of the eye's refraction

What will happen if a small spot of light is projected onto the center of an On-center ganglion cell's receptive field?

-The cell will respond with many action potentials

Fovea

-The center of the retina -A dark spot about 2 nm wide -A convenient anatomical reference point -Retina is thinner here than anywhere else

Wavelength

-The difference between successive peaks or troughs

Guanylyl cyclase

-The enzyme that produces cGMP in rods

Eye's orbit

-The eye socket -The eyeball sits in this

Lateral geniculate nucleus (LGN)

-The first synaptic relay in visual perception -A cell group on the dorsal thalamus -From here, information ascends to the cerebral cortex where it is interpreted and remembered

Dark current

-The movement of positive charge across the membrane, which occurs in the dark -Na+ constantly flowing through rods

Ray

-The name for a wave of electromagnetic radiation that travels in a straight line in a vacuum

Frequency

-The number of waves per second -The higher this is the higher the energy of the wavelength is

Macula

-The part of the retina for central vision -A darker-colored region with a yellowish hue and an abundance of large blood vessels that arc from the optic disk (a specialization that improves central vision quality)

Superior

-The part of the retina that lies above the fovea

Inferior

-The part of the retina that lies below the fovea

Temporal

-The part of the retina that lies close to the temple than the fovea

Nasal

-The part of the retina that lies closer to the nose than the fovea

What will happen when a shadow passes over a photoreceptor?

-The photoreceptor will depolarize, releasing more transmitter molecules

Retinal

-The pre-bound agonist in rhodopsin -A derivative of vitamin A

Pupillary light reflex

-The pupil continuously adjusts for different ambient light levels -Constriction of pupil increases depth focus -Involves connections between the retina and the neurons in the brain stem that control the muscles that constrict the pupils -It is consensual

Opsin

-The receptor protein in rhodopsin -Has seven transmembrane alpha helices typical of g-protein coupled receptors throughout the body

Tapetum lucidun

-The reflective layer beneath the photoreceptors that nocturnal animals like cats and raccoons have -Why their eyes shine in photographs -More sensitive to low light levels at the expense of reduced acuity

Refraction by the cornea

-The site of most of the refractive power of the eyes because it is made of water and is the first part of the eye that light hits -The eye uses refraction to focus light into a point on the retina -Light that strikes perpendicularly goes straight to the retina but light that strikes at an angle is bent towards the back of the retina

Optics

-The study of light rays and their interactions

Visual field

-The total area in which objects can be seen in the side (peripheral) vision as you focus your eyes on a central point -The structure of the eyes and where they are positioned limits how much we can see -It is completely inverted

Do photoreceptors depolarize or hyperpolarize in response to light?

-They hyperpolarize (become more negative) -Light reduces cGMP and causes Na+ channels to close

Young-Helmholtz trichromacy theory

-Thomas Young showed in 1802 that all visible colors, including white, could be mixed using only red, green, and blue -He proposed that the retina contains three receptor types -Confirmed by Hermann von Helmholtz in 1851 after he invented the ophthalmoscope

Lens

-Transparent and located behind the iris -Suspended by ligaments (zonule fibers) attached to the ciliary muscle -Helps to focus light and images on the retina -Divides the interior of the eye into two compartments (the aqueous humor and the vitreous humor)

Light rays

-Travel in straight lines until they interact with atoms and molecules around them

Light adaptation

-When dark-adapted eyes go back into bright light they are temporarily saturated -Takes 5-10 minutes -Because the cones are hyperpolarized in the dark, in the light Ca 2+ enters the cones through the sodium channels and inhibits guanylyl cyclase, stopping the synthesis of cGMP, and causing Na+ to enter the cell, raising the cell membrane potential to about -35 mV

Outer plexiform layer

-Where the photoreceptors make synaptic contact with the bipolar and horizontal cells

What does it mean if you have 20/20 vision?

-You can recognize a letter that subtends an angle of 0.083 degrees

What are the 5 steps in the transduction of light by rods?

1. Light activates (bleaches) rhodopsin 2. Transducin (the g-protein) is stimulated 3. Phosphodiesterase (PDE) (the effector enzyme) is activated 4. PDE breaks down cGMP 5. Na+ channels close and the cell membrane hyperpolarizes

What are the three important points to remember about the direct visual pathway?

1. With the exception of a few rare ganglion cells, the only light sensitive cells in the retina are photoreceptors. 2. Ganglion cells are the only source of output from the retina. 3. With the exception of a few amacrine cells, ganglion cells are the only retinal neurons that fire action potentials, an essential process for transmitting information from outside of the eye.

Color-blindness

-Results when one or more cone photopigment type is missing

Consensual

-Shining a light in only one eye will cause the constriction of both pupils -When the pupils are not the same size it is a sign of serious neurological disorder involving the brain stem

Photoreceptors

-Specialized cells that include rods and cones -Respond to light and influence the membrane potential of the bipolar cells connected to them

Amplitude

-The difference between wave trough and peak

Focal distance

-The distance from the refractive surface to the point where parallel light rays converge -Depends on the curvature of the cornea -The tighter the curve, the shorter this is

To distinguish small words in a book, what ganglion cells would most likely be involved?

-P-type ganglion cells