Chapter 14 Eye and Vision

Question 1

electromagnetic spectrum

Answer 1

a spectrum of the different range of energies

Question 2

visible light

Answer 2

energy range in the electromagnetic spectrum that humans are able to see; between 400 and 700 nm, a range of energy that can engage in significant and sustainable interactions with molecular and cellular structures in the body

Question 3

retina

Answer 3

complex structure consisting of a layer of light-sensitive photoreceptor cells and several layers of interconnected nerve cells; heavily crisscrossed with blood vessels, as the photoreceptor and other neural cells require a robust supply of biochemical fuel; derived from the Latin word “rete,” meaning “net,” referencing the complex network

Question 4

fovea

Answer 4

the center region of the retina that receives light coming from the center of the visual field; density of photoreceptor cells is highest at the fovea, so visual acuity is best for the region of space where we are directly looking

Question 5

blind spot

Answer 5

a small region of the retina where the large number of nerve fibers occupy so much space that there is no room for any photoreceptor cells, thus any light falling on the blind spot will not be detected (however we are usually unaware of this blindness because if both eyes are open and functioning the visual system can use the information received in one eye to fill in the blind spot for the other eye)

Question 6

rods

Answer 6

photoreceptor cells that are rod-shaped, found throughout most of the retina, and sensitive to even very small amounts of light; contains protein molecule rhodopsin

Question 7

cones

Answer 7

photoreceptor cells that are cone-shaped, mostly located at the fovea, and respond to higher intensity rather than very dim light; three types of cone cells, each responding to a different range of light wavelengths; contains protein molecule cone opsins

Question 8

rhodopsin

Answer 8

G protein coupled receptor (GPCR) that absorbs light and initiates the process of transformation of the light energy into a neural signal; found in rod photoreceptor cells; extremely sensitive to light, and thus enables vision in low-light conditions

Question 9

cone opsin

Answer 9

absorb light and initiate the process of transformation of the light energy into a neural signal; absorb light in slightly different regions of the visible light spectrum (short-wavelengths [S] correspond to blue cones, medium [M] to green, and long [L] to red)

Question 10

retinal achromatopsia

Answer 10

a genetic or developmental anomaly that results in loss of all functional cone cells; people with this condition have no experience of color (see the world in black, white, grey), sometimes report an appreciation of gradations of contrast, shadow, and texture more nuanced than normal vision

Question 11

outer segment of photoreceptor cells

Answer 11

contain the rhodopsin and cone opsin photoreceptor proteins

Question 12

inner segment of photoreceptor cells

Answer 12

contain nuclei, mitochondria, and other structures necessary for the functioning of the cell; synaptic ending

Question 13

retinal

Answer 13

small molecule (not an amino acid) attached to the opsin protein via a covalent bond with a nitrogen atom in a specific lysine amino acid within the protein; named after retina; absorbs the light and begins the cascade of events leading to a neural signal; occurs in the rhodopsin and in the various different cone opsins; the human body cannot make retinal from scratch - retinal is made from closely related molecules that we eat: vitamin A and carotenoids

Question 14

retinol

Answer 14

differs from retinal by the addition of hydrogen to the oxygen atom at the end of the chain

Question 15

beta-carotene

Answer 15

carotenoid; widespread in plants and are the most abundant chemical precursors to retinal and retinol in nature; also gives carrots their orange color; the human body divides a molecule of beta-carotene in half and chemically modifies it to produce two molecules of retinal

Question 16

photoisomerization of retinal (cis to trans) (light-induced isomerization)

Answer 16

when the retinal molecule is bound to the protein in rhodopsin or one of the cone opsins, it occurs in a form called the 11-cis isomer of retinal, where the carbon-chain portion of the molecule is bend or kinked; absorption of a photon of light by the retinal molecule triggers a change in the shape of the chain so that it rotates around the double bond where the kink is and straightens out, into a form called the all-trans isomer of retinal

Question 17

GPCR intracellular cascade

Answer 17

when a photon of light is absorbed by 11-cis retinal in rhodopsin, the retinal isomerized to the all-trans form, shape-shifting the opsin protein and thereby “activating” it, the activated opsin is available to bind to and activates an intracellular G-protein, which then activates an enzyme called cGMP phosphodiesterase, hydrolyzing it to noncyclic GMP, the interaction of cGMP with certain ion channels in the cell membrane keeps these channels open (when the concentration of cGMP in the cell decreases, these ion channels close, the membrane potential and thus the cell excitability changes, which changes the amount of neurotransmitters being released at the synapse between the photoreceptor cell and the other cells in the retina)

Question 18

bipolar cells

Answer 18

form synapses with rods and cones in the retina but can also form synapses with horizontal cells (transmits neural signals from photoreceptors to ganglion cells)

Question 19

ganglion cells

Answer 19

receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina amacrine cells; axons of ganglion cells bundle together to form the optic nerve, optic chiasm, optic tract (enables neural signal to flow to the brain); each retina has about one million ganglion cells; transmit image-forming and non-image forming visual information from the retina in the form of action potential to several regions in the thalamus, hypothalamus, and mesencephalon, or midbrain

Question 20

horizontal cells

Answer 20

laterally interconnecting neurons having cell bodies in the inner nuclear layer of the retina of vertebrate eyes. They help integrate and regulate the input from multiple photoreceptor cells. Among their functions, horizontal cells are responsible for allowing eyes to adjust to see well under both bright and dim light conditions. Horizontal cells are inhibitory interneurons that release GABA upon depolarization

Question 21

amacrine cells

Answer 21

interneurons in the retina; named from the Greek roots a– (“non”), makr– (“long”) and in– (“fiber”), because of their short neuritic processes; inhibitory neurons that project their dendritic arbors to the inner plexiform layer (IPL), there interacting with retinal ganglion cells and/or bipolar cells

Question 22

receptive field

Answer 22

an important property of neural cells in the visual system beginning with the photoreceptor, bipolar, and ganglion cells in the retina; receptive field of an individual sensory neuron is the particular region of the sensory space (e.g., the body surface, or the visual field) in which a stimulus will trigger the firing of that neuron

Question 23

visual map (world to retina to cortex)

Answer 23

information represented by the neural signals exit the eye and travel along the optic nerve toward the brain (a short distance behind the eyes) where two optic nerves intersect in a structure called the optic chiasm, at the chiasm, fibers from the two optic nerves divide into two new groups with axons from each of the eyes that gather information from the left half of visual space to the right half of the brain, and axons from each of these eyes that gather info from the right visual space to the left half of the brain

Question 24

LGN (lateral geniculate nuclei)

Answer 24

cells that send axons into the rearmost region of the cerebral cortext (the posterior occiptial lobe) where they form synapses with cortical neurons; axons carrying information from the right (left) visual field go to the left (right)-side LGN

Question 25

visual cortex

Answer 25

the region of the brain that is involved in the analysis of visual information (occipital lobes and posterior regions of the temporal lobes)
V1: responds to edges of objects
V4: responds to specific colors and are less influenced by things like shape and movement
V5: respond to movement and its speed and direction

Question 26

superior colliculus

Answer 26

a part of the midbrain where about 10% of the optic nerve axons go; this pathway is heavily involved in very rapid responses to sensory stimuli in ways that do not involve awareness

Question 27

scotoma

Answer 27

caused by a lesion in V1; a blind spot in a specific region of space

Question 28

hemianopia

Answer 28

complete loss of vision for an entire half of the visual field, the half of visual space contralateral to the side of the brain in which the lesion is located; caused by a complete damage of V1 in one hemisphere of the cortex

Question 29

cortical achromatopsia

Answer 29

the condition related to a cortical lesion rather than to a retinal condition; causes color disruption, varying from washed out or faded color perception to a complete loss of color awareness

Question 30

motion blindness

Answer 30

akinetopsia, caused by a lesion in V5; the person is unaware of movement in some region or regions of visual space (world appears as a series of snapshots)

Question 31

prosopagnosia

Answer 31

a specific type of agnosia; clinical condition brought on by sustaining a lesion in the inferior and medial temporal lobe; causes great difficulty of even a complete loss of ability to recognize faces

Question 32

lesion

Answer 32

a region in an organ or tissue that has suffered damage through injury or disease

Question 33

agnosia

Answer 33

a general neurological syndrome; may cause difficulty recognizing all or nearly all visual objects; often associated with lesions in the region where the occipital, temporal, and parietal lobes come together