Animal phys. topic 12

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How do the camera eyes of an animal such as an octopus differ from the camera eyes of vertebrates?

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Since most animals have the ability to respond to light, how do they accomplish this when they occur in a variety of environments and have very different body forms?

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What are the major features of the compound eyes possessed by insects and other arthropods?

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________ have the highest visual acuity and light reaches them without passing through bipolar and other cells

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how do rods and cones differ with respect to sensory processing of light?

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how do the visual capacities of different types of animals match their environments?

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how does the retina process light?

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what are the major functional components of the retina?

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what are the major functional components of the vertebrate eye?

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what are the major structural differences associated with seeing through air and seeing through water? with seeing under dim light conditions?

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where are visual impulses that originated in the retinal ganglion cells transmitted and how are the various aspects of visual information processed?

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light adaptation

Pupils constrict; Rhodopsin rapidly breaks down; Decreased sensitivity of rods to light

dark adaptation

Pupils dilate; Photopigments broken down during light exposure gradually regenerate; Increased sensitivity of rods to light; Night blindness is caused by dietary deficiency of vitamin A (retinene is a derivative of vitamin A)

tapetum lucidum

a layer of reflecting material enhances vision in dim light in some species

in the presence of light

a retinene molecule absorbs a photon; retinine changes shape from cis to trans conformation; triggers enzymatic activity of opsin; activates a G protein called transduction; Phosphodiesterase degrades cyclic GMP, causing Na+ channels to close; Hyperpolarizing receptor potential reduces glutamate release

Adjustments in lens focusing power for near vision in mammals is called

accommodation and are accomplished by the ciliary muscle.

Rods

are more abundant in nocturnal animals because they have greater light sensitivity; might function as tiny lenses; numerous activate one bipolar cell/ganglion cell combination, decreasing visual acuity at night; "burned out" by bright light; shades of gray;

cones

are present in concentrated retinal areas in diurnal animals. one activates one bipolar cell/ganglion cell combination, greatly increasing visual acuity during the day; three types (trichromatic) respond maximally to different light wavelengths

Retina may be

avascular (some reptiles), moderately vascularized (mammals) or have specialized oxygen-delivery structures called pectens (birds)

region where optic nerve exits the eye (optic disc) is the

blind spot

"visual streak"

broadens the region of high visual acuity

detachment of avascular retina

can lead to impaired vision

as an extension of the brain, the modified neurons in the retina

carry out preliminary processing of the light from the visual field before being transmitted by the optic nerve; This includes differential activation of on-center and off-center ganglion cells and lateral inhibition of nearby receptive fields.

some annelids alter the distance between the lens and photoreceptors by

changing the fluid volume of the optic chamber

Most animals have means of detecting light, there are two types of photoreceptors

ciliary and rhabdomeric

curvature of the lens is adjusted by the

ciliary muscles

vertebrates have ___________ with an outer fibrous layer (cornea and sclera), middle vascular layer (choroid, ciliary body, iris) and an inner lining of modified neurons (retina)

complex camera eyes

circular muscle

constricts pupil in response to light

inner segment

contains the metabolic machinery of the cell

autonomic nervous system

controls the iris muscles

in the absence of light

cyclic GMP concetration is hight in photoreceptors; Na+ channels are OPEN -> depolarization; Ca2+ channels in synaptic terminal remain open; glutamate is released

compound eye

densely packed units (ommatidia), each having its own lens and photoreceptors; arthropods

outer segment

detects the light stimulus

Different populations of cortical cells are

devoted to depth perception, form/movement, or color.

Light ________ depolarizes rhabdomeric photoreceptors (cf. with vertebrates, in which rods and cones stop releasing an inhibitory neurotransmitter in response to light).

directly

as in vertebrates, mechanisms for lateral inhibition

enhance visual contrast

at the optic chiasm

fibers from the medial half of each retina cross over, wile fibers from the lateral half remain on the same side

specific cone types respond to light of specific

frequencies

light must pass through the ______ and _______ layers before reaching the photoreceptors

ganglion and bipolar

cephalopod eyes

have cornea, lens, and retina

sensing polarized light helps

honeybees compute the location of food sources and butterflies to find a mate

visual development is regulated by the

hox gene known as Pax6

light moves

in the opposite direction of neural information flow in vertebrate eyes, which would not seem to be the best arrangment

Nocturnal arthropods possess superposition eyes,

in which visual info from separate ommatidia blend to some extent, increasing light sensitivity.

radial muscle

increases pupil size in dim light

Focusing of an image on the retina in air-seeing vertebrates involves

initial primary focus by the cornea, blocking out extraneous light by the iris, and fine focus adjustment by the lens.

parasympathetic

innervate circular muscle

sympathetic

innervate radial muscle

the result of separate transduction unites (rhabdomeric photoreceptors)

is a mosaic rather than a continuous image

optic tracts project to the

lateral geniculate nucleus of the thalamus

light rays from the _______ of the visual field fall on the right half of the retina

left half

Conversely, in fish the _______ is primarily responsible for focus.

lens

camera eye

lens enhances light-gathering power; many phyla, including vertebrates and cephalopods

eyespots

less than 100 photoreceptor cells lining an open cup; permits animal to locate a light source

synaptic terminal

lies closet to the eye's interior

phototransduction

light energy causes an isomerization reaction linked to neuron depolarization

three layers of

neurons

fibers terminate the visual cortex in the

occipital lobe

individual _____________ transduce light separately from each other and have their own individual visual fields

ommatidia

information from ach half of the visual field is brought together on the

opposite side of the brain

almost all of the visual systems employ pigments composed of

opsin and retinene.

axons of ganglion cells form the

optic nerve

The visual fields of some animals __________ to a significant extent (binocular vision, allowing depth perception) while in others they _____________ at all (monocular vision).

overlap , do not overlap

three retinal layers in the vertebrate eye, moving inward

photoreceptors (rods and cones); modified neurons (bipolar, amacrine, and horizontal cells); and ganglion cells

The eyes of cephalopod molluscs possess what seems to be a more "sensible" arrangement in which the

photoreceptors are in front of the neurons. Otherwise they are similar, with a cornea, iris, lens, etc. (see Fig. 6-42).

iris

pigmented ring of smooth muscle; controls the amount of light entering the eye

_________ of reticular cells contain the photopigment rhodopsin

rhabdoeres

when dark,

rods release an inhibitory neurotransmitter (glutamate) that hyper polarizes and bipolar cells

when light,

rods stop releasing glutamate and bipolar cells are allowed to depolarize

pupil

round central opening

pinhole eye

size of cup aperture is reduced; permits formation of an image

Not all retinal ganglion cell axon impulses are dedicated to perception of visual images:

some set the biological clock in the hypothalamus, others maintain cortical alertness.

some animals use ultraviolet light for

spatial orientation and prey detection, among other things

in nocturnal animals, the ________ _________ apparently reflects light throughout he retina a second time

tapetum lucidium

convex surfaces of the cornea and lens determine

the eye's refractive ability

Visual information flows from

the optic nerve to the lateral geniculate nucleus (LGN of thalamus). Axons of neurons originating in the LGN give rise to the optic radiations -> cerebral cortex.

fovea

the point of greatest visual acuity

image detected on the retina is

upside down and backwards

the number and distribution of cones depends on

where an animal lives


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