W7 - Visual System

second half

properties of the aqueeous humor

thin and watery

what is the purpose of the aqueous humor

to supply nutrients to the anterior structures

what are the 3 layers of the eye from innermost to outermost

retina, choroid, sclera

properties of the vitreous humor

thick and gelatinouos

what is the purpose of the vitreous humor

helps maintain the structure of the eye and keeps it clear of debris via phagocytic cells

what is the cornea

outer layer in front of eye with transparent tissue for light to enter through

where is the vitreous humor

between the lack of the lens and the surface of the retina

what are the 3 continuous structures of the uveal tract

choroid, ciliary body, iris

what is the choroid

capillary bed that nourishes photoreceptors

in what part of the uveal tract is there a high concentration of melanin?

choroid

what is the purpose of the ciliary body

adjusting the refractive power of the lens

what do the muscles of the iris control

pupillary constriction and dilation

the ciliary muscles use __ ___ that attach to the lens to change the shape of it

zonule fibers

what happens when the ciliary muscle contracts? what is it good for?

zonule fibers go on slack and the lens gains curvature; good for closer focus

what happens when the ciliary muscle relaxes? what is it good for?

zonule fiibers go on tension and the lens flattens; good for further focus

what controls the ciliary muscle?

the autonomic nervous system (ANS)

what is accommodation

changing the shape of the lens to help see near/far

what are the primary functions of the optical components of the eye

efficiently transmit light energy and generate a focused image on the retina

what 2 structures are primarily responsible for bending of light onto the retina?

cornea and lens

when does the lens have the most refractive power

when it is thick and round (zonule fibers on slack)

what do adjustments in the pupil do for the eye?

contribute to clarify images and adjust amount of light entering the eye

the retinal circuitry acts on ___ potentials

graded

what are the 2 types of photoreceptors

rods and cones

what is the most direct path of information flow to the optic nerve?

photoreceptor to bipolar cell to ganglion cell

where are photoreceptors located in reference to light?

furthest away from the light source

what is the purpose of horizontal and amacrine cells

to modulate activity at synapses between other cells in the retina

which part of the photo receptor has a short lifespan and is rapidly replaced?

outer segment

what is the trigger starting the cascade from a photoreceptor to send a signal to the brain?

absorption of light

what cells form the optic nerve

ganglion cells

light stimulus does what to photoreceptors

hyperpolarization

what is the relationship of light absorption to neurotransmitter release in photoreceptors?

more light = less neurotransmitter release

true or false: photoreceptors fire action potentials when they absorb a sufficient amount of light

false

why do photoreceptors use graded potentials?

because there are short distances involved, so there isn’t an issue with the signal fading

what is the resting membrane potential of a photoreceptor

-40mV

what is the threshold for photoreceptors

-65mV

in the dark, the number of open calcium channels is (high/low). when stimulated by light, the number of open calcium channels (increases/decreases) to hyperpolarize the cell

high; decreases

what is the ratio of cones to bipolar cells?

1:1

what is the ratio of rods to bipolar cells?

15-30:1

why are cones better at detecting spatial resolution than rods?

because of the 1:1 ratio with bipolar cells

rods are sensitive to ___ and good for ___ and __ __

light, sensitivity, low light

cones have high ___ ____ and good for __

spatial resolution, color

why are rods better at detecting light than cones?

the convergence of several rods on one bipolar cell

what are the 3 types of cones

blue, green, red (short, medium, long wavelength)

how are cone types defined

by the photopigment they contain

what is the purpose of on- and off-center retinal ganglion cells

helps us to detect edges in our vision

what do on-center ganglion cells prefer (what do they fire in response to)

light in the center of the cell

what do off-center ganglion cells prefer (what do they fire in response to)

no light in the center of the cell

why is it important for us to detect edges?

to determine contrast and thus tell what is in front of us

the bipolar cell that a cone/several rods are synapsing with is either:

an on-center or off-center cell

more light absorbed by a photoreceptor causes it to _____ which results in __ glutamate release

hyperpolarize, less

what does glutamate do to an on-center bipolar cell

inhibit it so it can’t excite the on-center ganglion cell

what does glutamate do to an off-center bipolar cell

excite it so it can excite the off-center ganglion cell

on-center bipolar cells get (hyper/de)polarized in (high/low) light conditions

de, low

off-center bipolar cells get (hyper/de)polarized in (high/low) light conditions

hyper, low

hyperpolarization of photoreceptors results in (more/less) neurotransmitter release

less

when there is less glutamate released by the photoreceptor because it was __polarized, the (on/off)-center bipolar cells depolarize, and the (on/off)-center bipolar cells hyperpolarize.

hyper, on, off

the more __ there is, the more the neurons will fire

contrast

what is luminance contrast

most retinal ganglion cells are better at detecting differences between the illumination on the center vs the surround of a receptive field

what is lateral inhibition

horizontal cells influence the amount of glutamate that the photoreceptor releases onto the bipolar cell

glutamate has a __polarizing effect on horizontal cells, and horizontal cells have a _polarizing effect on photoreceptor terminals

de, hyper

why are horizontal cells important

they tell the brain what is worth paying attention to by indicating when the contrast the photoreceptor is picking up is not as sharp of a contrast as they thought

damage to what structure causes monocular anopia

optic nerve

damage to what structure causes bitemporal hemianopia

optic chiasm

damage to what structure causes contralateral homonymous hemianopia

optic tract

damage to what structure causes contralateral superior quadrantanopia

Meyer’s Loop

what are the 4 targets of the ganglion cell axons making up the optic nerve

hypothalamus, pretectum, superior colliculus, LGN

what is the primary target of the optic nerve

LGN

what is the purpose of sending information from the optic nerve to the superior colliculus

orientation of head and eye movements (saccades)

what is the purpose of sending information from the optic nerve to the pretectum

reflex control of pupil and lens

what is the purpose of sending information from the optic nerve to the hypothalamus

circadian rhythms

true or false: neurons in the primary visual cortex have a preferred orientation that they will respond more to when presented with it

true

what are the stops in the dorsal stream

V1 - V2 - MT

what does the dorsal stream tell the brain

where something is in the environment

what are the stops in the ventral stream

V1 - V2 - V4

what does the ventral stream tell the brain

high resolution visual information; the what of what you’re seeing

the ventral stream goes from the ____ cortex to the ___ part of the __ lobe

striate, inferior, temporal

the dorsal stream goes from the ___ cortex to the __ lobe

striate, parietal

eyes are always ___, and thus need __ ___

moving, continuous stimuli

eyes move to find:

the greatest contrast

eye movements are attracted to the areas of most ___

detail

what is a microsaccade

fast directed involuntary movement

why do we have microsaccades

to provide continuous sensory stimuli for the retina

without continuous stimuli to the retina, what happens?

perception fades away

what is visual adaptation

after a period of time you have decreased response to constant input

what does visual adaptation help with

ignoring irrelevant stimuli

which of the 3 eye muscle nuclei innervates the contralateral eye’s muscle?

trochlear nucleus (CL superior oblique)

what are the 3 stimuli of the PPRF?

head movement, loud noise, purposive gaze

what are the 5 ways the eyes move?

saccades, smooth pursuit, vergence, vestibulo-ocular, optokinetic

what is optokinetic eye movement

stabilizes gaze to allow us to follow an object in motion when the head remains stable

saccades involve what part of the cortex, and for what purpose?

posterior parietal cortex for locating potential saccade targets

what part of the cortex is responsible for generating horizontal movements?

PPRF (paramedian pontine reticular formation)

what part of the brain is involved in implementing the motor action of the saccade

frontal eye field (FEF)

what are the roles of the basal ganglia in saccades?

gauge appropriateness of and initiate movement for the saccade by decreasing inhibition of the superior colliculus

what is the role of the superior colliculus in a saccade

stimulate appropriate eye movement

convergence is used to focus on (further/closer) objects while divergence is used to focus on (further/closer) objects

closer, futher

what is crucial to perform smooth pursuit?

tracking a target

what are the differences between optokinetic and vestibulo-ocular movements?

vestibulo-ocular is fast and based on head movement, while optokinetic is slow and based on image movement

every 1 degree of head movement …

has an equal and opposite movement response in the eyes