The Retina Flashcards
Light
vision is based on visible light - a band of energy within the electromagnetic spectrum
electromagnetic spectrum (EMS)
a continuum of electromagnetic energy that is produced by electric changes and is radiated as waves
the energy in the EMS can be described by its wavelength - the distance between the peaks of the electromagnetic waves
the eye
light reflected from objects in the environment enters the eye through the pupil (a hole whose size is controlled by the iris) and is focused by the cornea and the lens to form shartp images of the objects on the retina
the retina
the network of neurons that covers the back of the eye and contains photoreceptors (receptors for vision)
2 types of photoreceptors
rods and cones
visual pigments
light-sensitive chemicals that react to light and trigger electrical signals. they are made in the inner segment of the photoreceptor and stored in the outer segment
fovea
a part of the retina that contains only cones. when we look directly at an object, its image falls directly on the fovea
peripheral retina
all of the retina outside of the fovea. it contains many cones and even more rods
macular degeneration
a disease that destroys the cone-rich fovea and a small surrounding area. It creates a blind region in central vision -> when a person looks directly at something, they lose sight of it
retinitis pigmentosa
a genetic degeneration of the retina. It first attacks the peripheral rods and results in poor vision in the peripheral visual field. eventually, the foveal cones are also attacked, resulting in complete blindness.
blind spot
the place in the retina where the nerve fibers of the optic nerve leave the eye. logically, there are no photoreceptors there. the brain fills in the area served by the blind spot with a perception that matches the surrounding pattern
cornea
the transparent covering of the front of the eye. It cannot change place, so it cannot adjust its focus.
lens
can change its shape to adjust the eye’s focus for objects located at different distances The change in shape is achieved by the action of ciliary muscles, which increase the focusing power of the lens (its ability to bend light) by increasing its curvature.
Object far - eye relaxed
if an object is located more than 6 meters away, the light rays that reach the eye are parallel. The cornea-lens combination brings these parallel rays to a focus on the retina at point A.
object near - eye relaxed
if an object is located closer to the eye, the light rays reflected from it enter the eye at an angle, and this pushes the focus point back to point B. If the focus point is B, the light would be stopped by the retina before it reaches point B -> the image on the retina would be out of focus -> the person would see the object as blurred
accomodation
a change in the lens’ shape that occurs when the ciliary muscles tighten and increase the lens’ curvature so that it gets thicker. The increased lens curvature increases the bending of the light rays passing through the lens -> the focus point is pulled from B to A -> a sharp image is created on the retina
refractive errors
errors that can affect the ability of the cornea and/or lens to focus visual input onto the retina
presbyopia
age-related loss of ability to accomodate to close objects due to hardening of the lens and weaking of the ciliary muscles -> reading glasses
myopia (nearsightedness)
an inability to see distant objects clearly. It occurs when the optical system brings parallel rays of light into focus at a point in front of the retina -> the image that reaches the retina is blurred
refractive myopia
the cornea and/or lens bends the light too much
axial myopia
the eyeball is too long
Visual transduction
occurs at photoreceptors (rods and cones)
visual pigments
have 2 parts: a long protein called opsin (whose structure determines which wavelength of light the pigment absorbs) and a much smaller light-sensitive protein called retinal
isomerization
when incoming light hits the retina, the visual pigment molecule absorbs it and causes the retinal to change its shape
isomerization creates a chemical chain reaction which ultimately hyperpolarizes the photoreceptor.
-> causes less glutamate release at the synapse (graded potential)
dark adaptation
the process of increasing sensitivity in the dark when entering a dark room just after having been in a lit room
dark adaptation curve
a function that is used to measure the process of dark adaptation. It relates sensitivity to light to time in the dark, beginning when the lights are extinguished.
dark-adapted sensitivity
the sensitivity at the end of dark adaptation. The dark adaptation is about 100k times greater than the light-adapted sensitivity
light-adapted sensitivity
the sensitivity measured in the lit room.
sensitivity increases in 2 phases
1) rapidly for the first 3-4 minutes after light is extinguished
2) then it stays relatively constant for a short time
3) at about 7-10 minutes after entering, it begins to increase again and continue to do so until the person has been in the dark for about 20-30 minutes
rod monochromats
people who have no cones due to a rare genetic defect
rod-cone break
the time point when the rods begin to determine the dark adaptation curve instead of the cones
Visual pigment bleaching
after isomerization of a visual pigment, the retinal bends and then separates from the opsin part of the pigment. This causes the pigment to become lighter in color
Visual pigment regeneration
the process of reforming the visual pigment molecule by the retinal returning to its bent shape and becoming reattached to the opsin. The increase in visual pigment concentration that occurs as a result of visual pigment regeneration in the dark is responsible for the increase in sensitivity during dark adaptation
detached retina
a condition in which the retina becomes detached from the pigment epithelium (a layer that contains enzymes necessary for visual pigment regeneration)
Purkinje shift
the enhanced perception for short wavelengths during dark adaptation. It occurs because the rods are more sensitive to short-wavelength light than cones
visual pigment’s absorption spectrum
the amount of light absorbed by a visual pigment as a function fo the light’s wavelengths. The rod pigment (rhodopsin) absorbs best at wavelengths of 500nm
horizontal cells
connect different photoreceptors, so that signals can travel between them
amacrine cells
receive inputs from bipolar and other amacrine cells and send output to bipolar, amacrine and retinal ganglion cells
neural convergence
the phenomenon of several neurons synapsing onto a single neuron
why are rods more sensitive than cones?
1) it takes less light to generate a response from an individual rod than an individual cone
2) the greater convergence of rods makes them more likely to trigger an action potential in the ganglion cells and transfer a signal to the brain
visual acuity
the ability to see details. The cones have more acuity than the rods because of their lower convergence
receptive field
the region of the retina that must receive illumination in order to obtain a response (inhibitory or excitatory) in a neuron
center-surround receptive fields
ganglion cells have center-surround receptive fields, whose centers respond differently to light than the areas surrounding the centers. They are arranged like concentric circles in a center-surround organization
excitatory area
an area of the receptive field, which increases firing in response to light
inhibitory area
an area of the receptive field, which decreases firing in response to light
ON- center cell
has an excitatory-center, inhibitory-surround receptive field
OFF-center cell
has an inhibitory-center, excitatory-surround receptive field
Center-surround antagonism
a neuron responds best to a spot of light that is the size of its excitatory center of its receptive field. If it is smaller, the excitation would be smaller. If it is bigger, inhibition would also occur because of the inhibitory-surround.
lateral inhibition
the horizontal and amacrine cells transmit inhibitory signals laterally across the human retina. Lateral inhibition causes center-surround antagonism
edge enhancement
an increase in perceived contrast at borders between regions of the visual field
chevreul illusion
at the border between 2 colors (a lighter one and a darker one), the lighter color seems lighter than it is; and the darker color seems darker than it is
diffuse bipolar cell
a bipolar cell that receives input from many photoreceptors (e.g., in peripheral vision they can take input from ~50 photoceptors)
midget bipolar cell
a small bipolar cell in the fovea that receives information from a single cone
ON bipolar cell
depolarizes in response to an increase in light captured by the cone (lower postsynaptic glutamate level). In a photoreceptor-ON-bipolar cell synapse, glutamate is inhibitory
OFF bipolar cell
hyperpolarizes in response to an increase in light captured by the cone (higher postsynaptic glutamate level)
P ganglion cell
receives input from midget bipolar cells; sends output to the parvocellular layer of the lateral geniculate nucleus (LGN)
M ganglion cell
receives input from diffuse bipolar cells. Sends output to the magnocellular layer of the LGN
Koniocellular cell
some of them receive input from S-cones
