Physiology of Vision

Univarience

describes specialized function of neurons

Electromagnetic spectrum

the different wavelengths of light (humans can only see a small part of it; visible light)

Visible light

the light humans CAN see, ranges from 380nm to 780nm

Vertebrate eye

specialized sensory organ that detects light and enables vision; uses a single lens to focus light on the retina

Optical components

gathering/focusing light, focusing image

Neural components

convert optical image to neural code

Cornea

where light first enters the eye

Aqueous humor

second part of eye that light enters, liquid

Lens

third part of eye that light enters, focuses light on the retina

Vitreous humor

fourth part of eye that light enters, liquid

Retina

final part of eye that light enters, where all photoreceptors are (neural component)

Optic disc

aka the blind spot where the optic nerve exits the eye (lacks photoreceptors)

Fovea centralis

high concentration of cones, area with the sharpest vision

Ganglion cells

carry information from the eye to other parts of the brain, axons converge at the optic disk to form the optic nerve

Optic nerve

formed by all of the axons of ganglion cells

Bipolar cells

connect photoreceptos and ganglion cells

Horizontal cells

connect photoreceptos + bipolar cells, regulate communication

Amacrine cells

connects bipolar and ganglion cells, regulate communication

Photoreceptors

cells that detect light

Rods

type of photoreceptor, let’s us see black and white (only 1type)

Cones

type of photoreceptors, let’s us see colors (3 types)

Convergence

information from many photoreceptors will go to 1 ganglion cell

Synaptic region

where the photoreceptors connect to other retinal neurons

Cell body

part of the neuron that contains the nucleus and most of the cell’s organelles; metabolic center of the cell

Inner segment

packed full of organelles

Outer segment

all of membrane with light sensing materials

Disk membranes (rod)

~1000 disk membranes (intracellular), has rhodopsin (a light sensor)

Membrane infoldings (cone)

rhodopsin-like molecule; increases surface area

Photopigments

light-sensitive molecules that undergo a conformation change when exposed to light

Rhodopsin

light sensing molecule found in rod cells

Excitatory response

when a cell becomes more active after getting a signal

Inhibitory response

when a cell becomes less active after getting a signal

Membrane potential

difference in electrical charge between inside and outside the cell, ~-40mv

Resting potential

the steady electrical charge across a cell’s membrane when it is not sending a signal

Neurotransmitter

chemical messenger to help pass messages to other cells

Dark current

flow of charge in the dark, contributing to resting membrane potential

cGMP gated ion channel

in the outer segment of a rod, channel is closed in light (opens when cGMP is present; only in the dark)

Retinal

made using vitamin A, absorbs light

Opsin

a protein that binds to retinal; different versions allow us to see different colors

11-cis retinal

a form of retinal that binds to opsin

All trans retinal

the shape retinal conforms to after it absorbs light; triggers visual signal

Trimeric G protein (alpha, beta, gamma subunits)

a protein made of 3 parts that helps pass signals inside cells; activated when retinal changes shape

Transducin

trimeric G protein found in rod cells, helps close ion channels by reducing cGMP levels

cGMP phosphodiesterase

an enzyme activated by transducin that breaks down cGMP; closes ion channels

Dark adaptation

sensitivity to light depends on rods; the longer you stay in the dark the better you can see with lower light

Light adaptation

sensitivity to light depends on cones; eyes becoming less sensitive to light to see better

S (blue) cones

peak at 420 nm

M (green) cones

peak at 530 nm

L (red) cones

peak at 560 nm