Visual System Physiology Flashcards
What color is RPE
. Black pigment melanin
. Absorbs light not captured by photoreceptors preventing light form being reflected back to photoreceptors that would dull an image
Components of photoreceptors
. Outer segment: light sensitive portion where signal transduction takes place
. Inner segment: part of cell w/ nucleus and metabolic machinery
. Synaptic terminal: part of cell that transmits the electrical signal to next neuron in pathway
Photopigments
. Outer segments of rods and cones contain membranous discs that have high conc. Photopigments (rhodopsin in Rhodes, other names in cones)
. Disc structure enhances SA available to inc. photopigments available
. Each pigment has small light-absorbing (retinal) molecule attached to large protein (opsin)
. 4 types of pigment (1 in rod, 3 in cones)
. Retinal molecule is the same in all 4 pigments (aldehyde of vit. A) and only opsin proteins differ
Opsin protein function
Filtering action that determines which wavelengths of light at filtered and which are absorbed by retinal in a given photoreceptors
Phototransduction
. Mechanism where light energy is converted to an electrical signal
. Absorption of light triggers cascade of events that leads to altered ion. Channel activity across cell. Membrane of photoreceptors
Retinal response to light
. Retinal in dark exists as 11-cis configuration
. In dark it fits into binding site w/in opsin molecule and inhibits opsin activity
. When light strikes retinal molecule, retinal undergoes conformational change to all-trans-retinal
. This is only light dependent step in vision
. Retinal comes off opsin and diffuses away
. Removal of inhibitory retinal activates opsin molecule
Phosphodiesterase activation
. Activated opsin binds to a G-protein (transducin) which activates phosphodiesterase
Degradation of cGMP
. Phosphodiesterase hydrolzyes cGMP dec. its intracellular conc.
.cGMP levels are high in dark and low in bright light
Closure of cation channels
.cGMP-activated cation channels in photoreceptors membrane are open in dark due to cGMP binding
. Channels closed by dec. cGMP levels in response to light
. Results in reduction in depolarizing current mainly carried by Na
Hyperpolarization
. Reduction of depolarizing current results in hyperpolarization of cell
Decreased Ca. Channel activity
. Hyperpolarization of photoreceptor membrane spreads to synaptic terminal
. VG Ca channels in membrane are open when photoreceptor is depolarizer and close when it is hyperpolarized
Decreased NT release during photo signal transduction
. Hyperpolarization-induced closure of Ca channels dec. Ca influx
. Results in dec. Ca-dependent NT release from synaptic terminal
. In dark retinal goes back to original conformation, cGMP inc., channels open, and membrane potential is restored to -40 mV and NT release is high
Rhodopsin
. Rod photopigment
. Doesn’t discriminate wavelengths to rods response to all visible light as shades of gray
. Have huge convergence resulting in poor signal sharpness but higher sensitivity
. More sensitive to scatter light
Cone photopigments
. Respond to limited range of wave lengths to varying degrees for that specific wavelength
. Each cone has little convergence to inc. image sharpness but also makes it harder to reach AP threshold
. More sensitive to direct rays
Rod vs cone temporal resolution
. Rods: slow response, long integration time
. Cones: faster esponse, short integration time
Color blindness
. Inability to perceive differences btw some colors that most people can distinguish
. Usually genetic, more common in men
. Major causes: absence of cones, absence of 1 photopigment, non—functioning photopigment, alteration in spectral sensitive to photopigment
Red-green color blindness
. Most common
. Mutation in pigment of green photoreceptors that alters spectral sensitivity (shift to red end of spectrum)
. Individuals have difficulty distinguishing colors in red-yellow-green part of visual spectrum
Do bipolar cells fire APs?
. No, only have graded potentials
. Primary cells connecting the photoreceptors to ganglion cells
First cells in visual pathway to fire APs
. Ganglion cells
. Axons form optic n. And transmit visual into to brain where more complex integration of visual info occurs
What do bipolar cells respond to?
. Characteristics of visual images (color, intensity, shape, and movement)
Horizontal and amacrine cell function
. Interconnect photoreceptors
What control rate of firing for APs in ganglion cells?
. Patterns fo graded potential generated by lower order cells
. Allows considerable integration before signal leaves retina
Modulation of ganglion cell firing
. Ganglion cells fire APs at all times even in the dark
. Only thing that changes in frequency of firing determined by graded potentials impinging on ganglion cells
. Each ganglion cells respond to light in a defined circular area of retina (receptive field)
. Each ganglion cell monitors its receptive field
. Size of receptive field is important component of visual acuity (large field - high sensitivity to low light but low acuity)
On-center signaling
. Receptive field has circular center and area around center circle (the surround)
. On-center cells fire infrequently in the dark
. Respond to light signal to the center of the field w/ inc. in firing
Off-center signaling
. Cells fire when light shines on the surround
. Firing inhibited by light shining on center of field
Importance of on a dm off center signaling
. Provides parallel pathways for the processing of visual info
. The visual system is much better at discriminating contrasts than absolute light intensities
What is absolute light intensity determined by?
Ambient illumination
Thalamus in visual pathway
. Optic n. Fibers pass through lateral geniculate nucleus in thalamus to get to other parts of brain
Primary visual cortex
. Neurons in one subdivision of visual cortex respond only to stimuli oriented in a particular direction in visual field
.in another subdivision the neurons may respond primarily to movement across a visual field
. Other neuronal groups in cortex will respond mainly to color
. Complex interaction of many neurons in this cortex together w/ neurons in visual assoc. areas lead to visual picture
