10/30 Visual System - Dr. Fischer Flashcards
Objectives for Visual Lecture
- Describe the structure of the retina including layers, location of photorecetors, bipolar cells, and ganglion cells.
- Describe the basic circuitry of the retina: photoreceptor to bipolar cell to ganglion cell to lateral geniculate nucleus to visual cortex.
- Describe differences in activation of on-center and off-center bipolar cells.
- Describe the significance of the fovea.
- Define basic differences between the properties of rod and cone photoreceptors.
- Differentiate P and M ganglion cells.
- Describe the pathway from the retina to the lateral geniculate to the visual cortex including laterality of visual field.
- Differentiate simple and complex cells in the primary visual cortex.
- Differentiate and define ocular dominance column, orientation columns, blobs
- Differentiate between the “what” and the “where” pathways from the primary visual cortex.
The Retina is Layered
PHOTORECEPTORS:
RODS & CONES
> > > BIPOLAR CELLS
> > > GANGLION CELLS
> > > LATERAL GENICULATE
NUCLEUS
> > > CEREBRAL CORTEX
Light passes through layers to get to the photoreceptors.
The types of neurons in the retina are very diverse
Circuitry responsible for generating receptive field center responses of ______ .
Circuitry responsible for generating receptive field center responses of retinal ganglion cells.
In the dark, _____ have resting membrane potentials ~__mV;
In the dark, cones have resting membrane potentials ~-40mV;
When cones are depolarized, they release _____.
When cones are depolarized, they release glutamate.
Glutamate excites _____ cells and inhibits ____ cells.
Glutamate excites off-center bipolar cells and inhibits
on-center bipolar cells.
Light ______ cones resulting in ______ release of glutamate.
Light hyperpolarizes cones resulting in decreased release of glutamate.
With less glutamate, ____ cells are disinhibited (released from inhibition) and they depolarize (sign inverting).
With less glutamate, on-center bipolar cells are disinhibited (released from inhibition) and they depolarize (sign inverting).
Depolarization of the on-center bipolar cell causes ____ of glutamate at their terminals which _____ ganglion cells (sign conserving).
Depolarization of the on-center bipolar cell causes release of glutamate at their terminals which excites ganglion cells (sign conserving).
Light hyperpolarizes cones resulting in _____ release of glutamate.
Light hyperpolarizes cones resulting in decreased release of glutamate.
With ___ glutamate, off-center bipolar cells are disfacilitated (less excitatory input) and they
hyperpolarize.
With less glutamate, off-center bipolar cells are disfacilitated (less excitatory input) and they hyperpolarize.
Hyperpolarization of off-center bipolar cells results in less glutamate release at their terminals and ganglion cells decrease their firing rate.
Hyperpolarization of off-center bipolar cells results in less glutamate release at their terminals and ganglion cells decrease their firing rate.
Light hyperpolarizes cones resulting in decreased release of glutamate.
With less glutamate, on-center bipolar cells are disinhibited (released from inhibition) and they depolarize (sign inverting).
Depolarization of the on-center bipolar cell causes release of glutamate at their terminals which excites ganglion cells (sign conserving)
Light hyperpolarizes cones resulting in decreased release of glutamate.
With less glutamate, off-center bipolar cells are disfacilitated (less excitatory input) and they
hyperpolarize.
Hyperpolarization of off-center bipolar cells results in less glutamate release at their terminals and ganglion cells decrease their firing rate.
Receptive Field
the particular region from which a light stimulus will trigger the firing of a photoreceptor.
Simple cells
Neurons in Primary visual cortex that are orientation selective.
Input is from neurons in the lateral geniculate nucleus.
Represents input from multiple retinal ganglion cells and LGN cells that have particular alignment when put together.
Complex cells
Neurons in Primary visual cortex that respond primarily to direction of movement across the receptive field and increase or decrease in contrast.
Input is from multiple simple cells.
Orientation Columns
Organized regions of neurons that are excited by visual line stimuli of varying angles.
These columns are located in the primary visual cortex and span multiple cortical layers.
Blobs
Clusters of neurons interspersed between orientation columns.
Mostly concerned with color.
Ocular Dominance Columns
Stripes of neurons in the visual cortex that respond
preferentially to input from one eye or the other.
Alternate in visual cortex.
Primarily found in layer IV where LGN terminates.
Vision facts
(1) Nearly one-half of the Brain is involved in vision
(2) The human visual system recognizes a multitude of objects with ease. This task is still a “challenge” for computerized imaging systems
(3) The impact of vision disorders in the US was estimated at $51.4 billion in 2007 (The Impact of Vision Problems, National Symposium)
(4) Worldwide, about 39 million people are blind, about 230 million have some kind of vision impairment, and as many as 3.2 billion people have refractive error (need glasses)
(5) 80% of vision problems worldwide are avoidable or curable.
The retina receives an inverted image
Incident light is “focused” by the cornea, restricted by an aperture (the pupil), and focused again by the lens to project upon the retina
Incident light is “focused” by the ______
Incident light is “focused” by the cornea
aperture
an opening, hole, or gap.
Incident light is restricted by an aperture, the _____
Incident light is restricted by an aperture: the pupil
Incident light is focused again by the ____ to project upon the ____ .
Incident light is focused a second time by the lens to project upon the retina
Types of neurons can be distinguished based on…..
(1) morphology
(2) electrophysiology
(3) expression profile (calcium-bufferring proteins, neurotransmitters, neuropeptides, etc.)
The purpose of the fovea:
(1) Responsible for central vision and highest visual acuity
(2) Contains densely packed cone photoreceptors
– but contains NO ROD photoreceptors
= high acuity day-time vision
(3) Blood vessels excluded and inner retinal neurons are physically skewed to the side, hence “a pit” is formed, and light is not distorted by vessels or inner layers of the retina
rods
- high light sensitivity, long integration time
- more photopigment
- high amplification
- low temporal resolution (12 Hz)
- more sensitive to scattered light
- severely affected by quantum noise
cones
- low light sensitivity, short integration time
- less photopigment
- lower amplification
- high temporal resolution (55Hz)
- most sensitive to axial rays (+-15 deg)
- not affected by quantum noise
rod system
- low acuity: not present in the fovea
- highly convergent retinal pathways
- achromatic = one pigment only
cone system
- high acuity: concentrated in the fovea
- almost no convergence in the fovea
- chromatic, 3 types of cones, each with a distinct pigment that is most sensitive to a different part of the visible spectrum
Example of convergence:
The convergence of signals in the rod pathway is very large
Color vision deficiencies:
Congenital
affect photopigments
Color vision deficiencies:
Acquired:
lesions in visual pathways, usually after stroke
Red cone pigment missing
Protanope
Green cone pigment missing,
Deuteranope
only 2 active photopigments
dichromate
no color vision
achromatopsia
Blue cone pigment missingents
Tritanope
Types of neuronal electrical activity in the retina can be segregated to different cell types
Non-spiking and spiking (action potentials) of retinal neurons in different layers
“analog”
more finely graded response,
but becomes “noisy” over distances.
“digital”
more “steps”, but little or no degradation over long distances.
X-chromosomal recessive
Protanopia
Deuteranopia
Blue Cone Monochromacy
