Phototransduction & Visual System Physiology Flashcards
Rod-mediated vision is called _______ vision and cone-mediated vision is called _______ vision.
Scotopic
Photopic
This type of vision occurs when both rods and cones are activated by the light-levels of the environment.
Mesopic
T/F. People who lose rod-vision are legally blind.
False. People who lose cone-vision are legally blind. People who lose rod-vision experience night-blindness.
(CONES/RODS) can respond to a single photon. (CONES/RODS) need about 100 photons to respond.
Rods
Cones
Many rods converge into a single ________ cell. Many of these cells contact a single amacrine cell. This allows for the highest levels of sensitivity but sacrifices resolution.
Bipolar
One cone directly contacts only one _______ cell. This arrangement allows for the best resolution but sacrifices sensitivity.
Bipolar
Photoreceptor sensory transduction system is unique in that stimuli (light – photons) cause (HYPERPOLARIZATION/DEPOLARIZATION) of the cells and (MORE/LESS) release of the neurotransmitter glutamate. In conditions of darkness, the cells are consistently (HYPERPOLARIZED/DEPOLARIZED) and release (MORE/LESS) glutamate. This is what we call the “dark current”. Light suppresses the dark current.
Hyperpolarization
Less
Depolarized
More
Why do neuronal cells of the retina use graded potentials rather than action potentials (except in ganglion cells)?
Graded depolarization leads to increases in NT release at their synapses with ganglion cells. Therefore, they’re always releasing varying streams of glutamate, but that concentration is either very small (like a trickle) or very large (like a flood). If these cells relied on APs, then glutamate release would be binary, either it occurs or it doesn’t.
Bipolar cells have two subtypes, which are…
ON-center bipolar cells
OFF-center bipolar cells
This type of bipolar cell has activation of a photoreceptor in the center of this bipolar cell’s receptive field, which causes depolarization of the bipolar cell.
ON-center bipolar cells
ON-center bipolar cells work because they have a special version of the glutamate receptors, called ________. This receptor is a Gi GPCR-metabotropic receptor. When glutamate binds to this receptor it decreases the cation influx by closing the _______ _______ channels. Therefore, high concentrations of glutamate closes the channel and the ON-center bipolar cells remain inactivated, but low concentrations of glutamate allows the channel to open and the ON-center bipolar cell will depolarize.
mGluR6
cGMP-gated Na+
***Remember, when there is light then LESS glutamate is released. This is when the ON-center bipolar cells work.
This type of bipolar cell has activation of a photoreceptor in the center of tis bipolar cell’s receptive field, which causes hyperpolarization of the bipolar cell.
OFF-center bipolar cells
Put the following steps involving the ON-center bipolar cells in order:
A. Less Gi signaling.
B. Depolarizes the cell.
C. Light (photons) decrease the presence of glutamate.
D. Less activation of mGluR6 (metabotropic receptor) on the ON-center bipolar cell.
E. Less glutamate around.
F. Results in an increase in cation influx into the bipolar cell (opens cGMP-gated Na+ channels).
1) C.
2) E.
3) D.
4) A.
5) F.
6) B.
OFF-center bipolar cells express the normal, common version of the glutamate receptor known as _________ receptors (ionotropic). When glutamate is present, it binds to the receptor, the ion channel opens, and the cell depolarizes like normal.
Non-NMDA
This is a region of the visual field. When visual stimuli is presented to a neuron within this then the activity of the neuron changes.
Receptive field
There are lots of arborization of ganglion cell dendrites in the…
Inner Plexiform Layer
Ganglion cells are also ON-center and OFF-center varieties (whichever kind of bipolar cell it’s connected to). If the associated bipolar cell is depolarized, it releases ________ to subsequently depolarize the ganglion cell.
Glutamate
Ganglion cells express the typical glutamate receptors, either _______ or _______, that when activated with result in depolarization.
NMDA
Non-NMDA
T/F. Ganglion cells use action potentials rather than graded potentials.
True
Ganglion cell axons come the fibers of the…
Optic Nerve
In the cortex, the ganglion cells will themselves release ________.
Glutamate
These cells are generally inhibitory (release glycine or GABA) and suppress nearby activity. They are particularly important in allowing us to see in low-light conditions to enhance edges, shadows, and contrasting areas of luminance in our environment.
Amacrine cells (and other horizontal cells)
***What we term “rod-amacrine cells” are greatly utilized by stimulation of rod-photoreceptors and rod-bipolar cells pathways.
This is the direct target of the retina and is where retinotopic organization is maintained. The signals from the two eyes are kept apart at this level.
Lateral Geniculate Body
What are the functions of the Lateral Geniculate Body?
1) Control the motions of the eyes to converge on a point of interest.
2) Control the focus of the eyes based on distance.
3) Determine relative positions of objects to map them in space.
4) Detect movement relative to an object.
This creates a map of visual space to activate appropriate motor responses required to move the eyes into their intended position within the orbits. It coordinates head and eye movements to visual targets.
Superior colliculus
This is responsible for the reflex control of the pupil and lens, and sends projections to Edinger-Westphal then on to Ciliary Ganglion.
Pretectum
There are a small number of fibers that branch off the ______ _____, forming the retinohypothalamic tract, and terminate in the supraoptic, suprachiasmatic and paraventricular nuclei of the hypothalamus.
Optic tract
Visual input to the _________ drives the light-dark entrainment of neuroendocrine function and other circadian rhythms.
Hypothalamus
This contains neurons that respond selectively to the direction of a moving edge. It tracks the motion across a scene in terms of directionality and background/foreground context. It ignores color.
Medial temporal area (MT)
***Also called Visual association cortex V5
This consists of several small nuclei and function in advanced visual processing. It has an important role in eye movements of compensation and pursuit, particularly in alternation with saccadic-type eye movements responding to prolonged watching of large field motion.
Accessory optic system (AOS)
This is a striate cortex (“striped”) and constructs local image features including size, orientation, local direction of movement, and binocular disparity. It is used to identify “what” and “where”. Located on occipital lobe and has six main cell cortical layers.
Primary visual cortex V1
The major job of ______ is to identify the edges and contours of objects.
V1
In this area, depth perception occurs by analyzing the disparities between the two eyes, i.e, 3D stereograms.
Visual association cortex V2
In this area, it identifies that motion is occurring.
Visual association cortex V3
In this area, it does complete processing of color inputs.
Visual association cortex V4
These are in the primary visual cortex (V1) and they span all 6 layers of the cortex. They are a slab of cells that preferentially respond to input from one eye or the other (alternate right and left eye).
Ocular dominance columns
***Mapped in stripes (white is left eye, dark is right eye)
These are in the primary visual cortex (V1) and they span all 6 layers of the cortex. They are an organized region of neurons that are excited by visual line stimuli of varying angles. They’re oriented perpendicular to the cortical surface.
Orientation columns
***They can see things horizontal, diagonal, or vertical (depends on objects). Mapped in swirls, looks like a heat map.
These are in the primary visual cortex (V1) and they span all 6 layers of the cortex. They are an organized region of neurons that are sensitive to color assembled into cylindrical shapes. All 3 color-coding cones are required for accurate color detection.
Blobs
***Cytochrome oxidase stain shows blobs.
Recall that V1 as a primary visual cortex has a typical 6-layer arrangement of cells. The output as they relate to color projects to ______ where they finally become integrated to give you an image of all colors that make up your environment. They do so without regard to any motion of a colorful object. So, ______ is color and no motion, while _______ is motion with no color.
V4
V4
V5
In _________, the three types of cones in the retina function normally. It is damage to specific extrastriate cortical areas that renders the patient unable to see information suppled by the retina. In contrast, _______ _______ is a result of improper functioning at the level of the cones themselves.
Achromatopsia
Color blindness
These cells are intrinsically photosensitive and constitute a third class of photoreceptors in the mammalian retina. They do not project to the visual cortex, which makes them known as the “non-image-forming light-responsive system”. Those who experience shifts in light/dark cycles have been shown to benefit to light-therapy due to the non-image forming light-sensitive system input to the regulation of circadian rhythms, sleep, and mood.
Melanopsin Ganglion cells (MG)
The visual areas beyond the striate complex are broadly organized into two pathways: a ________ pathway that leads to the temporal lobe and a ________ pathway that leads to the parietal lobe. Both arise from _______ and pass through _______ but target different areas in their respective streams.
Ventral
Dorsal
V1
V2
The ________ pathway, also called the “what” pathway, arises from V1 and goes through ______ then ______, which leads to the temporal lobe.
Ventral
V2
V4
This pathway is primarily involved in interpreting images (recognizing or copying shapes, forms, faces) and complex patterns. Selectively activated by shape, color, texture, and object recognition.
Ventral Pathway
The _______ pathway, also called the “where” pathway, arises from V1, passes through _______, _______, and the ________ area before projecting toward the parietal lobe.
Dorsal
V2
V3
MT/V5
This is the primary path associating vision with movement. You also complete motor actions based on visual input through this pathway. Selectively activated buy directionality and speed of movement.
Dorsal Pathway
In this disease, a person can see an object, but is either unable to copy it or identify what it is. They cannot see parts of an object as contributing to its whole or cannot interpret, understand, or assign meaning to objects. Clinically, this appears as an inability to construct or draw visual stimuli or they cannot recognize a picture of an object.
Agnosia
This is a type of agnosia that results in the ability to identify a face as a face, recognize its parts, and detect facial expressions indicative of emotion but cannot recognize a particular face as belonging to a specific person.
Prosopagnosia
Damage to this area is a classic cause of agnosia or prosopagnosia due to the interruption of the ventral stream.
Inferior Temporal Lobe
An infarction in the medial temporal area (MT) can result in a condition known as ________ ________ or “motion blindness”. There are deficits in visualization of moving objects. Visual acuity and recognition/naming of objects, faces and colors are normal.
Cerebral akinetopsia
The major job of ______ is to identify whether motion is occurring.
V3a
This tracks motion across a scene in terms of directionality and context of the background and foreground. Does so by containing neurons that selectively respond to the direction of a moving edge.
MT/V5
If a patient KNOWS motion is occurring, but can’t track it, then where is the lesion?
MT/V5
***V3a would have been right if the patient did NOT know that motion was occurring and couldn’t track it.
When there is ______ damage, there is subconscious awareness of colors, but they are unidentifiable to you. Color processing remains separate from each other.
V1
***See neither color nor butterfly – do NOT know you can’t see color
In ______ damage, it integrates the color information from V1 and produces awareness of all colors that make up your environment. This is at a conscious level.
V4
***See butterfly but not color – you DO know that you can’t see color, and can still distinguish objects
When you lose rod-vision, it results in…
Night-blindness
When you lose cone vision, it results in…
Legal blindness
