~Chapter 2 - Lectures Flashcards
What are Dendrites?
Dendrites are the receiving part of the neuron. Incoming signals arrive here. Dendrites gather inputs
What does the cell body do?
The Cell Body (Soma) provides life support for the cell/neuron
What does the Axon do?
Axons transmit signals from one location to another.
What do the Terminal Boutons do?
The Terminal Boutons pass signals from one neuron to the next in the chain of info processing.
What signals do Neurons process and transmit?
The Bioelectric Signal
What is the Resting Potential of a neuron/cell?
The Resting Potential of a neuron/cell is about -70mV
What does it mean when a cell is at its Resting Potential?
When the cell/neuron is not stimulated or involved in passage of an impulse. This is when the neuron is just sitting there not doing anything
How do Neurons derive their electrical properties?
By separating ions
When a cell is in a resting state, where is most Na+?
Outside
When a cell is in a resting state, where is most K+?
Inside
How is separation of ions maintained?
By ATP-driven ion pumps or exchange mechanisms (e.g. Na+/K+ ATPase).
What does it mean if the cell is Polarized?
It means the cell is at rest, there is a separation of ion types, like the North and South poles.
What does Hyperpolarization mean?
Hyperpolarization is a change in a cell’s membrane potential that makes it more negative, it becomes even more polarized. (Example: -70mV → -90mV)
What does Depolarization mean?
Depolarization is a change in a cell’s membrane potential that makes it less negative, it becomes less polarized. (Example: -70mV → -20mV)
What are action potentials?
Action potentials are the way neurons/cells communicate with each other.
-AKA – spikes, nerve impulses
How do action potentials propagate?
Action potentials propagate when the axon becomes selectively permeable to ions
What are the steps of an action potential?
Sodium flows in → produces depolarization to about +40mV → Potassium flows out → re polarization/hyperpolarize → Returning back to its regular levels
What Causes Selective Permeability?
Ion channels
What are ion channels?
Little tunnels in the membrane that let only 1 type of ion pass through
How do neurons “talk” to each other?
Neurons talk to each other through synaptic transmission
Where does the sending neuron release chemical neurotransmitter from?
The sending neuron releases chemical neurotransmitter from its terminal boutons/synaptic boutons.
What is the synapse?
The synapse is the tiny gap between the two neurons,
What do neurotransmitters bind to after passing through the synapse?
Neurotransmitters will bind to receptors on the receiving neuron’s dendrites and cell body.
What are the two types of Neurotransmitters?
Excitatory and Inhibitory
What are two types of Excitatory Neurotransmitters?
Acetylcholine & Glutamate
What does an Excitatory Neurotransmitter do?
Causes a cell to become less negatively charged when these bind to their receptors, it produces Depolarization.
What do Excitatory Post-Synaptic Potentials (EPSP) do?
EPSPs increase the likelihood that the neuron will fire an action potential
What increases the likelihood that the neuron will fire an action potential?
Excitatory Post-Synaptic Potentials (EPSP)
What is a type of Inhibitory Neurotransmitter?
GABA
What does an Inhibitory Neurotransmitter do?
Causes a Cell to become more negatively charged (Hyperpolarization)
What do Inhibitory Post-Synaptic Potentials (IPSP) do?
IPSPs decrease the likelihood that the neuron will fire an action potential
What decreases the likelihood that the neuron will fire an action potential?
Inhibitory Post-Synaptic Potentials (IPSP)
What happens if the summated inputs are greater than the threshold of the cell at the axon hillock?
An action potential is produced.
How does a neuron handle all the inputs from its many dendrite and receptor sites?
All of the inputs that are coming into a neuron are summed together.
What Post-Synaptic Potential tries to make the cell spike?
EPSP’s pool together to try to make the cell spike
What Post-Synaptic Potential tries to stop the cell from spiking?
IPSP’s pool together to stop the cell from spiking
How do neurons make calculations on incoming signals?
Summation of signals
Can EPSP’s and IPSP’s be combined?
Yes. Neurons can sum different signals and weight them over their inputs and over time, and combine EPSPs and IPSPs. So they’re performing calculations before they send an output down to another area of the brain.
What does an Action Potential being Non-Attenuating mean?
The Action potential remains the same size as it propagates from the cell body to the terminal boutons.
- This is why you can have very long axons.
- Example: You can have motor neurons leaving your spinal cord, going all the way down your leg, allowing you to wiggle your toes.
What is All-or-None Rate Coding?
The Action potential remains the same size no matter how intense the input. Instead of creating bigger action potentials, the neuron creates more action potentials for stronger stimuli.
What is the Refractory Period?
The period where the neuron cannot fire one action potential after it has just fired an action potential. The minimum time period after one action potential fires before the next one can fire
How long is a refractory period?
A refractory period is typically about 1 millisecond
What is a typical neuron firing rate?
100 spikes/second
What is the upper limit on a neurons firing rate?
500-800 spikes/second
What is Spontaneous Activity?
Many neurons fire action potentials at a low rate even when there is no input. This occurs in most neurons
What is Spontaneous Firing?
The period before and after a stimulus is presented, the neuron is still firing action potentials, this is called spontaneous firing.
- Spontaneous firing allows the neuron to signal both increases and decreases in activity.
- Generally, this is the way that neurons, especially in the cortex, will act.
What is light?
Light can be described as both a particle (known as a photon) and a wave
What is the distance of a Wavelength of a light?
The Wavelength of a light is the distance of one complete cycle of the wave.
Can humans see all wavelengths of light?
No. Humans can only see a certain wavelength of light that we call Visible Light
What are the wavelengths of Visible Light?
Visible light has wavelengths from about 400nm (blue-ish light) to 700nm (red-ish light). Wavelengths longer and shorter than this are invisible to us.
How is wavelength related to the ability to view colour?
Wavelength of light is related to its perceived color.
What is the Electromagnetic Spectrum?
The electromagnetic spectrum is the range of frequencies of electromagnetic radiation and their respective wavelengths and photon energies
What kinds of things have shorter wavelengths than Visible Light?
Gamma Rays, X-Rays, and Ultra-Violet Rays
What kinds of things have longer wavelengths than Visible Light?
Infrared Rays, Radar, Broadcast Bands, and AC Circuits
How does white react to light/photons?
White absorbs little light, reflects all wavelengths of light equally, and reflects much of the light.
How does black react to light/photons?
Absorbs much light, about equally across wavelength. Reflects little to none of the light.
How does green react to light/photons?
Reflects green light (reflects medium wavelength more than short or long wavelengths)
How do coloured objects react to light/photons?
Coloured objects selectively reflect different wavelengths more than others
Why is the eye compared to a camera?
The eye works like a camera, using a lens to focus light onto a photo-sensitive surface at the back of a sealed structure
What are the optics at the front of the eye?
The Cornea and Lens
What focuses objects in the environment onto the retina?
The optics of the eye
What is Accomodation?
When the eye changes its optical properties. The lens becomes fatter, which then bends the light more, which allows the retinal image to come into focus right at the retina. It brings the focal point from behind the eye to right on the retina.
What optic of the eye provides the most focusing power?
The Cornea (80%)
What provides 80% of the eye’s focusing power?
The Cornea
Can the Cornea change shape?
No
What does the Lens do?
The Lens Provides ~20% of the focusing power. The Lens can change shape which then changes the focusing power.
What does it mean when the Lens is thin?
Less light bending/less refraction (for focusing on far objects)
What does it mean when the Lens is fat
More light-bending (for focusing on near objects).
What is the shape of the Lens controlled by?
The Ciliary muscles, a ring-shaped sphincter muscle
What happens when the Ciliary muscle is relaxed?
When the Ciliary muscle is relaxed, it creates a circle that has a large circumference. This creates a lot of tension on the zonule fibres, and it pulls the lens into a flattened thin shape.
Contracted Ciliary muscles = ?
Fat Lens
What happens during Accommodation?
During accommodation, the ciliary muscle contracts, and the sphincter muscle becomes a circle of a smaller circumference, which makes the zonule fibers slack, which then allows the lens to bunch up into a thick round shape.
What is Presbyopia?
The Ciliary muscles can only contract so far, and the Near-point moves farther away, as the Lens gets harder with age
What is Presbyopia?
The gradual loss of the eyes’ ability to focus on nearby objects, near point moves farther away. Bifocal glasses are a solution.
What is the maximum contraction (fattest Lens)?
The near point, 10-20cm
What is Nearsightedness?
Myopia. Sees near objects clearly, while objects in the distance are blurred. An image of a distant object becomes focused in front the retina.
What is Farsightedness?
Hyperopia. Sees far objects clearly, but near objects are blurred. The result of the visual image being focused behind the retina rather than directly on it.
Where are the photoreceptors in relation to the pupil?
The photoreceptors are furthest away from the pupil.
What is the Retina?
A flat structure that lays against the back of the eye.
Where does transduction occur?
Transduction occurs at the Retina in the photoreceptors in the rods and cones.
Where does Sensory Transduction occur?
Sensory transduction occurs in the photoreceptors in the Rods and Cones
What causes hyperpolarization in the photoreceptors?
Light
Where on the photoreceptors are Opsins located?
On the Disc Membranes.
Rods have ___ and Cones have ___?
Rhodopsin // Opsin
Which Opsin-type have sizes?
Cone Opsin’s, they come in small, medium, and large (S-, M-, L-)
Where are on the photoreceptors is the action of transduction started?
The Opsins
What an Opsin?
An Opsin is an opsin protein (7TM) that has a chromophore called retinal covalently bound to its seventh transmembrane domain.
What happens when light strikes the Retinal?
When light strikes the retinal, it undergoes photo Isomerization and changes the shape of the rhodopsin. The kink in the resting case retinal straightens out.
What does the activated Opsin allow?
Activated rhodopsin has a conformation (shape) that exposes a binding pocket which interacts with the G protein Transducin (Gαt)
Why is the Active shape of an Opsin important?
This active shape of an Opsin is what allows it to interact with transducin, which starts the phototransduction cascade.
What are the steps of the Transduction Cascade?
Light-activated Opsin activates Transducin → the Transducin activates an enzyme called Phosphodiesterase → which decreases the concentration of a cytoplasmic molecule called cGMP, which is responsible for keeping cGMP-gated channels open → the cGMP channels close → the closing of the channels causes hyperpolarization.
What is the function of cGMP
cGMP acts kind of like a doorstop, keeping the ion channel open, allowing Na+ to come in
What happens when the concentration of cGMP decreases
cGMP-gated channels close, preventing Na+ from entering
What is happening in neurons in the dark?
Ions are flowing through and cGMP doorstop is functioning to hold the ion channel open. There is lots of influx of Na+. The membrane potential is relatively depolarized
What is happening in neurons in the light?
The cGMP doorstop is removed and the ion channels close, preventing Na+ from coming in. You have relative hyperpolarization.
Who surmized that we could detect a single photon of light?
Selleck Heckt
What happens when 100 photons of light are flashed at the eye/
About 50% is reflected at the corneal surface, only 50% getting through → Some are absorbed in the vitreous humor → Only about 7 photons make it to the retina → Each photon is only going to hit a single photoreceptor
Why are we able to be so sensitive to light?
Because a single isomerization of retinal (which activates rhodopsin) starts an enzymatic cascade. (Like a snowball going downhill gathering power as it goes)
The periphery contains low density of ___?
Cones
The highest density of Cones is located in the ___?
Fovea
Where in the eye are there no Rods?
The Fovea
When something is in your center view, where is the image falling?
Onto the Fovea
Where are Rods the densest?
In the periphery
What does eccentricity mean?
The different eccentricities refer to how far away from the center of the eye, for example, the Fovea is at 0 degree eccentricity
What is the makeup of Rods and Cones in the Near periphery?
There are very large cones and tightly-packed Rods
What is the Near periphery?
The area closet to the center of the eye, closer to the nasal region
What is the Far periphery?
The area furthest from the center of the eye, closer to the temporal region
What is the makeup of Rods and Cones in the Far periphery?
Both the Rods and Cones are larger, but the Cones are quite sparse, and there are more Rods.
What is Macular Degeneration?
An age-related disease that causes loss of central vision. Initial degeneration of the macula
What is the most common blinding disease in the elderly in Western world?
Macular Degeneration
What is Retinitis Pigmentosa?
A heritable disease that causes initial degeneration of rod photoreceptors. It effects the periphery first
What is the Optic Disc?
The Optic Disc is where all of the ganglion cell axons gather together and leave the retina, like an exit-ramp on a highway
Are there photoreceptors on the Optic Disc?
No
What causes the blind-spot in the eye?
The Optic Disc
Why can we sometimes still see things that fall onto the blind-spot in our eyes?
With more complex stimuli, the rest of your visual system fills in what it thinks is supposed to be there. This is a good example of Top-Down Perception.
What is Dark Adaptation?
The increase in sensitivity that occurs when illumination is reduced from light to dark
Why do we need dark adaptation?
In a normal cycle of day and night, the illumination at the earth’s surface varies over 11 orders of magnitude, making dark/light adaptation fundamentally important to the normal functioning of the vertebrate visual system
What method works best when trying to get a Psychophysical quantification of dark adaptation?
The Method of Adjustment, because this method is very quick
Are there any places on the Retina that contain no Rods?
Yes, the Fovea
What does it mean if you are a Rod Monochromat?
A genetic mutation that stop the cones from working, the retina contains only functioning rods
What does the Two Stage Dark Adaptation Curve experiment demonstrate?
Sensitivity increases for 3-4 mins then levels off .Sensitivity increases again after 7-10 mins it becomes very easy to detect dim lights as they are flashed. This is making use of both rods and cones
What happens to the sensitivity of Cones in the Cone Adaptation Experiment?
Test spot in the Fovea only,
sensitivity increases for 3-4 minutes, then levels off, and doesn’t get any better than this.
What happens in the Rod Monochromat experiment?
Sensitivity gradually increases over 20 minutes
Can Retinal reset itself?
No, it needs to be removed from the Opsin, and new, fresh Retinal needs to be put in its place
What happens when old Retinal separates from the Opsin?
This causes Pigment Bleaching, where the Retinal goes from dark purple to orange to yellow to transparent (if no new Retinal is being put in)
What is Pigment Bleaching?
When retinal absorbs light, it separates from the opsin and the colour of the retina changes from red to orange to yellow to transparent
What needs to happen before a bleached molecule can absorb light again?
The Retinal must reattach to the Opsin
What is the Retinoid Cycle?
The energy-intensive resetting process of the Retinal
What is Pigment Regeneration?
Before bleached molecule can absorb light again, retinal must reattach to the opsin -enzyme dependent (pigment epithelium)
Do Rods or Cones regenerate their Opsins with fresh Retinal faster?
Cones
How long does it take for Cones to regenerate their Opsins with fresh Retinal?
6 minutes
How long does it take for Rods to regenerate their Opsins with fresh Retinal?
30 minutes
Who developed Retinal Densitometry?
William Rushton
What does Retinal Densitometry do?
Measures the visual pigment in living subjects
Why do cats eyes flash when you take a photo of them with flash on?
Because Cats have a reflective layer at the back of their eye called Tapetum
Why does red-eye occur in photos?
Red-eye that occurs in flash photography is light reflected back towards the camera from the back of the retina.
What occurs in Retinal Densitometry?
Light is shone into the eye using an ophthalmoscopic apparatus and measures how bright the reflection is.
What colour is a Dark-Adapted Retina?
Dark purple
How will light react to a Dark Retina?
Light is going to be absorbed quite a bit, and little light is reflected out of the eye
How will light react to a Bleached Retina?
Absorbs little light, so more light is reflected out of the eye
Cones are ___ cells that are specialized for ___ situations
Low sensitivity // high intensity
Rods are ___ cells specialized for ___ situations
High sensitivity // low intensity
What sensitivity occurs when going from Rod-vision to Cone-vision?
Spectral Sensitivity
In what colour does Spectral Sensitivity occur in Cones?
Yellow
In what colour does Spectral Sensitivity occur in Rods?
As long as you make the test spot dim, you’re able to flash different wavelengths of light
What is the Purkinje Shift?
The change in perception of colours that occurs when going from Cone-vision to Rod-vision. You become relatively more sensitive to short-wavelengths, or blue.
When using Rod-vision, red appears more ___, and blue appears more ___
Dark // Light
How many types of Cone Pigments are there in the human Retina?
3
What are the 3 types of Cone pigments in the human Retina?
Short Cones - sensitive to short wavelengths
Medium Cones - in the green range
Long Cones - in the red range
What are the 3 types of Cone pigments in the human Retina?
Short Cones - sensitive to short wavelengths
Medium Cones - in the green range
Long Cones - in the red range
What allows us to have colour vision?
The 3 types of Cone pigments in the Retina
Where do Short Cones appear less?
In the Retina
Why do different species have different perceptions of colour?
Because of the sensitivity of their transduction systems, other species have different cone pigments.
Honey Bees perception of colour is different to ours in that they…?
Have cones that are sensitive to UV light.
Birds perception of colour is different to ours in that they…?
Have 4 or more types of cones, making them very sensitive to colour.
What is Convergence?
Many inputs combining into a single receiving neuron
What is a 1:1 neural connection?
One sending neuron to one receiving neuron
What is Divergence?
Relatively few or a single sending neuron and many receiving neurons
What are the different types of neuronal connections?
Convergence, Divergence, and 1:1
What type of neuronal connection is the most rare?
1:1
How many Rods are there in the human Retina?
120 million
How many Cones are there in the human Retina?
6 million
How many Ganglion cells are there in the human Retina?
1 million
Where does Convergence and Divergence most commonly occur?
Where the axons and terminal boutons mingle with the dendrites of these different cell types. This is called neuropill. This is where all the connections are made.
Which photoreceptor has more Convergence?
There is more Convergence from Rods than from Cones
What type of connection occurs at the Fovea?
At the Fovea, Cones essentially have a 1:1 connection with Bipolar and Ganglion cells.
Which photoreceptor is more sensitive to dim light?
Rods
What allows the Rod system to be more sensitive than the Cone system?
Convergence allows the Rod system to be more sensitive than the Cone system
What is Acuity?
The ability to detect details
When you go to the eye doctor, and you are asked to say what you can see from the eye charts, what are they measuring?
Acuity
Which photoreceptor system is more acute?
Cone systems
What causes Rod systems to be less acute than Cone systems?
Convergence
What is Phototopic Vision?
Daytime vision
Phototopic Vision is dominated by ___?
Cones
What type of vision is used for high acuity colour vision
Phototopic Vision
What is Mesopic Vision?
Vision used in for fairly dim viewing such as dusk, dawn, natural situations, or in city life where there are many lights at night as well as light pollution.
Mesopic Vision uses ___?
Rods and Cones
What is Scotopic Vision?
Dim Light, high sensitivity, low acuity
Scotopic Vision is dominated by ___?
Rods
What type of vision doesn’t allow colour?
Scotopic Vision
There is much more convergence in the ___ than in the ___
periphery (rod dominated) // fovea (cone exclusive)
When you are looking at things directly with your Fovea, you will have ___
Highest acuity vision
When you are looking at things directly with your periphery, you will have ___
Very low acuity
Rods and Cones are more sparsely distributed in the ___
Periphery
