Systems
Circuits of neurons that interact to perform some function example: visual system, auditory system, the pain sensing system, memory systmes, motor systems, etc.).
Problems in understanding systems
1. The unknown are greater. 2. The complexity is vastly greater. 3. The operation of many systems is highly counterintuitive.
Neural codes (a fundamental concept: central importance)
The essential idea is that the meaning of activity (e.g., action potentials) by a neuron depends on the system it is part of. Examples: - action potential by a neuron in a pain system would yield the perception of pain.- action potential by neurons in the visual system would yield the perception of sight.- action potential by a neuron in motor systems would cause movement.- action potential by a neuron in a system underlying emotion might cause the feeling of fear or another emotion.
Subsystems
Systems also have subsystems- neurons in different subsystems code for different functions.Example:- in the visual system, different areas and groups of neurons perform different functions.- some neurons are responsible for detecting the form of a visual object.- others are responsible for detecting color (they code for color).- other for movement (they code fore movement)- others for location (they code to location of an object in the visual field). - Etc.
What do the action potential produced by different neurons mean?
The action potential produced by these different neurons mean different things. - i.e. they "code" for different perceptions/meanings etc.
What is a neuron coding?
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What attribute of the visual world is a neuron in the visual system encoding?
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All that you measure when you record membrane voltage is action potentials, but action potentials in different neurons mean different things. How do neuroscientists determine the function of a neuron or of a system of neurons?
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The visual system
The retina
Retina
Consists of several thin layers of cells distributed across the inside of the eye.
Fovea
The portion of the retina where light falls from an object that you are looking directly at.
Which portion of the retina has the highest acuity (sharpness)?
The fovea
Which portion of the retina has the ability to resolve fine detail and patterns of light?
The fovea
Are acuity and sensitivity the same thing?
No, they are different.
Optic Disk
The retinal location where axons from a type of retail cell collect and exit the eye and form the optic nerve.
Which location of the retina contains the blind spot?
The optic disk
Why is the blind spot in the optic disk?
Because there are no photoreceptors in the optic disk.
Diagram of the eye and how a visual image is 'mapped' onto the retina:
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A neuron's receptive field
The location in the environment (or the surface of the body) from which an appropriate stimulus will change that cell's activity.Example: light at 'A' (the tip of the flame on the diagram) will affect the activity of retinal cell in location 'a' in the retina.
Where do cells in different locations in the retina have receptive fields?
In different locations in the visual field.
What are the five cell types in the retina?
1. Photoreceptors2. Bipolar cells (BPs)3. Retinal ganglion cells (RGCs)4. Horizontal cells (HCs)5. Amacrine cells (ACs)
What is the first stage in the visual system?
Photoreceptors
Photoreceptors
The only cell type in the visual system that is directly sensitive to light.
What are the two types of photoreceptors?
1. Rods2. Cones
Rods
a) There are about 120 million rods in the human retina.b) Rods are highly sensitive to light and are responsible for vision in very dim light.c) Rods are bleached in bright light and thus unresponsive in bright light.d) Rods are not responsible for high acuity vision (not good for fine detail).e) Rods are achromatic (insensitive to colors).f) Rods only exist outside of the fovea.
Cones
a) There are about 6 million cones in the human retina.b) Cones are less sensitive to light intensity and are inoperative in dim light.c) Cones are sensitive to color. There are three subtypes, selectively sensitive to red, blue, and green wavelengths of light.d) Cones are most concentrated in the fovea.
How many rods are in the human retina?
About 120 million
How many cones are in the human retina?
About 6 million
Which of the two photoreceptors is highly sensitive to light?
Rods
How do rods respond to dim light?
They are responsible for vision in very dim light.
Which of the two photoreceptors is less sensitive to light?
Cones
How do cones respond to dim light?
They are inoperative in dim light.
During bright light, what happens to rods?
They are bleached and thus unresponsive in bright light.
Rods are not responsible and are not good for what?
They are not responsible for high acuity vision.- not good for fine detail.
What are cones sensitive to?
Color
What are the three subtypes of color that cones are selectively sensitive to?
Wavelengths of light:1. Red2. Blue 3. Green
Which photoreceptors is achromatic?
Rods, meaning they are insensitive to colors.
Where do rods exist?
They only exist outside of the fovea.
Where are cones most concentrated?
In the fovea.
Which cell do photoreceptors project to?
bipolar cells
Retinal ganglion cells (RGCs)
a. RGCs are the only output cell type in the retina.b. RGCs are the only means by which information from the eye gets to the rest of the visual system and their axons form the optic tract.
Which cells in the retina is the only output cell type?
Retinal ganglion cells (RGCs)
How does information from the eye get to the rest of the visual system?
RGCs are the only means by which information from the eye gets to the rest of the visual system and their axons form the optic tract.
Why is it that only the RGCs have axons?
1. Axons are needed for long-distance transfer of information. In the retina, the cells are very close together and so don't need action potentials or axons. Also, communication by PSPs may be able to convey information that is more subtle (precise) than can be conveyed by the AP frequency code (recall that PSPs are graded).2. Only the RGCs and ACs generate action potentials.
Which cells in the retina generate action potentials?
Only the RGCs and ACs generate action potentials.
What do the rest of the cell types use to release neurotransmitter to the next cell?
They use graded depolarization to release neurotransmitter to the next cell.
Does depolarization increase or decrease neurotransmitter release?
It increases neurotransmitter release.
Small depolarizations cause:
Small release of neurotransmitter.
Large depolarizations cause:
Large release of neurotransmitter.
The relationship between different cell types in the retina:
1. The retina is "inside-out" with the photoreceptors furthest away from the light (at the very back of the eye) and the RGCs the closest to the light. Thus light must pass through the other cell types to reach the photoreceptors. 2. This works because all the cells in the eye, except the photoreceptors, are translucent (allowing light but not detailed images to pass through).3. Also, at the fovea pit are cell types, except the photoreceptors, are pushed out of the way.
Which cell type is located furthest away from the light (at the very back of the eye)?
Photoreceptors
Which cell type is located closest to the light?
RGCs
How does light travel in the retina?
Light mist pass through the other cell types to reach the photoreceptors. - this works because all of these cells are translucent (allow light but not detailed images to pass through).
What happens to the cell types at the fovea pit?
All cell types, except photoreceptors, are pushed out of the way.
How light is absorbed by photoreceptors
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Phototransduction
How light energy leads to a change in membrane potential.
Diagram of how light changes photoreceptor membrane potential:
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The resting membrane potential (RMP) of photoreceptors is:
About -30 mV
Is the RMP of photoreceptors in the light or dark?
DARK
The maximum hyperpolarization is down to what mV?
-65 mV
The maximum hyperpolarization is produced by dark or bright light?
BRIGHT LIGHT
Glutamate
The neurotransmitter used by photoreceptors.
How does light intensity affect the amount of glutamate and neurotransmitter released?
Glutamate release < as light increases.- The greater the intensity of light, the less neurotransmitter released.
How does light produce the graded hyperpolarization?
A. The ligand-gated Na+ channels in the outer segment membrane are open in the dark, causing depolarization (to the "resting membrane potential" of -30 mV). B. These ligand-gated channels are like receptors, but they are "inside-out", meaning that they bind their ligand cGMP to a binding site on the intracellular face of the Na+ channel and this opens the channel.
When are ligand-gated Na+ channels in the outer segment membrane open? What does this cause?
They are open in the dark, causing depolarization (to the "resting membrane potential" of -30 mV).
How are these ligand-gated channels different from receptors? What does that mean?
These ligand-gated channels are like receptors, but they are "inside-out", meaning that they bind their ligand cGMP to a binding site on the intracellular face of the Na+ channel and this opens the channel.
How does light decrease the concentration of cGMP?
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What color is the photopigment in the dark?
It is purple in the dark.
The photopigment is highly concentrated in?
The photopigment is highly concentrated in the membrane of the disks in the outer segment of the photoreceptors.
What happens when the photopigment absorbs light?
When it absorbs light the photopigment is bleached to a pale yellow.
What is the photopigment called?
Rhodopsin
What two parts does rhodopsin consist of?
1. Opsin2. Retinal
Opsin
Is a protein.
Retinal
Is the only light sensitive molecule anywhere in the visual system.
The precursor of retinal is?
Vitamin A
Retinal exists in what 2 conformations?
1) in the dark it is 11-cis-retinal.2) a photon of light will switch it to the all trans-retinal conformation.
What are the steps in cGMP activation?
a. Opsin passes through the membrane seven times (i.e. it is a metabotropic or G-protein-coupled receptor). b. The release of retinal from opsin allows opsin to change shape and this activates a G-protein (transducin).c. The G-protein (G) dissociates and travels along the membrane and activates an enzyme (cGMP phosphodiesterase).d. cGMP phosphodiesterase converts cGMP to GMP, and thus lowers the concentration of cGMP.e. In the dark, cGMP is bound to the Na+ channel. Light decreases the concentration of cGMP, causing cGMP to disassociate from the channel. Consequently, the channels close and the photoreceptor cells hyperpolarize.
Why do we have this type of system (what is the advantage)?
a. The increased surface area and increased photopigment produced by having the photopigment molecules on the stacked disks, instead of on the Na+ channels, increases the chance of the light being detected by a rod. This system is so sensitive that a single photon can produce a detectable change in membrane potential of a rod type photoreceptor.b. The use of G-proteins allows for amplification; each molecule of opsin can activate many G-proteins, each of which, in turn, can activate many enzymes of cGMP phosphodiesterase, each of which can, in turn, convert many molecules of cGMP into GMP.