Sensory system- phototransduction
Learning outcomes
1. Knowledge of the structure and organization of the retina in mammals
2. Knowledge of the structure and properties of photoreceptor cells in vertebrates
3. Detailed knowledge of the mechanisms of phototransduction in vertebrate photoreceptor cells.
Introduction
• Diagram showing structure of the mammalian eye and retina
• Will be looking at the input side of the nervous system,
• Looking at phototransduction in mammals
• Will focus on the mechanism by which sensory information enters the nervous system.
- Focusing on the first cells of the sensory system, the sensory cells
• The most well studied sensory system is the visual system (photosensory system)
• Will look at phototransduction in 2 highly studied systems, mammals (deutestromian) and drosophila (protestomian)
• How and are the mechanism that animals employ in the visual system
Phototransduction in mammals (humans and vertebrates)
• Phototransduction occurs at the back of the eye in the retina
• The optic nerve is relaying the electrical signals to the brain
• The process of phototransduction:
- Turning the energy of light into an electrical signal occurs in the retina
• Image below shows the cellular organisation of the retina
• The retina in vertebrate is around the wrong way because the photoreceptor cells, which are doing the job of
phototransduction are located on the inner side of the retins. So the photoreceptor cells are at the back and the light is
coming in at the front
• Light is coming through a layer of neurons before light gets to the photoreceptor cells. It seems as an odd arrangement
but it is how the visual system is organised in vertebrates.
,Vertebrate photoreceptor
• Will not mention much about the neurons that are postsynaptic to the photoreceptor cells but will briefly touch on
them later on
• Main focus on the photoreceptor cells: rods and cones
• Rods are cells that are involved in the detection of light under low light condition or in the dark
• Cones are important in colour vision typically during daylight
• Rods divided into:
- Outer segment
- Inner segment
- Synaptic terminal
• The synaptic terminal is where the neurotransmitter is released to signal to the bipolar cells, which are postsynaptic to
these photoreceptor cells.
• Cones and rods contain:
- Nucleus (genetic information)
- Mitochondria (source of energy to maintain activity of cells)
- Cytoplasm
• The most important part for the function of photoreceptor cells are the discs (intracellular membrane bound
compartments that are filling the cytoplasmic region of the rod outer segment, which is where the process of
phototransduction is occurring)
• Discs are a specialisation of rods to increase the surface area for light to be absorbed and transfused into an electrical
signal
• Cones are similar in structure but slightly different, mostly in the outer segment
• The outer segment of cones in comprised of invagination of cell membrane. The photoreceptive membrane is not an
intracellular membrane compartment; it is an invagination of the cell surface.
A rod (left) and cone (right) from tiger salamander retina
• Images of rod and cone from a tiger salamander
,• Regarding the process of phototransduction, will be focusing on the rod and cone cells
Light hyperpolarises vertebrate photoreceptors
• How does light affect the activity of the rod and cone cells, electrophysiologically?
• To know the answer to this question, need to lace an electrode inside photoreceptor cell and see what happens to the
cell when light is shinned upon.
• The electrode is inserted inside the cell body of a rod cells
• When light is turned on can observe a hyperpolarisation of the membrane
• If increase the intensity of light, the magnitude of polarisation is greater
• Therefore, the first and most important observation is that light causes hyperpolarisation of photoreceptor cells in the
vertebrate brain.
Light causes hyperpolarisation of photoreceptor cells
• Graph shows recordings of responses of photoreceptor cells, showing the magnitude of the response with increasing
light intensity.
• Can see that there is a rapid response from when the light flashes and goes from least to most intense flash response
within 300 seconds.
• Is there anything unusual about the resting membrane potential of this photoreceptor cell?
- Typically, the resting membrane potential of neurons is ~ - 6o to 70 mV (millivolts).
- But in dark the photoreceptor cells, the resting membrane potential is ~ - 40 mV
- In neurons, once the resting membrane potential reaches ~ - 40 mV it is at the threshold, causing an action
potential
- This striking feature of photoreceptor cells, which provides us with a clue to the mechanism by which
phototransduction takes place.
• How does light cause hyperpolarisation of photoreceptor cells given that the resting membrane potential is at a
depolarised level with respect to what we normally expect from a typical neuron?
- Phototransduction closes cation channels in a rod outer segment
, Phototransduction closes cation channels in a rod outer segment
• Start with the photoreceptor proteins, which are directly activated by light and are called rhodopsin
- Photons directly activate rhodopsin, which is step 1
The photosensitive protein in rod is rhodopsin, which is a G-protein coupled receptor
• Rhodopsin is a G-protein coupled receptor
The human genome encodes ~ 800 GPCRs that belong to one of five main families
• G-protein coupled receptors are a huge family of proteins
• When the first animal (C.elegan) genome was encoded, it was found that there are more genes encoding G-protein
coupled receptor than any other protein type.
• Even in humans, there are hundreds of genes encoding G-protein coupled receptors.
Learning outcomes
1. Knowledge of the structure and organization of the retina in mammals
2. Knowledge of the structure and properties of photoreceptor cells in vertebrates
3. Detailed knowledge of the mechanisms of phototransduction in vertebrate photoreceptor cells.
Introduction
• Diagram showing structure of the mammalian eye and retina
• Will be looking at the input side of the nervous system,
• Looking at phototransduction in mammals
• Will focus on the mechanism by which sensory information enters the nervous system.
- Focusing on the first cells of the sensory system, the sensory cells
• The most well studied sensory system is the visual system (photosensory system)
• Will look at phototransduction in 2 highly studied systems, mammals (deutestromian) and drosophila (protestomian)
• How and are the mechanism that animals employ in the visual system
Phototransduction in mammals (humans and vertebrates)
• Phototransduction occurs at the back of the eye in the retina
• The optic nerve is relaying the electrical signals to the brain
• The process of phototransduction:
- Turning the energy of light into an electrical signal occurs in the retina
• Image below shows the cellular organisation of the retina
• The retina in vertebrate is around the wrong way because the photoreceptor cells, which are doing the job of
phototransduction are located on the inner side of the retins. So the photoreceptor cells are at the back and the light is
coming in at the front
• Light is coming through a layer of neurons before light gets to the photoreceptor cells. It seems as an odd arrangement
but it is how the visual system is organised in vertebrates.
,Vertebrate photoreceptor
• Will not mention much about the neurons that are postsynaptic to the photoreceptor cells but will briefly touch on
them later on
• Main focus on the photoreceptor cells: rods and cones
• Rods are cells that are involved in the detection of light under low light condition or in the dark
• Cones are important in colour vision typically during daylight
• Rods divided into:
- Outer segment
- Inner segment
- Synaptic terminal
• The synaptic terminal is where the neurotransmitter is released to signal to the bipolar cells, which are postsynaptic to
these photoreceptor cells.
• Cones and rods contain:
- Nucleus (genetic information)
- Mitochondria (source of energy to maintain activity of cells)
- Cytoplasm
• The most important part for the function of photoreceptor cells are the discs (intracellular membrane bound
compartments that are filling the cytoplasmic region of the rod outer segment, which is where the process of
phototransduction is occurring)
• Discs are a specialisation of rods to increase the surface area for light to be absorbed and transfused into an electrical
signal
• Cones are similar in structure but slightly different, mostly in the outer segment
• The outer segment of cones in comprised of invagination of cell membrane. The photoreceptive membrane is not an
intracellular membrane compartment; it is an invagination of the cell surface.
A rod (left) and cone (right) from tiger salamander retina
• Images of rod and cone from a tiger salamander
,• Regarding the process of phototransduction, will be focusing on the rod and cone cells
Light hyperpolarises vertebrate photoreceptors
• How does light affect the activity of the rod and cone cells, electrophysiologically?
• To know the answer to this question, need to lace an electrode inside photoreceptor cell and see what happens to the
cell when light is shinned upon.
• The electrode is inserted inside the cell body of a rod cells
• When light is turned on can observe a hyperpolarisation of the membrane
• If increase the intensity of light, the magnitude of polarisation is greater
• Therefore, the first and most important observation is that light causes hyperpolarisation of photoreceptor cells in the
vertebrate brain.
Light causes hyperpolarisation of photoreceptor cells
• Graph shows recordings of responses of photoreceptor cells, showing the magnitude of the response with increasing
light intensity.
• Can see that there is a rapid response from when the light flashes and goes from least to most intense flash response
within 300 seconds.
• Is there anything unusual about the resting membrane potential of this photoreceptor cell?
- Typically, the resting membrane potential of neurons is ~ - 6o to 70 mV (millivolts).
- But in dark the photoreceptor cells, the resting membrane potential is ~ - 40 mV
- In neurons, once the resting membrane potential reaches ~ - 40 mV it is at the threshold, causing an action
potential
- This striking feature of photoreceptor cells, which provides us with a clue to the mechanism by which
phototransduction takes place.
• How does light cause hyperpolarisation of photoreceptor cells given that the resting membrane potential is at a
depolarised level with respect to what we normally expect from a typical neuron?
- Phototransduction closes cation channels in a rod outer segment
, Phototransduction closes cation channels in a rod outer segment
• Start with the photoreceptor proteins, which are directly activated by light and are called rhodopsin
- Photons directly activate rhodopsin, which is step 1
The photosensitive protein in rod is rhodopsin, which is a G-protein coupled receptor
• Rhodopsin is a G-protein coupled receptor
The human genome encodes ~ 800 GPCRs that belong to one of five main families
• G-protein coupled receptors are a huge family of proteins
• When the first animal (C.elegan) genome was encoded, it was found that there are more genes encoding G-protein
coupled receptor than any other protein type.
• Even in humans, there are hundreds of genes encoding G-protein coupled receptors.
