Sensory Systems BIOL21341 Flashcards
What is the role of the premotor cortex?
Coordinates voluntary movements
What is the role of the primary motor cortex?
Voluntary movement.
What is the role of the primary somatosensory cortex?
The processing of somesthetic sensations and proprioception.
What is the role of the sensory association areas?
Integration of sensory information.
What is the role of the visual association areas?
higher vision processing.
What is the role of the primary visual cortex?
Vision
What is the role of Wernicke’s area?
Language comprehension
What is the role of the primary auditory cortex?
Hearing.
What is the role of the Limbic association cortex?
emotions, learning and memory.
What is the role of the olfactory cortex?
Smell.
What is the role of Broca’s area?
speech formation
What is the role of the prefrontal association areas?
idea and plan for voluntary movement, thoughts, personality.
What is the function of sensory systems?
Sense nature of the internal and external environment.
What is in charge of the sensors of the internal environment?
The vagus nerve.
What is somatosensation?
The physiological process by which neural substrates are activated by physical stimuli, resulting in the perception of what we describe as touch, pressure, pain, etc.
What is the sensation of the environment called?
Somatosensation
What is proprioception?
The sense of limb/body position and movement.
What is a sensory receptor?
A specialised excitable cell sensitive to a form of physical energy (modality)
What are the 4 modalities of sensory receptors?
Mechanical, Thermal, Chemical, Electromagnetic.
What are the 4 types of receptor (think modality).
Mechanoreceptor, Thermoreceptor, Chemoreceptor, Photoreceptor.
What is the law of specific nerve energies?
Receptors are (usually) specific to a particular modality.
What is meant by ‘adequate stimulus’ in reference to receptor specificity?
The modality to which a receptor responds best.
What do photoreceptors sense?
Photons of light
What do chemoreceptors for taste sense?
Chemicals dissolved in saliva.
What do chemoreceptors for smell sense?
Chemicals dissolved in mucus.
What do chemoreceptors for pain sense?
Chemicals in extracellular fluid
What do chemoreceptors for blood oxygen sense?
Oxygen dissolved in plasma.
What do chemoreceptors for blood pH sense?
Free hydrogen ions in the plasma.
What do thermoreceptors for warmth detect?
increase in temperatures between 30-43 degrees.
What do thermoreceptors for cold detect?
Decrease in temperatures between 35-20 degrees.
What are the 3 main types of mechanoreceptors?
Baroreceptors, Osmoreceptors, Hair cells.
What do baroreceptors sense and detect?
Blood pressure by measuring the stretch of specific blood vessels walls.
What do osmoreceptors sense and detect?
Osmolarity of extracellular fluid by the swelling (stretch) of the receptor cells.
What are the two things that hair cells sense and how do they detect them?
Sound - sound waves
Balance and equilibrium - acceleration.
What are the main components of neuronal structure?
Cell body (soma)
Neurites:
- dendrites
- axons
- synapses
What is a neurite?
any type of process or protrusion extending out from the cell body of a neuron
What is transduction?
Conversion of physical energy to a receptor potential in the receptor neuron.
What are the 5 types of receptors in the skin?
Meissner corpuscle
Pacinian corpuscle
Ruffini’s corpuscles
Merkel’s disks
Free nerve endings.
What is difference between the stimulus and the resulting receptor potentials?
Stimulus is a flat on/off, receptor potentials are graded potentials (gentle slope down through duration of the stimulus)
How do anaesthetics work?
They block the ion channels in the mechanoreceptors so that transduction cannot take place.
What are the two main types of transduction?
One in which the cells that receives also transduces and passes on the information (e.g. pacinian corpuscle).
One in which there are two cells involved in the process (think receptor cell which synapses an afferent neuron e.g., hair cell in the inner ear).
What is meant by afferent?
A neuron in the peripheral nervous system that conducts action potentials to the central nervous system
What is meant by sensory coding?
Representation of stimulus in terms of action potentials.
(e.g. small stimulus less frequent firing, larger = more firing)
Give an example of both single and two cell transduction.
Single = pacinian corpuscle
Two = hair cell in cochlea.
Briefly outline how cochlea hair cells receive and transduce information.
The vibrations in the cochlea fluid cause potassium ion channels to open/close on the cochlear hair cells depending on the type of vibration.
- Open, causes receptor potentials.
- The auditory afferents then transduce to the nervous system.
What is a sensory unit?
Receptor with an afferent neuron.
Outline the sensory pathway from stimulus at the PNS to CNS (all the way up).
PNS:
- stimulus
- receptors
- afferent (first order neuron)
CNS:
- Spinal cord/brainstem
- second order neuron
- Thalamus
- Third order neuron
- Cortex.
What does tonotopic organisation mean?
each sound frequency is represented in a narrow strip along the surface of the cortex in relation to its frequency.
Briefly outline how cochlear implants work.
A microphone protrudes from the side of the head, which is connected to electrodes that are implanted in the cochlea.
These inserted electrodes will stimulate the different parts of the cochlea depending on the wavelengths picked up by the microphone, thereby transducing the information for the cochlea.
Briefly outline the somatosensory pathway (7).
- Receptor
- Afferent neuron
- Spinal cord
- Second order neuron.
- Thalamus
- Third order neuron.
- Primary somatosensory cortex.
What is stereognosia?
Inability to identify objects by touch alone.
What is action defects?
Lacking fine motor skills.
What is the aim of the sensory systems?
To serve action.
What is the role of gustation?
To identify food and avoid noxious and toxic substances.
How many taste compounds can we recognise and give some examples.
Hundreds:
- Free protons (H+ acid)
- Complex organic compounds (sugars, amino acids)
How many dimensions of taste are there? What are they?
5
Sweet, sour, bitter, salty, umami
What produces the sensation of umami? Give an example of a substance.
It’s induced by glutamate.
MSG (monosodium glutamate)
What are the three types of papillae and where are they located on the tongue?
Fungiform - found from the tip to 3/4 the way back, all over medial surface of the tongue.
Foliate - Found laterally at the back of the tongue.
Vallate - Found back middle in a V shape (point being furthest back)
What are the three types of taste cell?
Type I, Type II, Type III
What is unique about taste cells as a sensory unit?
They can regenerate (if they die they can be replaced/grow back)
What is the organisation of taste cells in a taste pore?
Type I
- Thin and at the top, soma closer to the opening.
Type II
- Large and soma sits in the middle.
Type III
- Medium sized and soma sits closer to the body
What is the approximate number of taste cells per taste bud?
Roughly 100
Afferent projections of what type of nerves are found in taste buds?
Cranial nerves.
Provide a brief overview of how type II and III cells respond to stimulants in the taste bud.
The receptor cells respond to stimulants in the taste bud with graded changes in polarisation (and depolarisation) and neurotransmitter release.
DO NOT SIGNAL DIRECTLY TO BRAIN
How is resting potential established in type II and III taste cells?
Sodium/potassium exchanger ensures high intracellular K+ and high extracellular Na+
Leakage K+ channels allow K+ to leave the cell, establishing potential (a positive charge outside the cell)
How does synaptic release occur in type II and III taste cells?
Voltage gated cation channels (Na+ and Ca2+) open, allowing Na+/Ca2+ back into cell reducing potential (more positive inside now).
This causes neurotransmitter release into synaptic cleft and action potential transmitted.
What tastes do type II cells detect - what are the two types of type II cells?
Nasty
- bitter sensitive cells
Nice
- sweet and/or umami sensitive cells
What taste do type III cells detect?
Sour
What type of receptor are type II ‘nasty’ taste cells?
G protein coupled receptors. (Bitter)
Briefly outline the pathway of taste II receptors (think GPCR).
Ligand binds, changing confirmation.
G-alpha dissociates from beta/gamma subunits
Both these two different subunit clusters bind effector enzymes.
This starts a signalling cascade
This increases diffusible ‘second messengers’
(e.g. cAMP, cGMP, IP3)
Describe in detail the process of ligand binding to action potential in type II ‘nice’ taste cells.
Think receptor…
Pair of GPCR’s (Heterodimer) of T1R2 and T1R3.
The ligand binds the extracellular cleft formed by the heterodimer.
This change in conformation releases Gustducin (Gbetagamma)
This interacts with PLC which catalyses PIP2 to IP3 and DAG.
IP3 binds with its ligand gated Ca2+ channel (IP3 receptor).
This catalyses stores of Ca2+ in the EPR, increasing intracellular calcium which binds to TRPM5.
This allows the influx of calcium and causes depolarisation.
Describe in detail the process of ligand binding to depolarisation in type II ‘nice’ umami taste cells.
Simply type II nice cell pathway
Pair of GPCR’s (Heterodimer) of T1R1 and T1R3.
The ligand binds the extracellular cleft formed by the heterodimer.
This change in conformation releases Gustducin (Gbetagamma)
This interacts with PLC which catalyses PIP2 to IP3 and DAG.
IP3 binds with its ligand gated Ca2+ channel (IP3 receptor).
This catalyses stores of Ca2+ in the EPR, increasing intracellular calcium which binds to TRPM5.
This allows the influx of calcium and causes depolarisation.
Describe in detail the process of ligand binding to depolarisation in type II ‘nice’ umami taste cells.
OPTION 2
Think which receptor is it…
mGluR1 (metabotropic glutamate receptor)
The ligand binds the extracellular cleft.
This change in conformation releases Gustducin (Gbetagamma)
This interacts with PLC which catalyses PIP2 to IP3 and DAG.
IP3 binds with its ligand gated Ca2+ channel (IP3 receptor).
This catalyses stores of Ca2+ in the EPR, increasing intracellular calcium which binds to TRPM5.
This allows the influx of calcium and causes depolarisation.
Describe in detail the process of ligand binding to depolarisation in type II ‘nasty’ bitter taste cells.
T2R GPCR.
The ligand binds the extracellular cleft.
This change in conformation releases Gustducin (Gbetagamma)
This interacts with PLC which catalyses PIP2 to IP3 and DAG.
IP3 binds with its ligand gated Ca2+ channel (IP3 receptor).
This catalyses stores of Ca2+ in the EPR, increasing intracellular calcium which binds to TRPM5.
This allows the influx of calcium and causes depolarisation.
How many types of T2R are there?
30 types
How do type III sour cells maintain resting potential? How do they detect sour tastes? What neurotrasmitter is involved?
- Na+/K+ exchanger helps to maintain resting potential (K+ in, Na+ out).
- pH sensitive K+ leakage channel stops the channel from depolarising without stimuli.
H+ channels allow H+ ions to enter.
- This means that when acid is present, acidification of the cell occurs.
- This causes the closing of the K+ leakage channels which will cause depolarisation as charge inside the cell rises.
5HT (serotonin) transmits the signal.
How do type III salty ‘cells’ detect salt? What is the basic path to depolarisation.
Which neurotransmitter is involved?
LESS KNOWN!
ENaC channel (epithelial sodium channel).
The presence of salt extracellularly increases the conc gradient, causing more Na+ to move into the cell.
This causes depolarisation.
Serotonin transmits this info.
What are T2Rs and what do they detect?
GPCRs and bitter compounds
Why are there so many T2Rs ?
GPCRs recognise ligands based upon match between ligand 3D shape and chemical characteristics.
Therefore, as there are lots of bitter compounds, there needs to be lots of GPCRs to detect them
Outline the relationship between T2Rs and a single type II taste cell.
They typically express more than 1 type of T2R
What are isothiocyonates?
Bitter chemicals that are found in veg.
What is PTC (phenylthiocarbamide)? What is interesting about it?
An analog of isothiocynates which are also bitter.
Some people taste bitter from exposure to it, some people don’t.
What difference is evident between bitter and non bitter tasters of PTC (phenylthiocarbamide)?
Incidence of iodine deficiency higher in non-tasters.
What did Kim et al (2003) find in their study into molecular genetics of PTC sensitivity?
Most variation in human population was caused by polymorphisms at the TAS2R38 (bitter GPCR)
What did Wooding et al (2006) find in their study into molecular genetics of PTC sensitivity?
Chimps
Variation in Chimps also arise in TAS2R38 polymorphism, but not in the same one as humans.
What are the possible implications of the difference between the non-taster/taster TAS2R38 alleles.
Darwinnnnn
PTC non-tasters don’t lack TAS2R38, potentially it detects other bitter compounds which could lead to a heterozygote advantage (can detect more bitter compounds).
What can PTC teach us? (4 things)
Bitterness is about avoiding toxins.
Receptor affinity for different compounds is under strong selection.
Changes in amino acid sequence of the receptor alter the compound it detects.
Having a wide range of T2Rs allows a wide variety of toxins to be detected.
What shows that there is a fine tuning of receptor specificity by natural selection?
The varying levels of bitter taste threshold for quinine hydrochloride of various mammal feeding types (e.g., carnivore vs herbivore).
What does connections between neurons give an opportunity for?
Signal processing.
Outline the process from taste buds to the brain.
- Taste bud receives information.
- Its carried along the cranial nerves to the geniculate ganglion.
- Then it passes through the nucleus of the solitary tract.
- Up to the ventral posterior medial nucleus of the thalamus
- Arrives in the gustatory cortex (anterior insula-frontal operculum)
What are the two options of how taste information is transmitted to the brain?
Option 1:
Labelled line code
Option 2:
Combinatorial code
What is meant by labelled line code?
Information is transmitted as separate information streams - each neuron at each stage responds to a single dimension of taste.
What is meant by combinatorial code?
Information is transmitted as a single merged stream - individual neurons respond to a combination of taste dimensions.
What prediction does labelled line code make in regards to gustation?
Each receptor cell should detect a single taste dimension and the perceived taste should be defined by the identity of receptor cell activated.
Each postsynaptic neuron should receive input from receptor cells detecting the same dimension.
What are the problems of predicting gustatory information as being transmitted using labelled line code?
Sweet and umami tastes can co-occur, therefore there must be some merging.
Postsynaptic neurons are a combination of ‘specialists’ that respond to one taste and ‘generalist’ that respond to multiple tastes.
What conclusions can be made about the transmitting of information from taste cells to the brain?
Input to the brain is complex:
- Gustatory neurons in cranial nerves have different degrees of preference to singular taste dimensions.
- Some integration across dimensions at early stages.
Combinatorial code:
- taste discrimination occurs by the brain assessing activity across the population of neurons
Which three things influence your perception of flavour?
Use techincal terms
Gustation
Olfaction
Somatosensation
Name 3 things that influence our decision of what to eat.
Innate preferences
Learned associations
Satiety signals
Outline the two types of innate preferences that guide taste.
Innate aversions:
- Bitterness
Innate cravings:
- Sweet
- Umami
- Salty
(which is determined by our NaCl balance)
Give an example of an association influencing taste.
Having shellfish on a boat and then being sea sick can lead to you disliking the shell fish.
Name three key features of taste conditioning.
Timing and number of pairings
(can have up to an hour between)
Longevity
Stimulus generalisation
What does olfaction detect in land and aquatic animals?
Land: volatile hydrophobic compounds
Aquatic: Water soluble molecules
What are odorants?
Volatile, lipophylic organic compounds
Where does olfaction start?
The olfactory epithelium
Outline the structure of olfactory epithelium from nasal cavity out.
Cilia
Mucus
Dendrites
(supporting cells around them)
Olfactory sensory neuron (soma)
Axons
(basal cells around them)
Out to olfactory bulb.
Outline important information about olfactory sensory neurons.
They detect and signal to the brain - they don’t synapse to an afferent neuron.
They can regenerate throughout life.
They fire action potentials in response to application of odorants.
What is the role of the lateral nasal gland?
A thermoregulatory gland that keeps the nasal cavity moist.
What is the importance of binding proteins in olfaction?
They are released so that they can grab odorants, presenting them for sensory neurons.
What is the environment of the lateral nasal gland?
aqueous mucus environment
Who are Linda Buck and Richard Axel and what is the importance of them?
They won the nobel prize in 1991 for finding ‘A novel multigene family may encode odorant receptors: a molecular basis for odour recognition’.
Which receptor class is vital for odorant detection?
GPCR
Outline the process of receptor ligand binding to action potential in olfaction.
GPCR.
The odorant binds with the receptor protein.
G-olf activates at binding
Separates into alpha and beta-gamma subunits
Free alpha subunit activates Adenylate Cyclase
This increases cAMP
cAMP opens cation channels in membrane
This allows positive charge to influx (Na+ and Ca2+)
Depolarisation, therefore AP.
What is the role of calcium in modulating olfaction in the receptor? (2 main overarching)
Calcium activates calcium gated chloride channels which causes Cl- efflux, therefore increasing the speed of depolarisation.
Calcium activates calmodulin, which leads to activation of Phosphodiesterase E which deactivates adenylate cyclase
- This stops cAMP production, effectively switching off the whole system more and more the longer a odorant is present (hence why a smell ‘goes’ if you’re in it long enough)
Briefly outline the pathway from the first olfactory synapse to the specific areas of the brain olfactory information goes.
Olfactory bulb
Olfactory nerve
Olfactory tract
Olfactory cortex (paleocortex)
- Hippocampus
- Amygdala
- Hypothalamus
- Thalamus
— Frontal cortex
Which parts of olfaction are processed in the following areas of the brain?
- Hippocampus
- Amygdala
- Hypothalamus
- Thalamus/Frontal cortex
- Hippocampus: odorants activating memory
- Amygdala: Emotion
- Hypothalamus: Feeding behaviours
- Thalamus/Frontal cortex: perception of the smell
What is special about the olfactory bulb in the mouse brain?
It’s a proportionally large part of the brain (shows emphasis of smell in their perception).
Outline important information about odorants.
Thy are volatile compounds that are structurally distinct.
They are individual elements that combine to create and odour - complex.
How many odorants do humans detect and discriminate.
Detect hundreds of thousands of distinct odorants.
Discriminate a few thousand odorants.
Outline the relevance of human gene number and how odorant receptor proteins work.
Human genome has around 20k genes.
This means that we can’t have a specific GPCR per odorant we detect - therefore each odorant receptor protein must recognise >1 odorant.
Outline the relationship between receptors and how they detect odorants.
Each receptor detects multiple odorants.
Most odorants excite several receptors
- dependent on concentration (e.g. perfume)
- some odorants act as antagonists (combinations can antagonise)
Summarise the key information about how receptors and odorants interact for olfaction.
(6 points)
Not the specific pathway
- Odorants bound by odorant receptors according to 3D structure
- Each receptor protein responds to several odorants
- A huge number of odorants will be bound by one or more of these receptors
- A smaller (but still v large) number of odorants will have a unique receptor activation profile
- Complex odours contain numerous odorants … almost infinite variety in receptor activation profiles
- Conclusion: 1k olfactory receptor genes provide HUGE discriminatory power for odours
Outline the study into transgenic Lac-Z reporter mice.
SET-UP:
Mice were taken and genetically modified in their olfactory receptor protein genes with Lac-Z.
Lac-Z encodes b-galactosidase which metabolises X-gal to form a blue pigment.
This means that the receptor cells are labelled blue so we can see how they are expressed.
RESULTS:
The ~10k olfactory sensory neurons expressing a given olfactory receptor are distributed across the nasal epithelium.
Their axons converge on just 2 specific locations (glomeruli) in the olfactory bulb.
~2k glomeruli in the olfactory bulb.
WHAT THIS SHOWS:
An odour is translated into:
1.) a pattern of olfactory receptor activations, becomes
2.) a spatial pattern of activity in the olfactory bulb
‘Odour Map’
What did Lapil et al (2011) found out about Zones in terms of olfaction .
Zones of olfactory epithelium respond to structurally similar compounds.
Zones encode common perceptions
How is information about odours transmitted? (Type of code)
combinatorial code.
What are pheromones and what is their significance?
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species.
What is the name of the organ which detects pheromones? Outline the pathway to the brain of this information.
Vomeronasal organ.
Sensory neurons in the vomeronasal organ
Vomeronasal nerves
Accessory olfactory bulb
Hypothalamus
- regulates the response due to pheromones.
Outline the receptor used in pheromone detection. How many families are there? What are their names and important information about them?
GPCR
2 evolutionarily distinct families
V1r (about 150 genes; v high interspecific variation; mostly pseudogenes in primates)
V2r (about 100 genes)
Outline the process of receptor to depolarisation of pheromones.
plc
Vomeronasal sensory neurons
VR GPCR binds pheromone
G protein dissociates
G betagamma activates Phospholipase C (PLC)
Catalyses PIP2 into IP3 and DAG
DAG activates TRP2 cation channels
Cations influx
DEPOLARISATION :)
Briefly outline the behavioural effects of disrupting the vomeronasal system.
Mouse example
If a male mouse has a functional VNO it will respond to females by trying to mate and males by trying to fight.
If you take a male mouse with a non-functional VNO (knockout TRP2 gene (no depolarisation can occur)) they will try mount a male castrated mouse covered in male piss.
Shows that VNO and pheromones are important in social behaviour.
What is the eye?
Specialised photosensory organ.
What is a photoreceptor?
Cell specialised for light detection
What is a photopigment?
Protein + light absorbing cofactor.
Outline the important distinction about photopigments and their relationship to eyes and photoreceptors.
Many organisms have photopigments without having ‘photoreceptors’ or eyes.
Even in animals:
- not all photopigments are found in ‘photoreceptors’
- not all photoreceptors are found in eyes.
What is the role of the eye?
It’s the optical apparatus to provide spatial resolution to photoreception.
Outline photoreceptors in Xenopus.
Dermal photoreceptors (melanophores) detect ambient light levels.
They also adapt to light by pigments changing colour in response to light as a form of camouflage.
Outline an example of simple eyes and how they work.
Planarian eyes:
- Eyes exist in pigmented pits which limit the range of directions from which light can reach each photoreceptor
- By measuring photoactivity of different cells, they can detect where in the world is brighter.
What is -ve phototaxis? Give an example of an organism that uses it.
It is directed movement away from a source of light - planarian.
Briefly outline the structure of the compound eye of insects.
Are composed of thousands of units called ommatidia.
Outline the structure of ommatidia.
Have a crystalline cone and cornea which form a lens.
They have photosensitive neurons/photoreceptors in a column.
As light only enters from the front and it has pigmented cells which form a tube, this forms a fine collecting area.
What is meant by acuity in vision?
Capacity for seeing distinctly the details of an object.
Outline the acuity of the compound insect eye.
Medium acuity (closer objects = higher acuity)
Each ommatidium = 1 pixel
Therefore is limited to a few thousand
How do mirror eyes work? Give an example organism.
Scallop (has 60-100 small 1mm eyes)
Light enters the eye through opening (pigment around to stop light coming through anywhere else), and bounces off of a concave mirror at the back which focuses the image onto an array of photoreceptors (like a reflective telescope).
How does the compound eye deal with distance?
Acuity is limited by the collecting area of the ommatidium - which increases with the distance from object.
Simple inverse correlation between distance and acuity:
- the further away the object is, the large space the collecting area covers, therefore lower acuity.
How does the mirror eye deal with distance?
The curvature of the mirror is designed so that the image is reflected onto a specific point.
This means the focal length of the mirror is invariant (as it can’t change its shape)
Objects in focus at specific distances, but not at other.
Acuity strongly depends on viewing distance.
Outline the lens eye.
It has a very high acuity.
Cornea refracts 80%, lens variably refracts the other 20%.
This means light can be focused on the retina at varying distances.
In theory, each photoreceptor receives light from a different visual space (lots of pixels).
Which types of animals have the lens eye?
All vertebrates
Some invertebrates:
- Squid/Octopus
- Spider
Certain types of jellyfish.
Outline refraction in reference to the eye.
Focusing divergent rays from a point in visual space into a single point on the retinal surface.
What controls the changing size of the lens?
The ciliary muscle/body.
Outline the process of refraction in the lens eye at long distances.
Light rays are near parallel so require little refraction to focus.
Therefore lens is thin and needs low refractive power.
Actions taken to make this occur:
- Ciliary muscles relaxed
- Suspensory ligaments taught
- Lens pulled thin.
Therefore refracts distant objects to back of retina.
Outline the process of refraction in the lens eye at short distances.
Light rays are still diverging so require greater refraction to focus.
Therefore lens is thick and needs high refractive power.
Actions taken to make this occur:
- Ciliary muscles contracted
- Suspensory ligaments loose
- Lens small and thick.
What does accommodation mean in the eye?
Changing vision from distant to close:
- Ciliary muscles contract
- Lens thickens and refractive power increases
- Pupils constrict.
What causes refractive errors?
They arise from problems with corneal or lens refraction.
The distance from cornea to retina can be impacted by genes or experience.
What is myopia and how is it resolved?
Short sightedness.
Eyes too deep/long and/or the refractive power of the cornea is too great.
Near objects, reduction in accommodative change in lens can compensate.
Diverging (concave) lens corrects
What is hyperopia and how is it resolved?
Long sightedness.
Eyes too shallow/short and/or the refractive power of the cornea is too small.
Far objects, accommodative change in the lens can compensate.
Converging (convex) lens corrects
How many Opsin proteins are there per rod cell?
10^8
What are opsins?
Proteins that bind to light-reactive chemicals to underlie vision, phototaxis, circadian rhythms, and other light-mediated responses of organisms
What is retinal/retinaldehyde and what is its significance?
Retinaldehyde (which is a vitamin A derivative) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception.
What is a chromophore?
The factor of an opsin photopigment that absorbs light.
What are the two main types of retinaldehyde and what is the difference between them? What is their relationship?
11-cis retinaldehyde
- has an important kink in the carbon chain
all-trans retinaldehyde
- has no kink in carbon, linear version
11-cis retinaldehyde is activated into all-trans retinaldehyde due to light
They’re both derivatives of Beta-carotene (vitamin A)
Outline the structure and functions of opsin proteins.
7 transmembrane domain G protein coupled receptor.
Binds retinaldehyde.
Functions:
- translate isomerisation of retinaldehyde into a ‘biological’ signal
- determines which wavelengths the retinaldehyde absorbs
Outline the relationship between 11-cis retinaldehyde, 11-all trans retinaldehyde and GPCRs.
11-cis retinaldehyde is a inverse agonist - strongly inhibits the GPCR
11-all trans retinaldehyde activates the GPCR (occurs when light hits 11-cis retinaldehyde)
Outline the phototransduction cascade.
cGMP
Retinaldehyde absorbs light and changes opsin from inactive to active form
Activated opsin binds to transducin (g-protein) which breaks into alpha subunit and beta-gamma subunit
Alpha subunit binds to cGMP phosphodiesterase
Leads to reduction of cGMP.
Leads to closure of cGMP gated cation channels (cGMP opens cation channels, so long as there is lots of cGMP the cation channels stay open)
Outline how a single photon of light can have an impact in the phototransduction cascade.
Single photon of light can have a large effect.
Activation of a single rhodopsin leads to activation of lots of transducin, which can lead to activation of lots of cGMP phosphodiesterase, in turn leading to reduction of activity of cGMP.
Outline how photoreceptors act in the dark.
Gated cation channels are open so ions rush in.
Sodium potassium exchanger is in effect, increases the internal conc of potassium (whilst there is potassium leakage) and decreases internal Na+
Overall this leads to depolarisation.
Photoreceptors are excited and release glutamate in the dark.
What is the difference between rod and cone cells?
Rods have a long out segment, capture more photons and have a larger signal amplification cascade (more sensitive).
Cones adjust their sensitivity to be active under any light level.
They are never saturated - so you can always see.
Have a higher acuity
Provide colour vision.
Are located in the fovea.
Name the layers of the retina
Photoreceptor layer
Outer plexiform
Inner nuclear layer
Inner plexiform layer
Retinal Ganglion Cells
Outline the connections of the photoreceptor layer
Makes connections with second order neurons in outer plexiform layer
Outline the connections of the outer plexiform layer.
Photoreceptor layer and inner nuclear layer makes connections here.
No cell bodies here, just connections.
Outline the connections of the inner nuclear layer.
Makes connections with the outer plexiform layer
Outline the connections of the Inner plexiform layer.
No cell bodies, make connections with retinal ganglion cells in ganglion cell layer.
What is special about retinal ganglion cells.
In comparison to other retinal cells
Are the only cells in the retina with axons, and the only cells in the retina that can form APs.
Outline the role of retinal ganglion cells.
They bundle together to form the optic nerve.
They don’t just report the amount of light, they report visual info too.
Explain the reason that photoreceptors attending to a darker part of a visual space compared to a lighter part of a visual space are depolarised, whilst the inverse is hyperpolarized
Photoreceptors respond to light exposure with graded hyperpolarisation.
This results in a reduction in glutamate release at their synaptic terminals in the presence of light - cell is hyperpolarised.
In a darker area where glutamate is higher, depolarisation will occur due to the excitatory effects of glutamate.
Outline the basic process of what happens when an image is seen.
Eyes projects image onto photoreceptors.
Photoreceptors translate into a spatial pattern of glutamate release.
Bipolar cells convey signals from cones to RGCs
Retinal ganglion cells send to the brain using APs
Who did an experiment to discover that there where two types of bipolar cells?
Keffer Hartline.
What are the two types of bipolar cells?
ON: depolarised by a flash of light
OFF: are hyperpolarised by a flash of light
What are the two types of synapses that bipolar cells have?
Sign inverting synapse.
Sign conserving synapse.
Outline sign inversing synapses.
signalling cascade
They express a metabotropic glutamate receptor.
Glutamate activates signalling cascade.
CLOSING cation channels
When in light, glutamate signal is reduced, ability to close channels is descreased.
Bipolar cells depolarises.
Outline sign conserving synapse.
in light
Express an ionotropic glutamate receptor.
Cation channels OPENED by glutamate.
In light, glutamate signal is reduced, channels close.
Bipolar cell hyperpolarises.
Briefly summarise information about horizontal cells.
Cell bodies in inner nuclear layer.
Synapses in outer plexiform.
Allow information flow in the horizontal axis (apposed to up/down)
Outline Tortson Wiesels study into centre surround antagonism.
Took recordings from a single retinal ganglion cell:
A - small spot of light = lots of AP generation.
B - BIg spot of light = Less AP generation.
C - Anubus (spot of light with darkness in the middle) looks like an off ganglion cell.
What did Torston Wiesels study provide evidence for? Briefly explain why.
Provides evidence for centre surround theory due to horizontal cells:
Horizontal cells link cones within a region to the retina.
- Inputs from local cones sign conserving: hyperpolarised by light.
- Outputs sign inverting: antagonise the light response
Outline how horizontal cells enhance differences in diffuse light (large light spot).
Neighbouring cones are hyperpolarised.
Horizontal cells become hyperpolarised.
Signal (reduction in glutamate) from centre cone to bipolar cells damped.
Outline how horizontal cells enhance differences in light spot.
Neighbouring cone are depolarised.
Horizontal cells become depolarised.
Signal (reduction in glutamate) from centre cone to bipolar cells enhanced.
What do amacrine cells do in reference to information processing?
They provide an inhibitory link between bipolar cells and retinal ganglion cells - allows further modulation of response.
What did Horace Barlow find out about retinal ganglion cells?
The response of the retinal ganglion cells to spots of light is altered whether you’re looking at it moving from bottom to top or from top to bottom.
NEED TO ADD IMAGES + DETAIL ON SURROUND ANTAGONISM - will do once we have premium
Briefly outline circadian rhythms
They are 24 hour variations in physiology and behaviour.
They persist in the absence of any cyclic cue from the environment.
They need some external factor to keep the internal clock/rhythm in check.
Outline how a study into mimosa revealed circadian rhythms.
Two plants.
One exposed to light, one in a box separated from external cues.
Both plants had their leaves open in day time (presence of sunlight)
Both plants curled their leaves at night (to conserve water)
Both displayed circadian rhythms - the one without the cues proves the existence of a rhythm.
What is the difference in ambient illumination at midday compared to mignight?
10^9 more light at midday.
What is the most reliable environmental signal of time-of-day?
Ambient light.
Briefly outline the study into effects of light on rodent clocks.
Mice in a cage with a wheel - they love running on it at night as they are nocturnal.
3 conditions:
- normal light times (8am to 8pm)
- condition 2 (00am to 12pm)
- condition 3 (no light)
Study measures spins on the wheel as a measure of ‘night time nocturnal activity’.
- In the usual 8am-8pm, mice’s activity on the wheel was greatest in the hours of dark outside of this window.
- When the light pattern was changed after a period of time, mice’s wheel running activity shifted to the new ‘night time’ 12pm to around 8pm.
- In the presence of normal light pattern, behaviour returns to normal pattern of running outside the 8am-8pm light period.
- When there is NO light, the running activity of the mice slowly shifts earlier every day, showing that mice have an internal rhythm that is lower than 24 hours.
What are 24 hour variations in behaviour and physiology produced by?
Endogenous circadian clocks
Synchronised light:dark cycles.
What modulates circadian rhythm in mammals? Where is it located? What does it rely on for light information?
Suprachiasmatic nucleus (in the hypothalamus)
It is positioned above the optic chiasm and is innervated by the optic nerve..
Receives information from the optic nerve from the retinal ganglion cells (a small % that measure ambient light).
At the beginning of what period of night are the rod cells saturated? What does saturated mean in scientific terms?
At the beginning of twilight.
It means the rod cells are fully hyperpolarised.
What range of light intensity do cones cells cover from fully depolarised to fully hyperpolarised? What does this mean for the cones?
Over 1000x
This means that there needs to be a mechanism to allow the relative shifting of light range that cones can detect before becoming saturated.
Outline the photoreceptor adaptation process for getting brighter.
BRIGHT LIGHT:
- Once the signalling cascade occurs in response to light the effects are as follows:
- Closing of Ion channels, membrane hyperpolarisation, reduced neurotransmitter release, bipolar cells receive decreased input - reducing RGC activation.
- These lead to a decreased sensitivity of the cell to light.
- Once these effects have happened, cGMP will start to rebuild (no PDE to stop this)
- This returns the cell to a new sensitised position in which it can now respond to light again.
Effectively, the process of becoming saturated, and closing the channels etc, causes the cell to hit baseline again.
Once the cGMP begins to increase, the cell moves closer to depolarisation again, until light activates the signalling cascade again.
This process is not instant and occurs gradually as the system continuously adjusts to changes in light levels.
Outline the changes in relationship between light intensity and photoreceptor polarisation.
Under dark adapted conditions, even relatively dim light drives full hyperpolarisation.
Under light adapted conditions, photoreceptor can be partly depolarised even under much brighter light.
What does ‘adaptation’ of cones allow?
Adaptation allows cones to report changes in light intensity over space and time under all lighting conditions.
(Allows cones to shift sensitivity to the new level of ambient light intensities as it increases from the night to day time).
What does bilateral enucleation mean?
The removal of both eyes.
Outline the study into a house sparrow with the bilateral enucleation condition.
(insert actogram with premium)
House sparrow with both eyes removed tested to see its circadian rhythms, measured by its perch hopping behaviour (hops in daylight).
Two conditions:
1 - No light
2 - Day light (~8am-8pm)
1.
No light, perch hopping behaviour starts later every day (the birds get up later everyday) for the whole condition. Shows circadian rhythm is longer than 24 hours.
- Consistent light. The perch hopping behaviour starts every day at the start of the light and ends at the end of the light. They have no eyes, yet they manage to set their circadian rhythms to the light provided.
This suggest extraocular circadian rhythm detection.
Outline the study into a house sparrow with the bilateral enucleation condition + black ink under scalp.
(look at lecture slides for actogram)
- Consistent light. The perch hopping behaviour starts every day at the start of the light and ends at the end of the light. They have no eyes, yet they manage to set their circadian rhythms to the light provided.
- Black ink injected under scalp.
Circadian rhythm no longer set to light, birds get up later everyday and goes to their innate > 24 hour circadian rhythm.
This shows that there is some extra-retinal photoreception occurring underneath the scalp: the photoreceptors synchronising the clock must be in the brain.
What is the result of bilateral enucleation in mammals? What does this show>
Abolishes all responses to light in mammals.
This shows that ‘time of day’ responses originate in the retina.
Outline the study in mice to figure out which cells (other than rods/cones) are photoreceptors.
Rodents with rods and cones knocked out to test whether they will express circadian rhythms without these photoreceptors.
They measured the pupil responses in these mice in response to light.
(ADD PIC WITH PREMIUM)
It was found that in dim light the pupils were large, and in light they were small.
Therefore, even without the rods and cones, there is still photoreception to detect the light in the environment and bring about physiological changes as a result.
Outline the study in which dye was injected into the SCN, what was it trying to find?
SCN is where the circadian rhythm is modulated.
By injecting dye, the connections from the eyes will be revealed retrograde (back along the path) to where the ambient light information originates from.
This dye travels down axons of retinal ganglion cells and labels >1% of all ganglion cells.
What occurred in the two types of retinal ganglion cells when removed from the retina and exposed to light?
ON retinal ganglion cells still show light response when removed from the retina.
All other retinal ganglion cells didn’t respond by themselves.
Outline the retinal ganglion cells that project to the hypothalamus.
They are photoreceptors and they depolarise to light - detect ambient light levels.
What do the amino acid sequences of opsin like proteins in vertebrate genomes indicate about them?
Shared evolutionary history with rod and cone opsins.
They are membrane associated.
They are GPCRs
They bind chromophores.
What is a common opsin between non-mammals and mammals non-rod/cone photoreceptors?
Melanopsin.
What was the beginning proof that melanopsin is relevant in ambient light detection?
Melanopsin is in the retinal ganglion cells that project to the hypothalamus.
Outline briefly the two gene studies into melanopsin.
FIRST:
In mice, those with a melanopsin knockout had no RGC’s that respond to light.
Suggesting that melanopsin is key in ambient light detection.
SECOND:
Add melanopsin to see if it brings about light sensitivity in a nerve cell that is not light sensitive (Neuro2A - mouse neuroblastoma)
This makes cells light sensitive.
Shows that melanopsin does make cells light sensitive.
Outline melanopsin phototransduction (signalling cascade). Where does it occur?
PLC
Occurrs in non-rod/cone photoreceptors (search examples).
- Melanopsin activated by photon of light.
- Leads to an activation of a Gq/11 type G-protein.
- Gamma/beta subunit dissociates and activates effector enzyme PLC.
- Activates the degradation of PIP2 into IP3 and DAG.
- Leads to opening of TRPC6/7 channels and voltage gated Ca+ channels - allowing influx of Na+ and Ca+.
- Action potential generated
Glossary:
- PLC: Phospholipase C
- PIP2: Phosphoatidylinositol bisphospate
- IP3: Inositol triphosphate
- DAG: Diacylglycerol
What is the Thalamus (basic structure)
Large structure divided into multiple nuclei with distinct functions (some visual, some non-visual)
What is the Lateral Geniculate Nucleus (basic structure)?
- Primary visual part of the Thalamus
- Vast majority of RGCs (around 90%) project here in primate
Where can the Thalamus be found and what does it connect?
It is part of the Diencephalon - connects the midbrain to the cerebral hemispheres
What is the basic role of the Thalamus (3)
- Receive information from the periphery
- Process and communicates to cortex
- Essential link in transfer of sensory information
How many cone types does the human retina have? What is the difference between them
3
They are maximally responsive to different parts of the spectrum.
Outline the mechanism behind the basis of colour vision.
Comparing between the cones spectral sensitivity allows us to perceive the spectrum of colour:
Red vs Green
Blue vs Yellow (which is red and green cones)
What is colour perception about?
Observing the ratio between your colour photoreceptor activity.
What wavelengths does the visible spectrum run between?
roughly 400-700nm
Outline centre:surround for red/green colour.
(PREMIUM)
What are the two mechanisms for achieving colour vision?
Colour opponent centre : surround (green vs red)
Colour opponent ON and OFF bipolars (blue vs yellow)
Briefly outline Opsin proteins
7 transmembrane domain G protein coupled receptor
Binds retinaldehyde and determines the spectral sensitivity of it.
Translates isomerisation of retinal into a ‘biological’ signal.
Retinal ALWAYS absorbs light, if retinal is always the same in each opsin, then how are different cone cells sensitive to different wave lengths of light?
Retinaldehyde is most sensitive to UV light.
When retinaldehyde is bound to an opsin, the amino acids that surround it change the set electrochemical state of the retinal, which changes the efficiency of light absorption of the retinal.
