Receptors and sensory transduction Flashcards
What do sensory receptors do
Transduce stimulus energies into electrochemical energy in the form of APs
Receptor class for light
Photoreceptor
Receptor class for sound
Mechanoreceptor
Receptor class for gravity (balance)
Mechanoreceptor
Receptor classes for somatosensory system
Mechanoreceptors, thermoreceptors, chemoreceptors
Receptor class for gustatory and olfactory system
Chemoreceptors
What does each receptor detect
Its own stimulus energy
What is Muller’s law of specific nerve energies (1985)
Modality is a property of the sensory nerve fibres, each nerve fibre is activated by a specific type of stimulus
What does univariance mean
Regardless of how neurons are activated, they produce the same sensation
By what 2 forms of transduction do receptors transduce stimulus energy
Ionotropic and metabotropic
What are the principles of how receptors transduce a stimulus
Receptor detects specific stimulus, causes a change in ionic permeability of afferent nerve ending and thus change in membrane potential, causes an alteration in the receptor or APs in afferent nerve terminal
What happens atfter a stimulus has been transduced
Propagation of coding information to CNS if the threshold is reached, where info is decoded
What are receptor potentials
Graded, can lead to action potentials if threshold is reached
What is the impact of increase in stimulus intensity in the Pacinian corpuscle
Increased pressure on pacinian corpuscle causes a graded increase in receptor potentials, if threshold is reached an AP will be generated
How is intensity of stimulus endcoded in the Pacinian corpuscle
Frequency of action potentials
What is the effect of the encapsulated nerve ending of the Pacianian corpuscle
Capsule modifies the sensitivity of the bare mechanoreceptive axon, giving it phasic properties- without it, the nerve is more sensitive to static pressure (Loewenstein and Mendelson, 1965)
What do action potentials in the Pacinian corpuscle reflect
The temporal profile of the stimulus, Pacinian corpuscle has phasic properties
APs of Pacinian corpuscle in response to step stimulus
Medium response at the start, adaption to continued stimulation so spikes drop off
APs of Pacinian corpuscle in response to vibration stimulus
period response showing spike increases only at the start of each stimulus
APs of Pacinian corpuscle in response to fast ramp up of stimulu
Large spike response only at the start of stimulus due to speed ot the ramp, spike adaption ti continued stimulus
APs of Pacinian corpuscle in response to slow ramp up of stimulus
Small spike response as stimulus slowly continues to change, then spike adaption to continued stimulus
How do olfactory receptors work
Chemoreceptors are stimulated by odorants, receptor recognises a particular molecular feature
How many types of olfactory receptor proteins are there
Buck and Axel (1991)- over 1000 different odorant receptor genes in rodents, humans have about 350 odorant receptor genes
What is one theory about odour coding in the olfactory system
Weak shape theory- different receptors detect small components of the odorant and these are built up to form overall perception
What GPCR does olfactory receptor transduction use
G (olf)
What happens when G (olf) GPCR is activated by binding to an odorant ligand
A subunit of G (olf) activates adenylate cyclase that causes production of cAMP from ATP
What is the effect of the production of cAMP as a second messenger in olfactory transduction
cAMP gated cation channel is opened when cAMP binds, allows entry of Na+ and Ca2+ causing depolarisation of a receptor potential
Adaption to olfactory stimuli- - what is the biochemical cascade of adaption
Ca2+ that enters through the cAMP gated channel binds to calmodulin (CaM) to form a complex that activates enzyme phosphodiesterase (PDE) that converts cAMP to AMP
Adaption to olfactory stimuli- what is the effect of phosphodiesterase (PDE) converting cAMP to AMP
Some cAMP dependent channels close, less depolarisation and receptor potential despite continued exposure to the odorant, less AP spikes sent to the brain, less perception of an odour
What is the range of sound intensity the human auditory system ca hear
0db- threshold of human hearing
130dB- gunshot, metal concert
What part of the ear gathers sounds
Pinna
How are soudns directed to the tympanic membrane
Directed down the auditory canal towards the tympanic membrane
What is the effect of the vibration of the tympanic membrane of cochlear fluid
Ossicles vibrate and amplify the sound, contact inner ear fluid of the coclea via the oval window, cause vibrations of fluid in cochlea
How do receptors in the ear detect sound
Vibrations of the fluid in the cochlea cause bending of the sensory hair cell stereocilia in the organ of corti
What is the function of inner hair cells
Send most of the sensory info that the brain uses to perceive sound
What is the function of the outer hair cell
Used to control the sensitivty of the inner ear by altering their length
What is the effect of the alternating deflections of the hair cells in the ear
Open and close mechanically gated channels in the hair cell stereocilia
What is the effect of opening the mechanically gated channels on hair cells
Allows depolarisation and neurotransmitter release onto the auditory nerve
What is the effect of closing the mechanically gated channels on hair cells
Causes hyperpolariation, decreasing neurotransmitter release onto the auditory nerve
What is the effect of neurotrasmitters released onto the auditory nerve by hair cells
Causes APs to be generated in the auditory nerve and transmitted to the brain
What is the cochlear amplififer
The action of outer hairs in adapting the auditory system to be more sensitive to quiet sounds by mechanically amplifying the signal
What is prestin
A motor protein on the membrane of OHCs that can bind Cl- ions
What is the state of prestin when the OHC is hyperpolarised
Cl- is bound to prestin, meaing presting is elongated
What is the state of prestin when the OHC is depolarised
The +ve charge attracts the Cl- away from prestin, activating the prestin motor, and shortening the OHC
What is the effect of the prestin motor changing the length of the OHC
Increases the power of the high and low pressure elements of the soudn wave, meaning inner hair cell response is greater
What is the attenuation reflex
Adaption to sudden or high intensity noises, reflex causes contraction of muscles that reduce the transmission of sound
Protective function
How much does the attenutation reflex decrease sound intensity
30-40db
What is the delay of the attenuation reflex
50-100msec
What is the range of visible wavelengths of light
400-700nm
What are the 2 photoreceptor types in the retina
Rod and cones- selective for different light wavelengths
What actiates signal transduction in photoreceptors
Light is absorbed by the photopigment- rhodopsin (rods) or photopsins (from the 3 types of cone), causing photoisomeration of retinal
What happens to cGMP channels of photoreceptors in the dark
Na+ enters the cell through cGMP gated channels, depolarising it and releases glutamate
What happens to cGMP channels of photoreceptors in the light
cGMP gated Na+ channels close, causing hyperpolarisation and reduced glutamate neurotransmitter release from photoreceptors
How does light cause the cGMP gated Na+ channels to close
Light causes a conformational change in opsin, that activates a G protein called transducin, that triggers the enzyme phosphodiesterase (PDE) to break down cGMP and reduce cGMP levels
What happens when the membrane potential of photoreceptors hyperpolarises from -40 to -65mV
It is bleached- can no longer respond to light
What is the functional range of the visual system spread over
10 logarithmic units in lux
What is photopic vision
Pure cone vision, bright light
What is mesopic vision
Both cone and rod vision, twilight
What is scotopic vision
Pure rod vision, starlight
What is adaption
The ability of the eye to adjust to various levels of light
What are the mechanisms used for dark adaption
Pupil dilation, shift from cone to rod vision, re-synthesis of rhodopsin
What are the mechanisms used for light adaption
Pupil constriction, shift from rod to cone vision, re-synthesis of photopsin
How do you conduct an experiment to plot dark adaption
Subjects gaze at a bright light for 5 mins so all the photopigment in rods and cones are bleached- the threshold for detecting a spot of light is measured after a range of times in the darkness
Dark adaption- how fast to both types of photoreceptor regenerate their photopigments
Cones regenerate their pigment faster than rods but are much less sensitive to low light levels
Dark adaption- what is the sensitivity of rods vs cones after 25 mins in the dark
Rods are at their most sensitive as they can adapt to much dimmer light
Cones have a relatively high threshold, so aren’t very sensitive for dark-adapted vision
What us the result of all the cGMP channels being closed when photoreceptors are bleached by bright light
There is a drop in intracellular Ca2+, triggering changes in the signal transduction pathway that decrease sensitivity to the absoltue light level and allow contrast/detail to be seen
Dark adaption, effects of low intracellular Ca2+- guanylate cyclase
Low Ca2+ conc activates guanylate cyclase responsible for cGMP production, increasing cMP leels and reopning the channels to they can respond to light again
Dark adaption, effects of low intracellular Ca2+- phosphodiesterase enzyme
Ca2+ inhibits the phosphodiesterase enzyme responsible for breaking down cGMP, increasing cGMP levels and reopening channels
Dark adaption, effects of low intracellular Ca2+-channel affinity
Ca2+ increases channel affinity for cGMP, making it more likely to open again
What 3 steps are common to all senses
Physical stimulus, set of events where stimulus is transduced into a emssage of nerve impulses, response to the message eg perception
How do sensory systems receive info from the environemnt
Through specialised cells at the periphery fo the body called receptors
Example of sensory info not reaching consciousness
eg reflex withdrawing a hand from a hot surface
What does our body use sensory info from within the body for
Regulate temp, blood pressure, heart rate
What does the body use info from the environment for
Perception, control of movement, regulating the function of internal organs, maintenance of arousal
What are the different forms of energy transformed into sensations by the nervous system
Light, mechanical, thermal, chemical
What do each of the sensory modalities have within them
Submodalities that constitute elementary senses that combine to form complex sensations
What are receptors when it comes to stimuli
Stimulus specific
What can most sensory receptors do in response to constant stimuli
Adapt
What are the submodalities of the somatosensory system
Touch, proprioception, temp, pain, itch
Where do different sensory modalities travel to
Make unique connections within the nervous system, have different sites of termination in the brain
What is the sensory threshold
The lowest stimulus intensity a subject can detect
What can influence the sensory threshold
Experience, fatigue or context eg threshold for pain raised in childbirth
What does the sensitivity of the sensory system to differences in stimulus strength depend on
The difference in strength of the 2 stimuli eg easier to distinguish 1kg and 2kg than 50 and 51kg
What is Weber’s law
Weber (1934)- difference in magnitude between 2 stimuli must increase in proportion to the strength of the reference stimulus for a difference to be detected
∆S=K×S
What does the formula for Weber’s law mean
∆S=K×S
∆S is the just noticable difference between 2 stimuli, K is constant, S is strength
What is the two-point threshold
Quantifies the minimum detectable distance between 2 stimuli
How does transduction make life easier for the nervous system
All sensory systems share a common signalling meaning (electrochemical energy)
What are the 2 stages of the transfer of stimulus energy into neural discharge
Stimulus transduction (stimulus energy->de/hypolarisation), then neural encoding (signal evokes APs that represent stimulus information
Which sensory systems have a primary sensory neuron as the receptor
Somatosensory and olfactory system- terminal portion of the neuron transduces stimulus energy and axon conveys APs to CNS
What is a primary sensory neuron
Acts as a receptor in the somatosensory and olfactory system, the first neuron receptors project to in the gustatory, visual, auditory and vesitbular systems
Which sensory systems don’t have a primary sensory neuron as the receptor
Gustatory, visual, auditory and vestibular systems- receptors are separate epithelial cells that communicate with the primary sensory neuron via a mechanism like synaptic transmission
What are the 5 major types of sensory receptors in animals
Chemoreceptors, mechanoreceptors, thermoreceptors, photoreceptors, nociceptors
What is the adequate stimulus
The paticular type of stimulus energy a receptor is sensitive to
What is a receptive field
The range around a sensory receptor/primary sensory neuron in which a stimuli can active them
What does the size of a receptive field determine for a sensory system
The spatial resolution of the sensory system, as greater resolution is made possible by cells with smaller receptor fields
Where do the central nerve branches of primary sensory converge onto
Second-order neurons in the CNS that in turn converge on higher order neurons
What are interneurons
Activated by sensory fibres, contribute to the processing of sensory information
By what mechanisms is sensory information fine-tuned to achieve maximal discriminative capacity
Lateral inhibition
How are primary sensory neurons depolarised
Produced by the opening of channels for Na+, K+ and Ca2+ by the impact of the stimulus on the receptor
How is the receptor potential created in primary sensory neurons transmitted to the cell’s trigger zone
Potential is a graded response proportional to the strength of the stimulus, its amplitude decreases as it propagates passively down the axon as a function of distance from the site of transduction
What happens when the graded potential in primary sensory neurons reacehs the cell’s trigger zone
APs are generated if the amplitude of the receptor potential is above the cell’s threshold for firing- these APS are transmitted over the length of the axon to a central neuron
What is the effect of stimulus strength on ghe amount of transmitter released by receptor cells (in visual, vestibular, auditory and gustatory)
The amount of transmitter released by the receptor onto the primary sensory neuron (or the reduction in photoreceptors) is proportional to the strength of the stimulus
Where are stimulus transduction and neural encoding performed in the somatic sensory and olfactory systems
Stimulus transduction and the transmission of info are both performed in specialised regions of the primary sensory neuron
Where are stimulus transduction and neural encoding performed in the visual, auditory, vestibular and gustatory systems
Stimulus transduction is done by the receptor cell, and neural encoding is done by the primary sensory neuron it communicates with via a chemical synapse
What represents the neural code for the intensity and duration of the stimulus
Discharge patterns of APs- may be the pattern in a single neuron or an entire population of neurons
How are sensory neurons topographically organised
Sensory pathways are anatomically arranged to preserve spatially organised maps of the receptive surfaces, allowig us to localise stimuli- the location of a stimulus is encoded by the distribution of active neurons in the population of sensory neurons
By what 2 codes is info about stimulus strength conveyed
Frequency and population codes
What is the frequency code
The discharge frequecy of a primary sensory neuron increases with the strength of the stimulus, creating a neural code for stimulus strength
Why do stronger stimulu increase discharge frequency of a primary sensory neuron
Stronger stimuli means receptor potentials with faster rates of rise and greater amplitude, evoking trains of APs with higher frequencies
What limits the range of stimulus strength a primary sensory neuron can respond to
The transductive capabilities of the cell (eg no of channels) and the conductive properties of the axon that limit its discharge rate
Why is the max stimulus strength one neuron can encode much lower than the maximal strength the entire sensory system can register
A stimulus activates more receptors the higher the stimulus, meaning the size of the population of responding neurons provides a population code for stimulus strength
How do sensory receptors all adapt to constant stimulation
The receptor potential ivariably decreases in amplitude in response to a persistent stimulus
What are rapidly adapting receptors
Respond transiently to stimulation and only at the onset and offset of the stimulus
What are slowly adapting receptors
Fire throughout application of the stimulus
How does receptor adaption act as a signal filter
Eliminates neural signals related to the steady or slow components of a stimulus, focusing the neuron’s responses on abrupt changes in stimulus intensity
What is feature extraction
An aspect of coding where certain stimulus features are selectively detected and accentuated eg adaption to constant stimuli
What is the labelled line code
The most important coding mechanism for stimulus modality- enabled by receptor specificity defining the modality of the entire sensory pathway from receptor to cortex
What is lateral inhibition
A mechanism in the sensory system that enhances the contract of stimulus features, allowing us to discriminate one stimulus from another
What components of receptive fields of sensory relay neurons are there
Excitatory and inhibitory region, meaning they receive convergent excitatory input from many presynaptic neurons
What creates a gradient of excitation within the excitatory field of a sensory relay neuron
Primary sensory neurons carrying info from the central portion of the relay neuron’s receptive field discharge the neuron mroe effectively than those at the receptive field periphery
What reinforces the gradient of excitation in the excitatory field of a sensory relay neuron
Receptor neurons also contact inhibitory interneurons- receptor neurons innervating the centre of the field as less vulnerable to the efefcts of the inhibition than those at the periphery as the strength of the excitatory connection is stronger
What is the effect if the gradient of excitation in the excitatory field of sensory relay neurons
Reduced likelihood that stimuli in the periphery of the field will activate the relay neuron, focusing the size of the receptive field and enhancing the acuity of the sensory system
What are 2 other types of inhibition in sensory systems other than lateral inhibition
Feed-forward and distal inhibition
What does feed-forward inhibition allow
What Sherrington called a singleness of action, which ensures only one of two or more competing responses is expressed
What is the structure of neurons involved in feed-forward inhibition
Creates a central zone of intensity activity surrounded by a ring of lesser activity in the inputs to higher order neurons
How does feed-forward inhibition work
By enhancing/amplifying the contradt between highly active cells and their neighbours, these circuits contribute to selective perception where we attend to one stimulus and not another
Where are feed-forward inhibition pathways used
local circuits only, involving only the cells within one relay nucleus
What is distal inhibition
The mechanisms by which neurons from higher centres eg motor cortex can inhibit and control the flow of info into relay nuclei
What are the different type of receptors for taste
Each type of taste - saltiness, sourness, sweetness, bitterness and umami- each have special receptors
Where are taste receptors on the tongue
50-150 taste receptor cells on each taste bud, taste buds are located on the bumps on the tongue called papillae
How do taste stimuli reach threshold
Must be of a high enough concentration
What is the structure of taste receptor cells
The apical ends have microvilli that project into a taste pore aka small opening on the tongue surface
What do taste receptor cells form synapses with
The endings of the gustatory afferent axons near the bottom of the taste bud
Also form electrical and chemical synapses on some basal cells, some of which synapse onto the sensory axons to form a simple info-processing circuit within each taste bud
What ion causes depolarisation in taste receptor cells
Ca2+ entering the cytoplasm
How is transmitter released dependent on taste receptor cell type
Sour and salty taste cells release serotonin, sweet, bitter and umami taste cells release ATP
Evidence for taste receptive cells being selective
Sato (1980)- shows action potential firing rates of 4 different primary gustatory nerve axons in a rat, one responds strongly only to salt, one to sweet, and two to all but sweet
What causes the differences in selectivity of different receptor cellsl
Transduction mechanisms present
What different taste transduction processes are there
Taste stimuli may directly pass through ion channels (sweet and sour), bind to and block ion channels (sour) or bind to GPCRs that open ion channels via second messenger systems (bitter, sweet, umami)
What is the prototrpical salty compoent of food
NCL- salt sensitive taste cells use a Na+ selective channel
Evidence for Na+ channels opened by salty food
Na+ selective channels are blocked by the drug amiloride, a diuretic used to treat hypertension/heart disease by helping the body excrete salt
What are the receptors for the olfactory system
Olfactory receptor cells- genuine neurons with axons that penetrate into the CNS
How do odorants reach the olfactory receptor cells
Sniffing brings air to pass over the olfactory epithelium lining the nasal cavity, where odorants dissolve in the thin mucus layer to reach the olfactory receptor neurons
Evidence for the role of olfactory receptor cells in smell
The size of the olfactory epithelium and density of receptors indicates an animal’s olfactory acuity- dogs have an olfacotyr epithelium that is almost 20x the SA of humans wit 100x more receptors per sqcm
How do olfactory receptor cells project into the CNS
They have thin unmyelinated axons that collectively constitute the olfactory nerve- after leaving the epithelium, small clusters of the axons penetrate the cribiform plate then course into the olfactory bulb in the brain
How do odorants activate olfactory receptor neurons
Olfactory receptor neurons have a single thin dendrite that ends in a small knob at the epithelium surface with several long cilia waving from it
Odorants bind to the cilia surface and activate the transduction process
Clinical evidence for the role of olfactory axons as primary sensory neurons
Traumatic injury can create force between the cribiform plate and surrounding tissue that severs the olfactory axons, resulting in anosmia (inability to smell)
