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Neurophysiology > Receptors and sensory transduction > Flashcards

Receptors and sensory transduction Flashcards

1
Q

What do sensory receptors do

A

Transduce stimulus energies into electrochemical energy in the form of APs

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2
Q

Receptor class for light

A

Photoreceptor

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3
Q

Receptor class for sound

A

Mechanoreceptor

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4
Q

Receptor class for gravity (balance)

A

Mechanoreceptor

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5
Q

Receptor classes for somatosensory system

A

Mechanoreceptors, thermoreceptors, chemoreceptors

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6
Q

Receptor class for gustatory and olfactory system

A

Chemoreceptors

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7
Q

What does each receptor detect

A

Its own stimulus energy

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8
Q

What is Muller’s law of specific nerve energies (1985)

A

Modality is a property of the sensory nerve fibres, each nerve fibre is activated by a specific type of stimulus

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9
Q

What does univariance mean

A

Regardless of how neurons are activated, they produce the same sensation

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10
Q

By what 2 forms of transduction do receptors transduce stimulus energy

A

Ionotropic and metabotropic

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11
Q

What are the principles of how receptors transduce a stimulus

A

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

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12
Q

What happens atfter a stimulus has been transduced

A

Propagation of coding information to CNS if the threshold is reached, where info is decoded

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13
Q

What are receptor potentials

A

Graded, can lead to action potentials if threshold is reached

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14
Q

What is the impact of increase in stimulus intensity in the Pacinian corpuscle

A

Increased pressure on pacinian corpuscle causes a graded increase in receptor potentials, if threshold is reached an AP will be generated

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15
Q

How is intensity of stimulus endcoded in the Pacinian corpuscle

A

Frequency of action potentials

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16
Q

What is the effect of the encapsulated nerve ending of the Pacianian corpuscle

A

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)

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17
Q

What do action potentials in the Pacinian corpuscle reflect

A

The temporal profile of the stimulus, Pacinian corpuscle has phasic properties

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18
Q

APs of Pacinian corpuscle in response to step stimulus

A

Medium response at the start, adaption to continued stimulation so spikes drop off

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19
Q

APs of Pacinian corpuscle in response to vibration stimulus

A

period response showing spike increases only at the start of each stimulus

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20
Q

APs of Pacinian corpuscle in response to fast ramp up of stimulu

A

Large spike response only at the start of stimulus due to speed ot the ramp, spike adaption ti continued stimulus

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21
Q

APs of Pacinian corpuscle in response to slow ramp up of stimulus

A

Small spike response as stimulus slowly continues to change, then spike adaption to continued stimulus

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22
Q

How do olfactory receptors work

A

Chemoreceptors are stimulated by odorants, receptor recognises a particular molecular feature

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23
Q

How many types of olfactory receptor proteins are there

A

Buck and Axel (1991)- over 1000 different odorant receptor genes in rodents, humans have about 350 odorant receptor genes

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24
Q

What is one theory about odour coding in the olfactory system

A

Weak shape theory- different receptors detect small components of the odorant and these are built up to form overall perception

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25
Q

What GPCR does olfactory receptor transduction use

A

G (olf)

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26
Q

What happens when G (olf) GPCR is activated by binding to an odorant ligand

A

A subunit of G (olf) activates adenylate cyclase that causes production of cAMP from ATP

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27
Q

What is the effect of the production of cAMP as a second messenger in olfactory transduction

A

cAMP gated cation channel is opened when cAMP binds, allows entry of Na+ and Ca2+ causing depolarisation of a receptor potential

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28
Q

Adaption to olfactory stimuli- - what is the biochemical cascade of adaption

A

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

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29
Q

Adaption to olfactory stimuli- what is the effect of phosphodiesterase (PDE) converting cAMP to AMP

A

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

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30
Q

What is the range of sound intensity the human auditory system ca hear

A

0db- threshold of human hearing

130dB- gunshot, metal concert

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31
Q

What part of the ear gathers sounds

A

Pinna

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32
Q

How are soudns directed to the tympanic membrane

A

Directed down the auditory canal towards the tympanic membrane

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33
Q

What is the effect of the vibration of the tympanic membrane of cochlear fluid

A

Ossicles vibrate and amplify the sound, contact inner ear fluid of the coclea via the oval window, cause vibrations of fluid in cochlea

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34
Q

How do receptors in the ear detect sound

A

Vibrations of the fluid in the cochlea cause bending of the sensory hair cell stereocilia in the organ of corti

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35
Q

What is the function of inner hair cells

A

Send most of the sensory info that the brain uses to perceive sound

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36
Q

What is the function of the outer hair cell

A

Used to control the sensitivty of the inner ear by altering their length

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37
Q

What is the effect of the alternating deflections of the hair cells in the ear

A

Open and close mechanically gated channels in the hair cell stereocilia

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38
Q

What is the effect of opening the mechanically gated channels on hair cells

A

Allows depolarisation and neurotransmitter release onto the auditory nerve

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39
Q

What is the effect of closing the mechanically gated channels on hair cells

A

Causes hyperpolariation, decreasing neurotransmitter release onto the auditory nerve

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40
Q

What is the effect of neurotrasmitters released onto the auditory nerve by hair cells

A

Causes APs to be generated in the auditory nerve and transmitted to the brain

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41
Q

What is the cochlear amplififer

A

The action of outer hairs in adapting the auditory system to be more sensitive to quiet sounds by mechanically amplifying the signal

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42
Q

What is prestin

A

A motor protein on the membrane of OHCs that can bind Cl- ions

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43
Q

What is the state of prestin when the OHC is hyperpolarised

A

Cl- is bound to prestin, meaing presting is elongated

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44
Q

What is the state of prestin when the OHC is depolarised

A

The +ve charge attracts the Cl- away from prestin, activating the prestin motor, and shortening the OHC

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45
Q

What is the effect of the prestin motor changing the length of the OHC

A

Increases the power of the high and low pressure elements of the soudn wave, meaning inner hair cell response is greater

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46
Q

What is the attenuation reflex

A

Adaption to sudden or high intensity noises, reflex causes contraction of muscles that reduce the transmission of sound
Protective function

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47
Q

How much does the attenutation reflex decrease sound intensity

A

30-40db

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48
Q

What is the delay of the attenuation reflex

A

50-100msec

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49
Q

What is the range of visible wavelengths of light

A

400-700nm

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50
Q

What are the 2 photoreceptor types in the retina

A

Rod and cones- selective for different light wavelengths

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51
Q

What actiates signal transduction in photoreceptors

A

Light is absorbed by the photopigment- rhodopsin (rods) or photopsins (from the 3 types of cone), causing photoisomeration of retinal

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52
Q

What happens to cGMP channels of photoreceptors in the dark

A

Na+ enters the cell through cGMP gated channels, depolarising it and releases glutamate

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53
Q

What happens to cGMP channels of photoreceptors in the light

A

cGMP gated Na+ channels close, causing hyperpolarisation and reduced glutamate neurotransmitter release from photoreceptors

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54
Q

How does light cause the cGMP gated Na+ channels to close

A

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

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55
Q

What happens when the membrane potential of photoreceptors hyperpolarises from -40 to -65mV

A

It is bleached- can no longer respond to light

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56
Q

What is the functional range of the visual system spread over

A

10 logarithmic units in lux

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57
Q

What is photopic vision

A

Pure cone vision, bright light

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58
Q

What is mesopic vision

A

Both cone and rod vision, twilight

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59
Q

What is scotopic vision

A

Pure rod vision, starlight

60
Q

What is adaption

A

The ability of the eye to adjust to various levels of light

61
Q

What are the mechanisms used for dark adaption

A

Pupil dilation, shift from cone to rod vision, re-synthesis of rhodopsin

62
Q

What are the mechanisms used for light adaption

A

Pupil constriction, shift from rod to cone vision, re-synthesis of photopsin

63
Q

How do you conduct an experiment to plot dark adaption

A

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

64
Q

Dark adaption- how fast to both types of photoreceptor regenerate their photopigments

A

Cones regenerate their pigment faster than rods but are much less sensitive to low light levels

65
Q

Dark adaption- what is the sensitivity of rods vs cones after 25 mins in the dark

A

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

66
Q

What us the result of all the cGMP channels being closed when photoreceptors are bleached by bright light

A

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

67
Q

Dark adaption, effects of low intracellular Ca2+- guanylate cyclase

A

Low Ca2+ conc activates guanylate cyclase responsible for cGMP production, increasing cMP leels and reopning the channels to they can respond to light again

68
Q

Dark adaption, effects of low intracellular Ca2+- phosphodiesterase enzyme

A

Ca2+ inhibits the phosphodiesterase enzyme responsible for breaking down cGMP, increasing cGMP levels and reopening channels

69
Q

Dark adaption, effects of low intracellular Ca2+-channel affinity

A

Ca2+ increases channel affinity for cGMP, making it more likely to open again

70
Q

What 3 steps are common to all senses

A

Physical stimulus, set of events where stimulus is transduced into a emssage of nerve impulses, response to the message eg perception

71
Q

How do sensory systems receive info from the environemnt

A

Through specialised cells at the periphery fo the body called receptors

72
Q

Example of sensory info not reaching consciousness

A

eg reflex withdrawing a hand from a hot surface

73
Q

What does our body use sensory info from within the body for

A

Regulate temp, blood pressure, heart rate

74
Q

What does the body use info from the environment for

A

Perception, control of movement, regulating the function of internal organs, maintenance of arousal

75
Q

What are the different forms of energy transformed into sensations by the nervous system

A

Light, mechanical, thermal, chemical

76
Q

What do each of the sensory modalities have within them

A

Submodalities that constitute elementary senses that combine to form complex sensations

77
Q

What are receptors when it comes to stimuli

A

Stimulus specific

78
Q

What can most sensory receptors do in response to constant stimuli

A

Adapt

79
Q

What are the submodalities of the somatosensory system

A

Touch, proprioception, temp, pain, itch

80
Q

Where do different sensory modalities travel to

A

Make unique connections within the nervous system, have different sites of termination in the brain

81
Q

What is the sensory threshold

A

The lowest stimulus intensity a subject can detect

82
Q

What can influence the sensory threshold

A

Experience, fatigue or context eg threshold for pain raised in childbirth

83
Q

What does the sensitivity of the sensory system to differences in stimulus strength depend on

A

The difference in strength of the 2 stimuli eg easier to distinguish 1kg and 2kg than 50 and 51kg

84
Q

What is Weber’s law

A

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

85
Q

What does the formula for Weber’s law mean

A

∆S=K×S

∆S is the just noticable difference between 2 stimuli, K is constant, S is strength

86
Q

What is the two-point threshold

A

Quantifies the minimum detectable distance between 2 stimuli

87
Q

How does transduction make life easier for the nervous system

A

All sensory systems share a common signalling meaning (electrochemical energy)

88
Q

What are the 2 stages of the transfer of stimulus energy into neural discharge

A

Stimulus transduction (stimulus energy->de/hypolarisation), then neural encoding (signal evokes APs that represent stimulus information

89
Q

Which sensory systems have a primary sensory neuron as the receptor

A

Somatosensory and olfactory system- terminal portion of the neuron transduces stimulus energy and axon conveys APs to CNS

90
Q

What is a primary sensory neuron

A

Acts as a receptor in the somatosensory and olfactory system, the first neuron receptors project to in the gustatory, visual, auditory and vesitbular systems

91
Q

Which sensory systems don’t have a primary sensory neuron as the receptor

A

Gustatory, visual, auditory and vestibular systems- receptors are separate epithelial cells that communicate with the primary sensory neuron via a mechanism like synaptic transmission

92
Q

What are the 5 major types of sensory receptors in animals

A

Chemoreceptors, mechanoreceptors, thermoreceptors, photoreceptors, nociceptors

93
Q

What is the adequate stimulus

A

The paticular type of stimulus energy a receptor is sensitive to

94
Q

What is a receptive field

A

The range around a sensory receptor/primary sensory neuron in which a stimuli can active them

95
Q

What does the size of a receptive field determine for a sensory system

A

The spatial resolution of the sensory system, as greater resolution is made possible by cells with smaller receptor fields

96
Q

Where do the central nerve branches of primary sensory converge onto

A

Second-order neurons in the CNS that in turn converge on higher order neurons

97
Q

What are interneurons

A

Activated by sensory fibres, contribute to the processing of sensory information

98
Q

By what mechanisms is sensory information fine-tuned to achieve maximal discriminative capacity

A

Lateral inhibition

99
Q

How are primary sensory neurons depolarised

A

Produced by the opening of channels for Na+, K+ and Ca2+ by the impact of the stimulus on the receptor

100
Q

How is the receptor potential created in primary sensory neurons transmitted to the cell’s trigger zone

A

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

101
Q

What happens when the graded potential in primary sensory neurons reacehs the cell’s trigger zone

A

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

102
Q

What is the effect of stimulus strength on ghe amount of transmitter released by receptor cells (in visual, vestibular, auditory and gustatory)

A

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

103
Q

Where are stimulus transduction and neural encoding performed in the somatic sensory and olfactory systems

A

Stimulus transduction and the transmission of info are both performed in specialised regions of the primary sensory neuron

104
Q

Where are stimulus transduction and neural encoding performed in the visual, auditory, vestibular and gustatory systems

A

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

105
Q

What represents the neural code for the intensity and duration of the stimulus

A

Discharge patterns of APs- may be the pattern in a single neuron or an entire population of neurons

106
Q

How are sensory neurons topographically organised

A

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

107
Q

By what 2 codes is info about stimulus strength conveyed

A

Frequency and population codes

108
Q

What is the frequency code

A

The discharge frequecy of a primary sensory neuron increases with the strength of the stimulus, creating a neural code for stimulus strength

109
Q

Why do stronger stimulu increase discharge frequency of a primary sensory neuron

A

Stronger stimuli means receptor potentials with faster rates of rise and greater amplitude, evoking trains of APs with higher frequencies

110
Q

What limits the range of stimulus strength a primary sensory neuron can respond to

A

The transductive capabilities of the cell (eg no of channels) and the conductive properties of the axon that limit its discharge rate

111
Q

Why is the max stimulus strength one neuron can encode much lower than the maximal strength the entire sensory system can register

A

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

112
Q

How do sensory receptors all adapt to constant stimulation

A

The receptor potential ivariably decreases in amplitude in response to a persistent stimulus

113
Q

What are rapidly adapting receptors

A

Respond transiently to stimulation and only at the onset and offset of the stimulus

114
Q

What are slowly adapting receptors

A

Fire throughout application of the stimulus

115
Q

How does receptor adaption act as a signal filter

A

Eliminates neural signals related to the steady or slow components of a stimulus, focusing the neuron’s responses on abrupt changes in stimulus intensity

116
Q

What is feature extraction

A

An aspect of coding where certain stimulus features are selectively detected and accentuated eg adaption to constant stimuli

117
Q

What is the labelled line code

A

The most important coding mechanism for stimulus modality- enabled by receptor specificity defining the modality of the entire sensory pathway from receptor to cortex

118
Q

What is lateral inhibition

A

A mechanism in the sensory system that enhances the contract of stimulus features, allowing us to discriminate one stimulus from another

119
Q

What components of receptive fields of sensory relay neurons are there

A

Excitatory and inhibitory region, meaning they receive convergent excitatory input from many presynaptic neurons

120
Q

What creates a gradient of excitation within the excitatory field of a sensory relay neuron

A

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

121
Q

What reinforces the gradient of excitation in the excitatory field of a sensory relay neuron

A

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

122
Q

What is the effect if the gradient of excitation in the excitatory field of sensory relay neurons

A

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

123
Q

What are 2 other types of inhibition in sensory systems other than lateral inhibition

A

Feed-forward and distal inhibition

124
Q

What does feed-forward inhibition allow

A

What Sherrington called a singleness of action, which ensures only one of two or more competing responses is expressed

125
Q

What is the structure of neurons involved in feed-forward inhibition

A

Creates a central zone of intensity activity surrounded by a ring of lesser activity in the inputs to higher order neurons

126
Q

How does feed-forward inhibition work

A

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

127
Q

Where are feed-forward inhibition pathways used

A

local circuits only, involving only the cells within one relay nucleus

128
Q

What is distal inhibition

A

The mechanisms by which neurons from higher centres eg motor cortex can inhibit and control the flow of info into relay nuclei

129
Q

What are the different type of receptors for taste

A

Each type of taste - saltiness, sourness, sweetness, bitterness and umami- each have special receptors

130
Q

Where are taste receptors on the tongue

A

50-150 taste receptor cells on each taste bud, taste buds are located on the bumps on the tongue called papillae

131
Q

How do taste stimuli reach threshold

A

Must be of a high enough concentration

132
Q

What is the structure of taste receptor cells

A

The apical ends have microvilli that project into a taste pore aka small opening on the tongue surface

133
Q

What do taste receptor cells form synapses with

A

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

134
Q

What ion causes depolarisation in taste receptor cells

A

Ca2+ entering the cytoplasm

135
Q

How is transmitter released dependent on taste receptor cell type

A

Sour and salty taste cells release serotonin, sweet, bitter and umami taste cells release ATP

136
Q

Evidence for taste receptive cells being selective

A

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

137
Q

What causes the differences in selectivity of different receptor cellsl

A

Transduction mechanisms present

138
Q

What different taste transduction processes are there

A

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)

139
Q

What is the prototrpical salty compoent of food

A

NCL- salt sensitive taste cells use a Na+ selective channel

140
Q

Evidence for Na+ channels opened by salty food

A

Na+ selective channels are blocked by the drug amiloride, a diuretic used to treat hypertension/heart disease by helping the body excrete salt

141
Q

What are the receptors for the olfactory system

A

Olfactory receptor cells- genuine neurons with axons that penetrate into the CNS

142
Q

How do odorants reach the olfactory receptor cells

A

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

143
Q

Evidence for the role of olfactory receptor cells in smell

A

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

144
Q

How do olfactory receptor cells project into the CNS

A

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

145
Q

How do odorants activate olfactory receptor neurons

A

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

146
Q

Clinical evidence for the role of olfactory axons as primary sensory neurons

A

Traumatic injury can create force between the cribiform plate and surrounding tissue that severs the olfactory axons, resulting in anosmia (inability to smell)

Neurophysiology (23 decks)

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