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Somatosensory System - Receptors & Transduction Flashcards

1
Q

Give 2 examples of propioception.

Why is it important?

A
  • Golgi Tendon Organ + Muscle Spindles
  • Control of Movement
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2
Q

What are the 3 types of sensory inputs that can arrive via sensory neurones of the DRG?

A
  • Cutaneous (e.g. touch & pain)
  • Propioceptive (e.g. muscle length)
  • Visceral (e.g. enteric pain & arterial O2)
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3
Q

What does somatosensation include?

A

Does not include special senses (e.g. olfaction & taste & hearing)

  • Touch (light)
  • Touch (discriminative / 2-point)
  • *Nociception**
  • Temperature
  • Vibration Sense
  • etc.
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4
Q

What can the composition of a spinal nerve be?

A
  • Mixed Nerve
  • Motor Nerve
  • Sensory Nerve

(come from muscle joints or skin)

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5
Q

What are different categories of sensory neurones surrounded by?

A
  • Satellite Cells
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6
Q

What is found in the Dorsal Root Ganglia (DRG)?

A
  • Cell Bodies of All Sensory Neurons
  • Satellite Cells
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7
Q

Give an example of the fastest neuron that is myelinated.

Give an example of a non-myelinated fibre.

A
  • 1a Afferents (muscle spindles)
  • C-Fibres (nociceptive fibres)
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8
Q

What are the 4 skin receptors called?

How are they arranged?

A
  • Merkel’s Disk
  • Meissner’s Corpuscle
  • Pacinian Corpuscle
  • Ruffini’s Ending

Arranged as two superficial receptors and two deep receptors

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9
Q

What are the two superficial receptors?

A
  • Merkel’s Disk (quite small)
  • Meissner’s Corpuscle (encapsulated)
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10
Q

What are the superficial receptors able to detect?

A
  • Low Intensity Mechanical Events on the skin surface
  • Touch Receptors
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11
Q

What are the deep receptors?

A
  • Pacinian Corpuscle (encapsulated)
  • Ruffini’s Ending
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12
Q

What is a good way of remembering the layers?

A
  • Germans –> SUPERFICIAL
  • Italians –> DEEP
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13
Q

What do mechanical receptors respond to?

A
  • Respond to mechanical events
  • Such as contact & pressure
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14
Q

When are deep receptors activated?

Does this impact sensitivity?

A
  • More substantial pressures & deformations on the surface –> to activate the deep receptors (than for superficial receptors)
  • This does not mean they are less sensitive –> just need a larger amplitude depression –> to press down on tissue enough –> to get an activation from the deep receptors
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15
Q

What codes for stimulus strength?

What drives this?

A
  • Impulse Frequency encodes for stimulus strength
  • Receptor potential drives impulse discharge
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16
Q

Why is saltatory conduction fast?

A
  • Because it is passive in the myelinated regions
  • Only active in the node regions –> where active ion exchange takes place (slow phase)
  • Active regions are there to renew AP so it does not fade away
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17
Q

How is a stimulus generated at a receptor generally?

A
  • Mechanical disturbance created by depression –> causes a change in receptor potential (can measure using recording electrode)
  • This causes threshold level to be acheived –> initiates a spike –> AP produced and propogates down axon
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18
Q

What is the frequency proportional to?

What is the nature of this proportionality?

A
  • Mechanical Disturbance
  • Log Relationship (plateus eventually)

(most neuronal coding has a log relationship between stimulus strength & firing rate)

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19
Q

What are the two types of sensory receptors?

A
  • Slow Adapating (SA)
  • Rapidly Adapting (RA)
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20
Q

Why are there two receptor types?
Where are they found?

A
  • Cover a range of stimulus intensity –> requiring accurate coding
  • This is typical in many different sensory modalities
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21
Q

How do Static / Slow Adapating level receptors operate?

A
  • Measure slowly changing levels or maintained levels
    (e. g. pressure on skin that is sustained - can work for a few seconds only though - not forever)

This can report onset of a stimulus but not the dynamics of the onset (i.e. how its changing/rate)

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22
Q

How do Dynamic / Rapid Adapating receptors operate?

A
  • Measure the changes during the onset of stimuli
    (e. g. initial contact stimuli)
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23
Q

What are the properties of slowly adapting (SA) receptors?

A
  • Higher the Depression = Higher the Average Firing Rate
  • Receptor can respond throughout the stimulus duration
  • Moderately stable but not completely stable (it trails over time)
  • Stimulus extended for a long time –> would expect decline
  • Slowly adapting
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24
Q

What are the properties of rapidly adapting (RA) receptors?

A
  • Capable of detecting onset of the stimuli
  • Ramped –> I.e. they have different rates of reaching the asme pressure (measures rates - bunched up AP or spread out?)
  • Not much firing rate in sustained phase
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25
Q

How is vibration related to palpation?

A
  • Run fingers over a rough surface
  • This sets up vibrations frequency
  • This is related to the speed the person is moving their fingers across
  • Detecting texture by palpating surface –> as the receptor is set up to recognise frequencies & textures (e.g. using vibrations)
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26
Q

Why is a rapid & slow adapting receptor needed?

A
  • Economy (only need to know about changes)
  • Confers special sensitivity to high frequency vibration used in tactile discrimination (e.g. by palpating surfaces)
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27
Q

How many classes are there of PNS sensory afferent fibres?

What do they begin with?

What are they classified by?

What are their properties?

A
  • 4 classes
  • Begin with A or C
  • Diameter + Myelination status
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28
Q

What is the only sensory afferent fibre that is not myelinated?

A
  • C-fibres
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29
Q

What are C-fibres receptors for?

A
  • Pain
  • Itch
  • Temperature
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30
Q

What sensory modalities are A-delta fibres for?

A
  • Fast Pain
  • Temperature
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31
Q

What sensory modalities are A-beta fibres for?

A
  • Skin Mechanoreceptors (touch)
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32
Q

What sensory modalities are A-alpha fibres for?

A
  • Propioceptors
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33
Q

What is the largest & smallest sensory afferent myelinated fibres?

A
  • A-alpha –> largest
  • A-delta –> smallest
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34
Q

Which PNS sensory category (due to cross-over) are alpha motor neurones in?

A
  • A-alpha category
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35
Q

What is the class I-IV for?

A
  • Deep afferents from muscles & joints

(partial overlap - e.g. class I = A-alpha)

36
Q

What cutaneous receptor fibres convey pain?

What is the difference?

A
  • C-fibres –> slow (2 m/s)
  • A-delta –> fast (5-30 m/s)
37
Q

Where do pain responses come from?

A
  • First –> centrally –> in the spinal cord
  • Second –> somatosensory cortex
38
Q

What are A-deltas involved in?

A
  • Flexion Reflex
39
Q

What two effects does the stimulation of nociception have?

A
  • Local Effect (first)
  • Central Effect (second)

Both are capable of turning up sensitivity of the response

40
Q

What causes a release of Substance P?

A
  • Branching/collaterals from the afferent fibre goes back to the local area near periphery –> leads to release of substance P
41
Q

What effects does Substance P have?

A
  • Affects Mast Cells –> causes release of histamine
  • Histamine –> causes sensitisation of the receptor-ending itself
  • This promotes inflammation & swelling around the stimulus site
  • Promotes the local release of inflammatory mediators.

(this is only caused by a strong stimulus that is causing pain - thus if it is enduring & causing local inflammatory response to promote swelling)

42
Q

What does the release of histamine cause?

A

Release of:

  • Bradykinin
  • Potassium (K+)
  • Prostaglandin

These influence receptors causing a positive feedback loop within the detection system

43
Q

What effect does Substance P have?

A
  • Relaxation of Blood Vessels
  • Allowing for an increase in blood flow & inflammation
44
Q

What is the ‘Axon Reflex for Nociceptors’?

A
  • After Tissue Injury –> receptors in the local area are more sensitive to local effects (due to local effects)
45
Q

What is meant by a ‘silent receptor’?
Where are these commonly found?

A
  • Receptors that are relatively silent until there is some kind of inflammation or injury –> which is induced by a local response –> then a central one
  • Visceral Receptors –> these are silent until there is an effect (e.g. ischaemia) –> where they start to signal
46
Q

What are the 2 proprties of silent nociceptors?

A
  1. High Threshold Mechanoreceptors
  2. Sensitised by Inflammation (e.g. viscera)
47
Q

Which fibre types carry temperature?

A
  • C-fibres
  • A-delta fibres
48
Q

What are the 2 types of temperature receptors?

How do they work?

A
  • Cold Receptors
  • Warm Receptors

They work similar to colour vision (but those have 3 types) –> to extract temperature information over a wide spectrum

49
Q

What is the pattern of a cold fibre?

A
  • Activity begins at 15 degrees
  • Peaks at 25 degrees
  • Trails down as it gets near 40 degrees
50
Q

How is temperature coded?

A
  • Firing Rate (temperature)
51
Q

How does the body overcome differentiating between signals on either side of the bell curve?

A
  • There is cross-over per frequency (e.g. 4 AP can mean two temperatures)
  • However we have warm & cold receptors
  • Warm receptor sensitivity range between 30-50 degrees
  • Therefore, if the warm receptor is also firing –> then you can tell it is the upper part of the range
  • Body takes into account activity of both receptor types –> to work out high/low temperatures
52
Q

What is meant by paradoxical cold?

How does it come about?

A
  • Cold Receptors –> begin to fire again at high 40s (after dropping to 0)
  • At 50 degrees –> warm receptors stop firing
  • Thus you can feel hot water between 65-70 degrees as a sensation of paradoxical cold (unsure if hot or cold)

This is due to EXCLUSIVE ACTIVATION of the COLD RECEPTORS

53
Q

What is the disstribution of the hot and cold fibres subcutaneously?

A
  • Mosaic Distribution
54
Q

What are temperature receptors also very useful for? (other than plain temperature)

A
  • Detecting the properties of objects
  • They are more than danger signals –> the specific heat of an object is an important tactile identifier (e.g. wetness, metal vs plastic etc.)

(e.g. ambient objects are usually colder thus we detect temperature difference)

(other signals like if they are shiny help us work out if things are metallic)

(temperature gives us an identifying factor when we palpate using out receptors)

55
Q

What sensory fibres carry mechanoreceptors?

A
  • A-beta
  • A-delta
56
Q

How are superficial receptor distributed?

A
  • Differently across the ridges of the superficial skin

(ridges give us our fingerprints)

57
Q

What are the two receptors found superficially?

A
  • Merkel’s Discs
  • Meissner’s Corpuscles
58
Q

Which is slow adapting and fast adapting between the superficial receptors?

A
  • Merkel’s Discs –> Slow Adapting
  • Meissner’s Corpuscles –> Fast Adapting
59
Q

What are the properties of Merkel’s Discs?

A
  • Sits in valleys between the two grooves
  • Slow adapting
60
Q

What are the properties of Meissner’s Corpuscles?

A
  • Situated alongside the ridges
  • This is where the grooves go down –> on either side of groove
61
Q

What sensation are Meissner Corpuscles good at sensing?

A
  • Vibration sense (e.g. during palpation of objects)
  • During palpation –> ridges are tickled –> these are situated besides the ridges –> they are rapidly adapting

Vibrating Sense –> must be Rapid Adapting Type

(these properties are confimred by their capsules)

62
Q

What is the structure of Merkel’s Discs?

A
  • Linked mechanically by adherent junctions to surrounding keratinocytes
  • Activates afferent fibres synaptically (with keratinocyte)
  • Receptor is simply naked ending of afferent fibre (attached to keratinocyte)
  • Slow Adapting

(no capsule properties)

63
Q

What is the structure of Meissner’s Corpuscles?

A
  • Bare nerve endings in fluid-filled capsules
  • Afferent fibres activated directly
  • Rapid adapting (due to capsule?)
64
Q

What property foes a capsule give and how does it work?

A
  • Good at detecting pressure change (onset)

Any pressure applied to capsule –> is distributed through the whole volume –> thus if push anywhere on capsule –> causes ripple (elasticity absorbs some) –> causing a ripple –> but then an even pressure across corpuscle (even if pressure maintained) –> therefore there is no more effect –> i.e. the effect came and went –> ripple is made & now stops –> it is not being pressed on

65
Q

Generally what does the capsule do?

A
  • Take out standing pressure changes –> reports back when pressure is ON or OFF by the INCREASE or DECREASE in PRESSURE

(touching –> increase pressure)

(not touching –> decrease pressure from high)

Encapsulated Receptors –> very good at detecting pressure changes

66
Q

What structure does the rapid adapting property relate to?

A
  • Capsule

(nerve ending itself is probably the same for both)

67
Q

What are the two deep mechanoreceptors?

A
  1. Pacinian Corpuscles
  2. Ruffini Endings

(can make pacinian corpuscle become a SA fibre just by removing capsule)

68
Q

What property does the capsule give?

A
  • Rapdily adapting properties
69
Q

What are the properties of Pacinian?

A
  • Sensitive to very high frequencies
  • Lie deep in the skin
70
Q

What is the distribution of Merkel’s Discs (superficial) & Ruffini (deep) in terms of receptive fields & functions?

A
  • Both are slow adapting
  • Merkel –> smallest receptor field + fine discriminated touch
  • Ruffini –> large receptor fields + directional sense
71
Q

How is the receptor field deteremined?

A
  • Area of Skin –> which when activated –> activates a single receptor

(deteremines level of acuity in a system)

72
Q

How is the receptor density & receptor field on the cutaneous skin surface?

(e.g. skin?)

A
  • They are shown differentially
  • Fingers –> have highest receptor density (regions for palpations)
73
Q

Generally, in terms of receptor fields, how are superficial receptors compared to deep receptors?

A
  • Deep Receptors –> sensitive to tissue depression/movements over a wider area (due to their depth)
  • Superficial Receptors –> sensitive to much higher acuity –> but need more to be arranged (density)
74
Q

What do Meissner & Pacinian have in common?

A
  • Both use capsules
  • These enhance response to vibration detection
  • But they cover very different distributions
75
Q

What are the properties of Meissner Corpuscles?

A
  • Small Receptor Field
  • Detects local protuberances
76
Q

What are the properties of Pacinian Corpuscles?

A
  1. Very Large Receptive Fields
  2. Very High Sensitivity

(stroked surfaces)

77
Q

What is the distribution like for Meissner & Pacinian corpuscles?

78
Q

What is the most accurate receptor used in Braille?

A
  • Merkel Activity

(precise in time & localisation)

(superficial SA receptor)

Meissner & Pacinian are more smeared due to encapsulation

79
Q

Overall summary of touch receptor properties

80
Q

What do propioceptors measure and do?

Why?

A
  • Measures muscle length & tension
  • Informs brain about body stance & progression of voluntary limb movement
  • Relays it unconsciously
81
Q

What do spindles do?

A
  • Detect muscle stretch
  • Gamma-efferents adjust spindle length to adjust sensitvity during contraction
82
Q

What do golgi tendon organs do?

A
  • Measure muscle tension
83
Q

What is another name for A-betas?

A
  • Type II (muscle spindles)
84
Q

What is another name for 1a afferents?

A
  • A-alphas

(used in spindles)

85
Q

Which is fast & which is slow adapting?

  • 1a afferents
  • Type II afferents
A
  • 1a Afferents –> FAST –> A-alpha
  • Type II afferents –> SLOW –> A-beta

Neuroscience (Yeo) (8 decks)

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