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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
How is **vibration** related to **palpation**?
* 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)
26
Why is a **_rapid_** & **slow** **adapting** **receptor** needed?
* **Economy** (only need to know about changes) * Confers **special** **sensitivity** to **_high_** **frequency** **vibration** used in **tactile** **discrimination** (e.g. by palpating surfaces)
27
How many classes are there of **PNS sensory afferent fibres**? What do they begin with? What are they **classified** by? What are their **properties**?
* 4 classes * Begin with A or C * Diameter + Myelination status
28
What is the **_only_** **sensory** **afferent** fibre that is not myelinated?
* C-fibres
29
What are **C-fibres receptors** for?
* Pain * Itch * Temperature
30
What sensory modalities are **A-delta** **fibres** for?
* Fast Pain * Temperature
31
What **sensory** **modalities** are **A-beta fibres** for?
* Skin Mechanoreceptors (touch)
32
What **sensory** **modalities** are **A-alpha fibres** for?
* Propioceptors
33
What is the **_largest_** & **_smallest_** **sensory** **afferent** **myelinated** fibres?
* A-alpha --\> **largest** * A-delta --\> **smallest**
34
Which **PNS sensory category** (due to cross-over) are **alpha** **motor** **neurones** in?
* **A-alpha category**
35
What is the class I-IV for?
* **Deep** **afferents** from **_muscles_** & **_joints_** (partial overlap - e.g. class I = A-alpha)
36
What cutaneous receptor fibres convey pain? What is the difference?
* C-fibres --\> **slow (2 m/s)** * A-delta --\> **fast (5-30 m/s)**
37
Where do pain responses come from?
* **First** --\> centrally --\> in the _**spinal** **cord**_ * **Second** --\> **_somatosensory cortex_**
38
What are **A-deltas** involved in?
* Flexion Reflex
39
What **two** **effects** does the **stimulation** of **nociception** have?
* **Local** **Effect** (first) * **Central** **Effect** (second) Both are capable of **turning** **up** **sensitivity** of the **response**
40
What causes a release of Substance P?
* **Branching/collaterals** from the **afferent** **fibre** goes back to the **local** **area** **near** **periphery** --\> leads to **release** of **substance P**
41
What effects does **Substance P** have?
* 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
What does the **release** of **histamine** cause?
Release of: * **Bradykinin** * **Potassium (K+)** * **Prostaglandin** These **influence** **receptors** causing a **positive** **feedback** **loop** **within** the **detection** **system**
43
What effect does **Substance P** have?
* **Relaxation** of **Blood** **Vessels** * Allowing for an **increase** in **_blood flow_** & **_inflammation_**
44
What is the **'Axon Reflex for Nociceptors'**?
* **_After_ Tissue Injury** --\> receptors in the **local** **area** are **more** **sensitive** to **local** **effects** (due to local effects)
45
What is meant by a **'silent receptor'**? Where are these commonly found?
* 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
What are the 2 proprties of **silent** **nociceptors**?
1. **High** **Threshold** **Mechanoreceptors** 2. **Sensitised** by **Inflammation** (e.g. viscera)
47
Which fibre types carry temperature?
* C-fibres * A-delta fibres
48
What are the **2 types** of **temperature** **receptors**? How do they work?
* **Cold** Receptors * **Warm** Receptors They work similar to **colour** **vision** (but those have 3 types) --\> to **extract** **temperature** **information** over a **wide** **spectrum**
49
What is the pattern of a cold fibre?
* Activity **begins** at **15 degrees** * **Peaks** at **25 degrees** * **Trails** **down** as it gets **near** **40 degrees**
50
How is temperature coded?
* Firing Rate (temperature)
51
How does the body overcome differentiating between signals on either side of the bell curve?
* 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
What is meant by **paradoxical** **cold**? How does it come about?
* **Cold** **Receptors** --\> begin to **fire** **again** a**t 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
What is the disstribution of the **hot** and **cold** **fibres** **subcutaneously**?
* Mosaic Distribution
54
What are **temperature** **receptors** **_also_** very **useful** for? (other than plain temperature)
* **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
What sensory fibres carry **mechanoreceptors**?
* A-beta * A-delta
56
How are superficial receptor distributed?
* **Differently** **across** the **ridges** of the **superficial** **skin** (ridges give us our fingerprints)
57
What are the two receptors found superficially?
* Merkel's Discs * Meissner's Corpuscles
58
Which is **slow adapting** and **fast** **adapting** between the **superficial** **receptors**?
* Merkel's Discs --\> **_Slow_** **Adapting** * Meissner's Corpuscles --\> **_Fast_** **Adapting**
59
What are the properties of **Merkel's** **Discs**?
* Sits in **valleys** between the **two** **grooves** * **Slow adapting**
60
What are the properties of **Meissner's** **Corpuscles**?
* Situated **alongside** the **ridges** * This is where the **grooves** go **down** --\> on **either** **side** of **groove**
61
What sensation are **Meissner** **Corpuscles** good at sensing?
* **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
What is the structure of Merkel's Discs?
* 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
What is the structure of **Meissner's** **Corpuscles**?
* **Bare nerve endings** in **fluid-filled capsules** * **Afferent** **fibres** activated **directly** * **Rapid** **adapting** (due to capsule?)
64
What property foes a capsule give and how does it work?
* 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
Generally what does the capsule do?
* **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
What **structure** does the **rapid adapting property** relate to?
* Capsule (nerve ending itself is probably the same for both)
67
What are the two deep mechanoreceptors?
1. Pacinian Corpuscles 2. Ruffini Endings (can make pacinian corpuscle become a SA fibre just by removing capsule)
68
What property does the **capsule** give?
* Rapdily adapting properties
69
What are the properties of **Pacinian**?
* Sensitive to **very high frequencies** * Lie **deep** in the **skin**
70
What is the distribution o**f Merkel's Discs** (superficial) & **Ruffini** (deep) in terms of **receptive** **fields** & **functions**?
* Both are **slow adapting** * Merkel --\> **smallest receptor field** + **fine discriminated touch** * Ruffini --\> l**arge receptor fields** + **directional sense**
71
How is the **receptor** **field** deteremined?
* **Area of Skin** --\> which when activated --\> activates a **single receptor** (deteremines level of acuity in a system)
72
How is the receptor density & receptor field on the cutaneous skin surface? (e.g. skin?)
* They are shown **differentially** * **Fingers** --\> have **_highest_** **receptor** **density** (regions for palpations)
73
Generally, in terms of **receptor fields**, how are **superficial** **receptors** compared to **deep** **receptors**?
* **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
What do **Meissner** & **Pacinian** have in common?
* Both use **capsules** * These **enhance** **response** to **vibration** **detection** * But they **cover very different distributions**
75
What are the properties of **Meissner** **Corpuscles**?
* Small Receptor Field * Detects local protuberances
76
What are the properties of **Pacinian** **Corpuscles**?
1. Very **Large Receptive Fields** 2. Very **High Sensitivity** (stroked surfaces)
77
What is the distribution like for **Meissner** & **Pacinian** **corpuscles**?
78
What is the most accurate receptor used in Braille?
* **Merkel Activity** (precise in time & localisation) (superficial SA receptor) Meissner & Pacinian are more smeared due to encapsulation
79
Overall summary of touch receptor properties
80
What do **propioceptors** measure and do? Why?
* Measures muscle **length** & **tension** * Informs brain about **_body stance_** & **_progression of voluntary limb movement_** * Relays it **unconsciously**
81
What do **spindles** do?
* Detect **muscle stretch** * **Gamma-efferent**s **adjust** **spindle** **_length_** to **adjust** **sensitvity** during **contraction**
82
What do **golgi tendon organs** do?
* Measure **muscle** **tension**
83
What is another name for **A-betas**?
* **Type II** (muscle spindles)
84
What is another name for **1a afferents**?
* A-alphas (used in spindles)
85
Which is fast & which is slow adapting? * 1a afferents * Type II afferents
* **1a Afferents** --\> FAST --\> **A-alpha** * **Type II afferents** --\> SLOW --\> **A-beta**

Neuroscience (Yeo) (8 decks)

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