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Year 2 - LCRS - Neuroscience and Mental Health > Session 4: Sensory Pathways > Flashcards

Session 4: Sensory Pathways Flashcards

18.10.2019

1
Q

Name 4 somatosensory modalities

A
  • mechanical
  • thermal
  • proprioceptive
  • nociceptive
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2
Q

What are the qualities of A-beta fibres?

A
  • large diameter
  • myelinated
  • fast signal transmission
  • innocuous mechanical stimulation
  • i.e. mechanoreceptors in skin
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3
Q

What are the qualities of A-delta fibres?

A
  • smaller diameter
  • myelinated
  • slower signal transmission
  • noxious mechanical and thermal stimuli
  • i.e. pain, temperature
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4
Q

What are the qualities of C-fibres?

A
  • small diameter
  • unmyelinated
  • slow signal transmission
  • noxious mechanical, thermal and chemical stimulation
  • i.e. temperature, pain, itch
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5
Q

What are the different types of sensory nerve endings?

A
  • free: thermoreceptors and nociceptors
  • enclosed: mechanoreceptors

=> individual axons of sensory nerves have modified terminals

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6
Q

Define: Sensory receptor

A

a transducer that converts energy from the environment into neuronal APs.

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7
Q

What are the different types of receptors?

A
  • mechanoreceptors (4 different ones)
  • nociceptors
  • thermoreceptor
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8
Q

What are the 4 types of mechanoreceptors?

A
  • Meissner corpuscle
  • merkel cells
  • Pacinian corpuscle
  • Ruffini endings
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9
Q

What so Meissner Corpuscles sense?

A

Fine discriminative touch, low frequency vibration

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10
Q

What do Merkel cells sense?

A

Light touch and superficial pressure

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11
Q

What do pacinian corpuscles sense?

A

Detects deep pressure, high frequency vibration and tickling

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12
Q

What do Ruffini endings sense?

A

Continuous pressure or touch and stretch

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13
Q

Stimulus threshold

A

‘’A threshold is the point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold)’’

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14
Q

Thermoreceptors

A
  • Aδ- and C-fibres
  • Free nerve endings
  • Transient receptor potential (TRP) ion channels
    • 4 heat activated:
      TRPV1-4
    • 2 cold activated
      TRPM8
      TRPA1
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15
Q

Stimulus intensity

A

Increased stimulus strength and duration = increased neurotransmitter release = greater intensity

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16
Q

What does a modality refer to?

A

The type of information encoded

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17
Q

What do different mechanoreceptors differ in?

A
  • location
  • structure
  • size
  • what they encode
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18
Q

Receptive field

A

the region on the skin which causes activation of a single sensory neuron when activated

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19
Q

Adaptation: tonic receptors

A
  • Detect continuous stimulus strength
  • Continue to transmit impulses to the brain as long the stimulus is present
  • Keeps the brain constantly informed of the status of the body
  • e.g. Merkel cells: Slowly adapt allowing for superficial pressure and fine touch to be perceived.

Tonic receptors - do not adapt or adapt very slowly

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20
Q

Adaptation: phasic receptors

A
  • Detect a change in stimulus strength
  • Transmit an impulse at the start and the end of the stimulus
  • e.g. when a change is taking place: The pacinian receptor -> Sudden pressure excites receptor
    Transmits a signal again when pressure is released
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21
Q

What are the properties of small and large receptive fields?

A
  • Small receptive fields allow for the detection of fine detail over a small area ->Precise perception
  • Large receptive fields allow the cell to detect changes over a wider area (less precise perception)
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22
Q

What are the receptive fields on the fingers like?

A

The fingers have many densely packed mechanoreceptors with small receptive fields

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23
Q

What is two-point discrimination?

A
  • Minimum distance at which two points are perceived as separate
  • Related to the size of the receptive field

e.g. hand detects 2 points at 4mm distance. Back: 40 mm

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24
Q

C5 dermatone

A

clavicle

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25
Q

C6 dermatome

A

thumbs

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26
Q

T4 dermatome

A

nipples

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27
Q

T10 dermatome

A

umbilicus

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28
Q

Where are cell bodies of the body and face found respectively?

A

Cell bodies are in the dorsal root ganglia (body) and trigeminal ganglia (face)

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29
Q

What two groups can neurones in the dorsal horn be divided into?

A
  • Those with axons that project to the brain (projection neurons)
  • Those with axons that remain in the spinal cord (interneurons)
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30
Q

What is lateral inhibition?

A
  • sometimes receptive fields overlap -> difficult to distinguish between 2 similar locations
  • in the dorsal horn there are inhibitory interneurones between sensory neurones
  • most intense stimulus comes through

=> to prevent overlap of receptive fields and to facilitate pinpoint accuracy in localisation of the stimulus

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31
Q

What are the two ascending pathways?

A
  • the dorsal column system (touch + vibration)

- the spinothalamic tract (pain, temperature, crude touch)

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32
Q

Dorsal column system

A
  • innocuous mechanical stimuli: fine discriminative touch, vibration)
  • A-beta fibres enter via the dorsal horn and enter ascending dorsal column pathway.
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33
Q

What are the 2 ascending dorsal column pathways?

A
  • fascilicus gracilis (lower limb and below T6)
  • fascilicus cuneatus (upper limb and above T6)

-> both ipsilateral

34
Q

What are the different neurones in the dorsal column system?

A
  • 1st order neurone: first synapse in the gracile nucleus / cuneate nucleus (medulla) -> travels ipsilaterally
  • 2nd order neurone: decussate in the caudal medulla and forms the contralateral medial lemniscus tract; terminate in the ventral posterior lateral nucleus of the thalamus (VPL)
  • 3rd order neurone: from the VPL, project to the somatosensory cortex
35
Q

Somatosensory homunculus

A
  • Size of somatotopic areas is proportional to density of sensory receptors in that body region
  • pain and temperature localisation is not as precise
36
Q

The spinothalamic (anterolateral) pathway

A
  • Pain and temperature sensations ascend within the lateral spinothalamic tract
  • Crude touch ascends within the anterior spinothalamic tract
37
Q

What fibres are for crude and for fine touch?

A
  • Crude touch is mediated by Adelta fibres (Free nerve ending)
  • Fine touch is mediated by Abeta fibres (Meissner’s corpuscles)
38
Q

What are the different neurones in the spinothalamic tract?

A
  • 1st oder neurone: terminates in the dorsal horn
  • 2nd order neurone: decussates immediately forming the spinothalamic tract; terminates in the ventral posterior lateral nucleus (VPL) of the thalamus
  • 3rd order neurone: projects into the somatosensory cortex
39
Q

VPL - topographic representation of the body

A

There is a topographic representation of the body in the VPL (lower extremities are lateral)

40
Q

What are the key differences in the dorsal column system and the spinothalamic tract?

A

Dorsal Colums:

  • decussates at the medulla
  • light touch, vibration, two point discrimination

Spinothalamic:

  • decussates immediately
  • pain, temperature, coarse touch
41
Q

What warm temperature is perceived as painful?

A

> 43 degrees C

42
Q

What would happen in an anterior spinal cord lesion?

A
  • Spinothalamic tract damage causes pain and temperature loss below the level of the lesion (bilateral)
  • Retained light touch, vibration and 2-point discrimination due to intact dorsal columns

(Blocked anterior spinal artery causes ischemic damage to the anterior part of the spinal cord)

43
Q

What is pain?

A
  • An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.
  • there are social, emotional and learning components to pain
44
Q

Which fibres mediate sharp, intense or first pain? What are the 2 types of nociceptors?)

A

A-delta

(Nociceptors:

  • Type 1: noxious mechanical
  • Type 2: noxious heat )
45
Q

Which fibres mediate dull, aching or second pain?

A

C-fibres

  • Noxious thermal, mechanical and chemical stimuli (polymodal)
46
Q

Pain pathways - what are the 2 components?

A
  • Lateral spinothalamic tract
    (sensory component)
  • Spinoreticular tract
    (emotional component)
47
Q

What are nociceptors?

A
  • a sensory receptor for painful stimuli
48
Q

What receptors do e.g. Wasabi and Caspaicin bind to?

A

Thermoreceptors (Wasabi: cold; Caspaicin: hot)

interesting fact: capsaicin stays bound

49
Q

Pain Matrix in the brain

A
  • fMRI have shown a cerebral signature for pain
- Cortex: 
      SI 
      SII 
      Insula cortex
      Anterior cingulate cortex 
- Prefrontal cortex 
- Amygdala 
- Cerebellum 
- Brainstem

There is no specific place for pain in the brain (in the US sometimes fMRIs are used to see if someone had pain in court - dangerous path to take)

50
Q

Gate control theory

A
  • Inhibition of primary afferent inputs before they are transmitted to the brain through ascending pathways
  • e.g. when you want to touch something that hurts
  • stimulates A-beta fibres which block C-fibers

also: strong emotions can inhibit pain, also hypnosis

51
Q

What is an example of emotional inhibition of pain?

A
  • ‘’75% of recently wounded soldiers with various significant injuries such as bone fractures and abdominal wounds reported pain that
    was insufficiently severe to require the use of analgesics
  • strong emotions can inhibit pain
52
Q

What is chronic pain?

A

> 3 months

53
Q

Descending control pathways of pain inhibition

A
  • strong emotions can inhibit pain
    (- you touch the body part that hurts?)
  • Periaqueductal grey (PAG)
  • placebo can activate descending inhibition
  • Facilitation and inhibition of nociceptive processing in the dorsal horn

Monoamines

  • Serotonin
  • Noradrenaline
54
Q

PAG

A
  • periaqueductal grey
  • a key area in the descending control pathway
  • can either be facilitative or inhibitory
  • monoamines play a role
  • Serotonin and noradrenaline are inhibitory -> they can reduce function of the C-fibres
55
Q

How do opioids work?

A

Opiods inhibit action at the RVM and at the PAG (preaqueductal grey)

56
Q

Nociceptive pain examples

A
  • noxious stimulation of a nociceptor (somatic or viscera)
  • arthritis
  • fractures
  • burns
  • headache
    ( - skin, muscles, ligaments, bone, joints, viscera)
57
Q

Neuropathic pain examples

A
  • lesion or disease of the somatosensory system
  • sciatica
  • diabetic
  • trauma
  • chemotherapy
  • post-surgical
58
Q

examples of mixed (nociceptive and neuropathic pain)

A
  • osteoarthritis

- lower back pain

59
Q

How many people in the UK have chronic pain?

A

28 million

60
Q

Allodynia

A

pain due to a stimulus that does not normally provoke pain

61
Q

Hyperalgesia

A

increased pain from a stimulus that normally provokes pain

 - Primary 
 - Secondary
62
Q

Peripheral sensitisation

A
  • tissue damage: inflammatory substances released
  • these chemicals (NT, glutamate, serotonin, ATP, adenosine peptides, bradykinin, cytokines, chemokines, lipids, proteases)
  • these chemicals modulate the excitability of nociceptors making them more sensitive

=> decreases threshold to peripheral stimuli at the site of injury

63
Q

Central sensitisation

A
  • C fiber sensitivisation in the periphery
  • Adjacent Aδ-fibre sensitisation in the dorsal horn
    => decreases threshold to peripheral stimuli at an adjacent site to the injury
64
Q

Diagnosis of neuropathic pain

A
  • symptoms have to be consistent with the site of injury
  • use questionnaire as help
  • quantitative sensory testing (QST)
65
Q

What are the 3 neuropathic pain clusters?

A
  • sensory loss
  • thermal hyperalgesia
  • mechanical hyperalgesia
66
Q

What drugs can target descending control pathways for pain relief?

A
  • opiods
  • antidepressants (TCA, SNRI, SSRI)

-> big difference in analgesic effect in the different antidepressants

67
Q

Descending control pathway in chronic pain - facilitation and inhibition

A

Noradrenaline - protective, inhibitory

Serotonin - harmful, facilitative

68
Q

Conditioned pain modulation

A
  • specific pain types at a different location can increase threshold or decrease painfulness
69
Q

transcranial direct current stimulation

A
  • non invasive primary motor cortex stimulation
  • activation of endogenous analgesic systems in the brain (PAG, anterior cingulate cortex)
  • analgesic mechanisms unclear
70
Q

How many different thermoreceptors are there in humans?

A

6

  • 4 heat activated: TRPV1-4
  • 2 cold activated (TRPM8, TRPA1)
71
Q

What is the difference between phasic and tonic receptors?

A
  • A tonic receptor is a sensory receptor that adapts slowly to a stimulus and continues to produce action potentials over the duration of the stimulus.
  • In this way it conveys information about the duration of the stimulus. Some tonic receptors are permanently active and indicate a background level. Examples of such tonic receptors are pain receptors, joint capsule, and muscle spindle.
  • A phasic receptor is a sensory receptor that adapts rapidly to a stimulus. The response of the cell diminishes very quickly and then stops.
  • It does not provide information on the duration of the stimulus
  • instead some of them convey information on rapid changes in stimulus intensity and rate.
  • An example of a phasic receptor is the Pacinian corpuscle.
72
Q

VPL

A
  • ventral posterior lateral nucleus of the thalamus
  • 2nd order neurones coming from the medulla (after crossing over) of the dorsal column terminate there
  • 3rd order neurones of the dc begin there and travel to the ss-cortex
  • topographic representation of the body in the VPL (lower extremities are lateral)
73
Q

Where to 2nd order neurones of the dorsal column decussate?

A
  • at the caudal medulla

- then travel up the contralateral medial leminiscus tract

74
Q

QST

A

quantitative sensory testing

  • e.g. you can test 2 point discrimination
75
Q

Which fibres mediate which type of pain?

A
  • Aδ fibers mediate sharp, intense or first pain
    Type 1: noxious mechanical
    Type 2: noxious heat
  • C-fibres mediate dull, aching or second pain
    Noxious thermal, mechanical and chemical stimuli (polymodal)
76
Q

Which NT is involved in the transmission of pain in the spinal cord?

A

glutamate

77
Q

What is the emotional component of the pain pathway?

A
  • spinoreticular tract

- has 4 levels of neurones

78
Q

Is pain objective or subjective?

A
  • Pain is a subjective process
  • only the patient can tell you something is painful
  • ## ouch pain threshold is highly variable
79
Q

RVM and PAG

A

RVM: rostral venteromedial medulla

PAG: periaqueductal gery

-> both areas that are affected by opioids to cause pain inhibition.

80
Q

What are the types of pain sensitisation?

A

Peripheral (decreases threshold to peripheral stimuli ar the site of injury)

Central (decreases the threshold to peripheral stimuli at an adjacent site to injury; expansion of receptive field; spontaneous pain).

81
Q

Primary and secondary hyperalgesia

A
  • Primary hyperalgesia describes pain sensitivity that occurs directly in the damaged tissues.
  • Secondary hyperalgesia describes pain sensitivity that occurs in surrounding undamaged tissues

Year 2 - LCRS - Neuroscience and Mental Health (21 decks)

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