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Year 2 Collin > Neuro 7 - Sensory pathways > Flashcards

Neuro 7 - Sensory pathways Flashcards

1
Q

What are sensory modalities?

A

Types of stimulus

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2
Q

Mechanoreceptors of skin are which sensory fibre?

A

A-beta (v fast)

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3
Q

Pain / temperature fibres are which sensory fibre

A

A-delta (fast-ish)

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4
Q

Slow pain/temperature/itch fibres are which sensory fibre?

A

C fibres

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5
Q

How are axons adapted to sensing different stimuli?

A

They have different modified nerve endings/terminals

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6
Q

What is the absolute threshold?

A

Stimulus strength which produces a positive response 50% of the time (i.e. is detected 50% of the time)

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7
Q

Increasing stimulus strength and duration does what?

A

Increases neurotransmitter release causing greater intensity

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8
Q

Describe TRP channels

A

They are free nerve endings with high thermal sensitivity - temperature changes activates TRP channels

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9
Q

How many heat activated TRP channels are there?

A

4

TRPV1-4

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10
Q

How many cold activated TRP channels are there?

A

2

TRPM8
TRPA1

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11
Q

Name the 4 mechanoreceptors

A
  1. Meissners corpuscle
  2. Merkel cells
  3. Pacinian corpuscle
  4. Ruffini endings
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12
Q

What type of touch are Meissners corpuscles for?

A

Fine discriminative touch

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13
Q

What type of touch are Merkel cells used for?

A

Light touch and superficial pressure

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14
Q

What type of touch are Pacinian corpuscles used for?

A

Detecting deep pressure, vibration and tickling

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15
Q

What type of touch are Ruffini endings for?

A

Continuous pressure / touch and stretch

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16
Q

Describe tonic receptors

A

Detect continuous stimulus strength - transmits impulses for stimulus duration

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17
Q

Give an example of a tonic receptor

A

Merkel cells

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18
Q

Do tonic receptors adapt?

A

No, or they adapt very slowly.

Keeps brain constantly informed of body status

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19
Q

Describe phasic receptors

A

These detect changes in stimulus strength - fire impulses at start and end of stimulus (i.e. when a change is taking place)

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20
Q

Give an example of a phasic receptor?

A

Pacinian corpuscle

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21
Q

What is a receptive field?

A

Region on the skin which activates one single sensory neurone when stimulated

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22
Q

If you want fine detail, how big should the receptive field be?

A

Small

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23
Q

Describe two point discrimination

A

Minimum distance at which 2 points are perceieved as separate - related to size of receptive field

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24
Q

What type of sensory fibres are nociceptors?

A

A-delta fibres

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25
Q

A-delta fibres are myelinated. There are 2 types - describe

A

Type 1: A-delta mechanoheat receptors (noxious mechanical/thermal stimuli)

Type 2: A-delta mechanoreceptors (noxious mechanical stimuli)

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26
Q

Which type of sensory fibre is unmyelinated?

A

C fibres

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27
Q

What modalities do C fibres respond to?

A

Thermal, mechanical and chemical stimuli

They are polymodal

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28
Q

In the sensory pathway, where are cell bodies present?

A

DRG and Trigeminal ganglia (face)

29
Q

How many rexed laminae are there?

A

7

30
Q

Where do mechanical stimuli terminate (A-beta and A-alpha) in the dorsal horn?

A

3-6 rexed laminae

Deep

31
Q

Where do pain and temperature (A-delta and C fibres) stimuli terminate in the dorsal horn?

A

Rexed laminae 1-2

Superficial

32
Q

What is the main excitatory neurotransmitter of the dorsal horn?

A

Glutamate

33
Q

How can overlapping receptive fields be overcome?

A

Via lateral inhibition - facilitates pinpoint accuracy in localising stimulus.

Thus facilitates enhanced sensory perception

34
Q

What mediates lateral inhibition?

A

Interneurons in the dorsal horn of SC

35
Q

S1 = Primary somatosensory cortex. Where is it located?

A

Postcentral gyrus

36
Q

S2 = Secondary somatosensory cortex. Where is it located?

A

Parietal operculum

37
Q

What is the posterior parietal cortex needed for?

A

Spacial awareness of body

38
Q

When innocuous mechanical stimuli enter the ascending dorsal column pathway (via A-beta) , what are the differences between lower limbs and upper limbs?

(AP1)

A

Below T6 = lower limb = travels ipsilaterally along gracile tract

Above T6 = lower limb = travels ipsilaterally along cuneate tract

39
Q

Where do first order neurone terminate?

AP1

A

In the medulla

40
Q

Within the medulla, gracile tract fibres have their first synapse where?

(AP1)

A

In the gracile nucleus

41
Q

Within the medulla, cuneate tract fibres have their first synapse where?

(AP1)

A

In the cuneate nucleus

42
Q

What do the second order neurons do?

AP1

A

They decussate in the caudal medulla, forming the contralateral medial lemniscus tract

43
Q

Where do second order neurons terminate?

AP1

A

In the Ventral Posterior Lateral Nucleus (VPL) of the nucleus

44
Q

Where does sensory information from lower limbs terminate in relation to the vPL

(AP1)

A

More laterally

45
Q

What do 3rd order neurons do?

AP1

A

3rd order neurons convey information from VPL to Somatosensory cortex

46
Q

What is meant by somatosensory humunculus

AP1

A

Area whereby size of somatotopic areas is proportional to density of sensory receptors in that body region

47
Q

What is AP1?

A

Ascending pathway 1 - innocuous mechanical stimuli

48
Q

AP2 =?

A

Ascending pathway 2 - pain/temperature/crude touch

49
Q

The spinathalamic pathway involves 2 aspects which are?

A

AKA anterolateral pathway

“Lateral” aspect of spinothalamic tract is where pain/temperature sensations ascend

“Anterior” aspect of spinothalamic tract is where crude touch sensations ascend

50
Q

In AP2, where do the primary afferent axons terminate?

What happens next?

A

Primary afferent axons (first order) terminate upon entering SC (in the dorsal horn)

Second order neurons decussate immediately and form spinothalamic tract

51
Q

Where do second order neurons terminate?

AP2

A

Ventral posterior lateral nucleus (VPL)

Lower extremities more lateral

52
Q

What are key differences between dorsal column and spinothalamic tracts?

A

Dorsal column = decussates in medulla

ST tract = decussates in SC

Dorsal column = light touch, vibration, 2 point discrimination

ST = Pain, temperature, coarse touch

53
Q

There is an emotional component to pain, where does the signal from the SC go to?

A

The parabrachial area

54
Q

How can we test the integrity of the ascending sensory pathways?

A

Quantitative sensory testing (QST)

55
Q

If someone gets an anterior SC lesion, they cannot feel pain and temperature below the level of the lesion. What can they feel?

A

Light touch and vibration below the level of the lesion

56
Q

What are the 6 different types of pain?

A
  1. Nociceptive (usually acute)
  2. Muscle
  3. Somatic
  4. Visceral
  5. Referred
  6. Neuropathic
57
Q

What is neuropathic pain?

A

Pain caused by a lesion/disease of the somatosensory nervous system

58
Q

Do analgesic drugs (e.g. opiates) have a good relieving effect for neuropathic pain?

A

No, poor response to analgesics

59
Q

What is the most common form of neuropathic pain?

A

Radicular low back pain (sciatica)

60
Q

What is allodynia?

A

Pain caused by a stimulus that usually does not cause pain

61
Q

What is hyperalgesia?

A

Increased pain from a stimulus that normally provokes pain

62
Q

What is paraesthesia?

A

Abnormal sensation (whether spontaneous or evoked)

63
Q

How can synaptic plasticity cause chronic pain?

or central sensitisation

A

NMDA receptor activation

causes Ca2+ mediated synaptic plasticity

meaning increased synaptic strength which reduces inhibitory influences on dorsal horn neurons

Constant NMDA activation therefore causes chronic pain development

64
Q

Name some neuropathic phenotypes

A 
Heat hyperalgesia
Cold hyperalgesia
Mechanical hyperalgesia
Mechanical allodynia
Loss of sensation
65
Q

What are the 2 inhibitory pathways for descending control of nociception?

A
  1. PAG-RVM axis (brainstem) which uses 5-HT (aka serotonin) as its inhibitory monoamine
  2. Locus cereleus (pons), inhibitory via noradrenaline
66
Q

How do endogenous opioids increase descending inhibition?

A
  1. PAG-RVM contains lots of miu-opioid receptors
  2. Endogenous opioids inhibit glutamate release (less spinothalamic neurones activated)

Opioids part of endogenous analgesic system

67
Q

Aside from opioids, what other class of drugs may have some efficacy in neuropathic pain?

A

Anti-depressants

68
Q

Why may SNRIs (class of antidepressants) enhance descending NA inhibition?

A

Stimulate NA and inhibitory influences within the synapse

69
Q

What is conditioned pain modulation used for?

A

To measure the level of descending control in patients

Year 2 Collin (114 decks)

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