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Somatosensory System Flashcards

1
Q

Sensory modality

A

Type of stimulus (hot,cold, touch)

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2
Q

Types of receptor for different stimuli

A

Mechnoreceptor- touch, pressure, vibration
Mechanoreceptor- proprioception (joint position, muscle length/tension)
Thermoreceptor- temp
Pain- nociceptor

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3
Q

3 main categories of sensory fibres

A

Mechanoreceptors- (Abeta)-very fast (large, myelinated) transmit mechanical stimulation

Pain, temperature(Adelta)-slightly fast (myelinated) transmit fast pain and temperature signals

Temperature, pain, itch (C)-slow transducing (no myelination) transmit slower, achy pain

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4
Q

Sensory nerve endings

A

C fibres - unmyelinated, specialised free nerve endings

Abeta (mechanoreceptors)- encapsulated nerve endings

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5
Q

Absolute threshold

A

Level of stimulus required to produce a positive response 50% of the time

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6
Q

What happens if the stimulus is stronger

A

Larger generator potential
Faster AP
More NT released at the end

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7
Q

Thermoreceptros- nerve endings and what channel activates them

A

Free nerve endings- high thermal sensitivity

Change in temp- activates transient receptor potential ion channels

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8
Q

Types of thermoreceptors

A

4 heat activated TRP- TRPV 1-4- ranging from noxious

2 cold activated TRP channels: TRPM8 and TRPA1

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9
Q

Types of mechanoreceptors and what their stimulus is

A

Meissner’s corpuscle- Fine discrete touch

Merkel’s Disks- light touch and superficial pressure

Pacinian corpuscle- Detects deep pressure, vibrations and tickling

Ruffini Endings- Continuous pressure or touch and stretch

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10
Q

Tonic receptors

A

Detect continuous stimulus strength
Do not adapt/ adapt very slowly
Continue to transmit impulse to brain as long as stimulus is present

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11
Q

Phasic Receptors

A

Detect change in stimulus strength
Transmit impulse at start and end of stimulus
Adapt quickly

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12
Q

Adaptation of receptors

A

Ability of receptors to change their sensitivity to a stimulus when exposed to stimuli for prolonged time period

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13
Q

Receptive field

A

Region of the skin that causes the activation of a single sensory neurones

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14
Q

What the size of receptive field determines

A

Perception

Small receptive field- precise perception

Large receptive field- less precise perception

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15
Q

Two point discrimination

A

Minimum distance at which 2 points are perceived as separate

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16
Q

Types of nociceptors and their stimuli

A

A-delta - mediates sharp intense pain

  • Myelinated – quite fast
  • Type 1: Aδ-mechano-heat receptors (noxious mechanical and thermal stimuli)
  • Type 2: Aδ-mechanoreceptors (purely noxious mechanical stimuli)

C fibre- mediate dull, persistent or second pain

  • Unmyelinated – SLOW
  • Respond to thermal, mechanical and chemical stimuli (polymodal)
  • Chemical stimuli include inflammatory mediators
  • Polymodal: respond to all the modalities – only one type of C fibre
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17
Q

Where afferent cell bodies are located

A 
Dorsal root ganglia (body)
Trigeminal ganglia (face)
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18
Q

Rexed laminae

A

System of 10 layers of grey matter in the spinal cord

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19
Q

Where certain types of afferent neurones enter the dorsal horn

A

Innocuous mechanical stimuli- Abeta fibres- terminate in limina III-VI (deep dorsal horn)

Pain and temperature- Adelta and C fibers terminate in lamina I-II (superficial dorsal horn)

20
Q

Main excitatory neurotransmitter released from pre-synaptic neurones in the dorsal horn

21
Q

Lateral inhibition

A

Receptive fields can overlap

Mediated by interneurones in the dorsal horn

The neurone activated the most- inhibits neighboured neurones for pinpoint localisation of the stimulus

22
Q

2 pathways of afferent neurones to the brain and what information they transmit

A

Dorsal column system- transmits touch and proprioception

Spinothalamic pathway- pain, temperature, crude touch

23
Q

Dorsal column system-route

A

1st order neurones terminate in the medulla

  • fibres in the Gracile tract synapse in the Gracile Nucleus
  • fibres in the Cuneate Tract synapse in the Cuneate Nucleus

2nd order neurones cross the caudal medulla- forming the contralateral medial lemniscus tract
These then terminate in the VPL nucleus of the thalamus

3rd order neurones from VPL project and terminate in the somatosensory cortex

The size of somatotropin areas is proportional to density of sensory receptors in that body region (somatosensory homunculus)

24
Q

What tract in the dorsal column pathway does the information conveyed from lower limbs travel along

A

Travels ipsilaterally along the gracile tract

25
What tract in the dorsal column pathway does the information conveyed from upper limbs travel along
Travels ipsilaterally along the cuneate route
26
Where the information of specific stimuli travel in the spinothalamic pathway
Pain and temperature sensations ascend within the lateral spinothalamic tract Crude touch ascends within the anterior spinothalamic tract
27
Spinothalmic pathway- route
1st order neurones terminate in the dorsal horn Second order neurones decussate immediately in the spinal cord and form the spinothalamic tract 2nd order neurones terminate in the ventral posterior lateral (VPL) nucleus of the thalamus 3rd project onto primary somatosensory cortex
28
Key differences between dorsal column and spinothalamic tract
-Dorsal column tracts: transmit light touch, vibration and 2-point discrimination – Cross in brainstem -Spinothalamic tracts: transmit pain, temperature and coarse touch – cross in spinal cord
29
The emotional component of pain
Pain has an emotional aspect, reflected anatomically. Pain is transmitted through a pathway from the spinal cord to the parabrachial area (in the brainstem), and then to the limbic system. This is called the spino-reticular system (emotional component of pain)
30
Quantitative sensory testing (QST)
Allows us to quantify sensory function in patients e.g temperature threshold, vibration sensitivity, brush sensitivity
31
Anterior spinal cord lesion
Spinothalamic tract damage causes pain and temperature loss below the level of the lesion Retained light touch and vibration sensation due to intact dorsal columns
32
Electrical perceptual thresholds
Use semi-automated method- electrical current is passed through the skin Ask patient whether they can feel it If spinal cord injury- lose sensation
33
Types of pain
* Nociceptive – tissue damage, typically acute pain (e.g. skin cut) * Muscle – lactic acidosis, ischaemia (e.g. stretching, fibromyalgia) * Somatic – well-localized (e.g. inflammation, infection) * Visceral – deep, poorly localised (e.g. stomach, colon, IBS) * Referred – from an internal organ/structure (e.g. Angina) * Neuropathic – dysfunction of the nervous system
34
Neuropathic pain
Pain caused by a lesion or disease of the somatosensory nervous system
35
Features of neuropathic pain
* Pain in area of neurological dysfunction (hyperalgesia and allodynia) * Typically sharp, burning, ‘electric shocks’ * Poor response to usual analgesic drugs (e.g. opiates)
36
Allodynia
Pain due to a stimulus that does not normally provoke pain
37
Hyperalgesia
Increased pain from a stimulus that normally provokes pain
38
Synaptic plasticity
Ability of synapses to strengthen or weaken over time, in response to increases or decreases in their activity
39
Mechanism of synaptic plasticity
NMDA activation at the synapse allows big post synaptic depolarisation- Ca2+ to enter the neurones. Ca is involved in activating signalling pathways- therefore this increases synaptic strength (efficacy)- increased sensitivity. The inhibitory interneurons in the dorsal horn can then become less influential Persistent activation of NMDA receptor can result in the development of chronic pain (e.g. arthritis)
40
How chronic pain can manifest allodynia and hyperalgesia
Inhibitory neurones would normally prevent touch (abeta fibres) causing pain – loss of inhibition causes allodynia. If someone has chronic pain (central sensitisation), it manifests as allodynia and hyperalgesia. Pain stimulation goes to the dorsal horn and stimulates the post-synaptic hormone. This pathway no longer has an inhibitory influence, so the response is greater and the pain is felt as stronger pain
41
Types of descending modulation of pain
The PAG-RVM axis and The locus cereleus (LC) Brain releases serotonin and NA which can inhibit spinal cord excitability
42
The PAG-RVM axis
The RVM contains serotonergic neurones, which project to the dorsal horn releasing serotonin. This can either inhibit or facilitate pain depending on which receptors are activated. 5-HT1a receptors: predominantly inhibitory. 5-HT3 receptors: predominantly facilitatory on NT release.
43
The locus cereleus
Located in the pons. The main noradrenergic nuclei involved with descending control of nociception in the dorsal horn. It is predominantly inhibitory and can be kind of thought of as pains braking mechanism, dampening down the response by binding to pre-synaptic alpha 2 receptors on primary afferents and projection neurons, ultimately reducing excitability.
44
Endogenous opioids causing analgesia
The PAG and RVM contain high concentrations of µ opioid receptors Endogenous opioids (enkephalin, dynorphin) enhance descending inhibition from the PAG-RVM axis They reduce pain transmission in the dorsal horn by inhibiting glutamate release
45
Targeting descending control system for pain relief
Opioids – work in the PAG and RVM Antidepressants treat neuropathic pain (opioids are ineffective) – TCA, SNRI, SSRI SSRI aren't effective, TCA and SRNs have better efficacy
46
Neuropathic pain mechanism
Lesions of neurones can cause increased excitability which can lead to central sensitisation
47
How SNRI inhibits pain
Binds to pre/post synaptic terminal in dorsal horn This causes inhibition of NA uptake- increasing NA in synaptic cleft These then act on alpha 2 receptors- which are inhibitory. This inhibits activity in the neurones and reduces pain

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