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Y2 Neuroscience and Mental Health > The Somatosensory System > Flashcards

The Somatosensory System Flashcards

1
Q

What are sensory receptors

A

Transducers that converts energy from the environment into neuronal action potentials

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2
Q

What are the types of sensory neurones

A

Aβ-fibres - innocuous mechanical stimualtion. Myelinated and large, very fast

Aδ-fibres - noxious mechanical and thermal stimulation. Myelinated and thinner than Ab, fast

C-fibres - noxious mechanical, thermal and chemical stimulation, non-myelinated, slow

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3
Q

What are the somatosensory modalities

A

Touch – detection of light mechanical stimuli

Thermal – detection of temperature

Nociceptor – noxious and pain

Proprioception – mechanical displacement of muscles and joints

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4
Q

What kind of nerve endings do receptors have

A

Thermo receptors - free nerve
nociceptors - free nerve

Mechanoreceptors - enclosed nerve endings

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5
Q

Describe thermoreceptors

A 
Aδ- and C-fibres 
Free nerve ending 
Transient receptor potential (TRP) ion channels 
4 are heat activated: TRPV1 - 4
2 are cold activated: TRPM8, TRPA1
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6
Q

Describe the mechanoreceptors

A

Meissner’s corpuscle - fine discriminative touch, low frequency vibration

Merkel cells - light touch and superficial pressure

Pacinian corpuscle - Detects depp pressure, high frequency vibration and tickling

Ruffini endings - continuous pressure or touch and stretch

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7
Q

What is a threshold

A

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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8
Q

What are tonic receptors

A

Detects continuous stimulus strength
Continues to transmit impulses to the brain as long as the stimulus is present
Keeps the brain constantly informed of the status of the body

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9
Q

Give an example of tonic receptors

A

Merkel cells

Slowly adapt allowing for superficial pressure and fine touch to be percieved

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10
Q

What are phasic receptors

A

Detects changes in stimulus strength

Transmits an impulse at the start and the end of the stimulus (when a change is taking place)

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11
Q

Give and example of phasic receptors

A

Pacinian receptors
Sudden pressure excites receptor
Transmits a signal again when pressure is released

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12
Q

What is a receptive field

A

Region on the skin which causes activation of a single sensory neurones when activated

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13
Q

Compare small to large receptive fields

A

small - allow for detection of fine detail over small areas, precise perception e.g. fingers have many densely packed mechanoreceptors with small receptive fields

Large - allows the cells to detect changes one a wider area, less precise

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14
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

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15
Q

Where are the cell bodies found for the body and the face

A 
Dorsal root ganglia (body)
Trigeminal ganglia (face)
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16
Q

What are the two types of dorsal horn neurons

A

Neurones with axons that project into the brain (projection neurons)
Neurons with axons that remain in the spinal cord (interneurons)

17
Q

What is lateral inhibition

A

prevents the overlap between two receptive fields
Facilitates pinpoint accuracy in localisation of the stimulus
Mediated by inhibitory interneurons within the dorsal horn of the spinal cord
Facilitates enhances sensory perception

18
Q

What is the somatosensory pathway for touch and proprioception for the lower limbs (which vertebral level)

A

Below T6

  1. Aβ fibres enter the dorsal horn
  2. Aβ fibres enter the ipsilateral ascending dorsal column pathway
  3. Information travels ipsilaterally along the gracile tract (inner)
  4. Synapse in the gracile nucleus
  5. Crosses/ decussate in the caudal medulla (second order) to form the contralateral medial lemniscus tract
  6. Projection to the ventral posterior lateral nucleus of the thalamus
  7. Projection to the somatosensory cortex (third order neuron)
19
Q

What is the somatosensory pathway for touch and proprioception for the upper limbs (which vertebral level)

A

Above T6

  1. Aβ fibres enter the dorsal horn
  2. Aβ fibres enter the ipsilateral ascending dorsal column pathway
  3. Information travels ipsilaterally along the cuneate tract (outer)
  4. Synapse in the cuneate nucleus
  5. Crosses/ decussate in the caudal medulla (second order) to form the contralateral medial lemniscus tract
  6. Projection to the ventral posterior lateral nucleus of the thalamus
  7. Projection to the somatosensory cortex (third order neuron)
20
Q

What is size of somatotopic areas proportional to

A

Density of sensory receptors in that body region (somatosensory homunculus)
Pain and temp. localisation is not as precise)

21
Q

Describe the somatosensory pathway for pain and temperature

A
  1. Sensations ascend within the lateral spinothalamic tract (crude touch in the anterior spinothalamic tract)
  2. Synapse in the spinal cord
  3. Decussate immediately in the spinal cord to form the spinothalamic tract
  4. 2nd order terminates in the ventral posterior lateral nucleus of the thalamus
22
Q

What are the key differences between dorsal column and spinothalamic tracts

A

Spinothalamic - Pain, temp., coarse touch

Dorsal - light touch, vibration, 2 -point discrimination

23
Q

What is crude touch and fine touch mediated by

A

crude - Aδ fibres (free nerve ending)

fine touch - Aβ fibres (meissner’s corpuscles)

24
Q

What is the clinical importance of the 2 point discrimination to assess posterior column potency

A

Psychophysical assessment

Evaluate loss and gain to thermal and mechanical stimuli Normative data for face, hand foot and back

25
Q

Describe an anterior spinal cord lesion

A

Blocked anterior spinal artery causes ischaemic damage to the anterior part of the spinal cord
Spinothalamic tract damage causes pain and temp. loss below the level of the lesion
Retained light touch, vibration and 2-point discrimination (dorsal column intact)

26
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

27
Q

What are the nociceptors

A

Aδ fibers - mediate sharp, intense or first pain

C-fibres - mediate dull, aching or second pain. Noxious thermal, mechanical and chemical stimuli (polymodal)

28
Q

What are the types of Aδ fibers

A

Type 1 - noxious mechanical

Type 2 - noxious heat

29
Q

Which NT is involved in spinal cord nociceptive processing (first synapse)

A

Glutamate released from sensory afferents in response to acute or persistent noxious stimuli

30
Q

Where are the sensory and components of the pain pathways carried

A

Sensory - lateral spinothalamic tract -> thalamus -> cortex

Emotional - spinoreticular tract -> parabrachial area -> hypothalamus/amygdala

31
Q

What is involved in the pain matrix

A

Cortex: SI, SII, insula cortex, anterior cingulate cortex, prefrontal cortex
Amygdala
Cerebellum
Brainstem

32
Q

What is gate control theory

A

Inhibition of primary afferent inputs before they are transmitted to the brain through ascending pathways

33
Q

Describe the descending control pathway

A 
Periaqueductal grey (PAG)
Facilitation and inhibition of nociceptive processing in the dorsal horn
Monoamines (seratonin, NA)
34
Q

What can nociceptive pain be associated with and what is it

A 
Skin
Muscles
Ligaments
Joints
Bones
Viscera

Noxious stimulation go a nociceptor (somatic or viscera)

35
Q

What can neuropathic pain be associated with and what is it

A 
Sciatica
Diabetes
Trauma
Chemotherapy 
Post-surgical

Lesion of disease of the somatosensory system

36
Q

Give examples of mixed nociceptive and neuropathic pain

A

Osteoarthritis

Low back pain

37
Q

Compare peripheral sensitisation to central sensitisation

A

peripheral - Reduced thresholds to peripheral stimuli at the site of injury

Central - Reduced thresholds to peripheral stimuli at an adjacent site to the injury
Expansion of receptive field
Spontaneous pain

38
Q

What is allodynia

A

pain due to a stimulus that does not normally provoke pain

39
Q

What is hyperalgesia

A

Increased pain from a stimulus that normally provokes pain
Primary - site of injury/pain
Secondary - area around it

Y2 Neuroscience and Mental Health (22 decks)

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