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NAB > 6b. Primary Afferents to Thalamus > Flashcards

6b. Primary Afferents to Thalamus Flashcards

1
Q

Sensory Axon Types

A
  • Aα and Aβ
  • Adelta fibres
  • C fibres
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2
Q

Sensory Axon Types

- Aα and Aβ Fibre Characteristics

A
  • Large
  • Myelinated
  • 40-80m/s conduction velocity
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3
Q

Sensory Axon Types

- Aβ Fibre Sensations

A
  • Touch
  • Proprioception

Pain is not elicited by stimulation of the Aβ fibres alone

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4
Q

Sensory Axon Types

- Adelta Fibre Characteristics

A
  • Intermediate
  • Myelinated
  • 5-30m/s conduction velocity
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5
Q

Sensory Axon Types

- Adelta Fibre Sensations

A
  • Cold

- Stabbing pain

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6
Q

Sensory Axon Types

- C Fibre Characteristics

A
  • Small
  • Unmyelinated
  • 0.5-2m/s conduction velocity
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7
Q

Sensory Axon Types

- C Fibre Sensations

A
  • Warmth
  • Itching
  • Burning pain
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8
Q

Sensory Axon

- Mild Anoxia

A

Affects larger Aβ fibres first, so the following sensations are lost:

  • Touch
  • Proprioception
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9
Q

Sensory Axon

- Prolonged Anoxia

A

Affects both Aβ and Adeta fibres, so the following sensations are lost:

  • Touch
  • Proprioception
  • Initial sharp pain
  • Cold
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10
Q

Sensory Axon

- Mild Anaesthetic

A

Affects C fibres, so the following sensations are lost:

  • Burning pain
  • Itch
  • Warmth
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11
Q

Sensory Axon

- Prolonged Anaesthetic

A

Affects C fibres and Adelta fibres, so the following sensations are lost:

  • Burning pain
  • Itch
  • Warmth
  • Cold
  • Stabbing pain
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12
Q

Sensory Axon

- Diabetes Mellitus nad Multiple Sclerosis

A

Myelin sheath of large diameter neurones degenerates resulting in slow conduction or failure of impulse transmission

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13
Q

Sensory Axon

- Anoxia Induced Hyperalgesia

A

Anoxia can elicit hyperalgesia because of:

  • Switch from A-delta to C-fibres bringing burning pain
  • Occlusion causes build up of chemicals which sensitise local nerve endings
  • Loss of large fibre sensation has opened spinal cord gates so pain messages pass through more easily to consciousness
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14
Q

Gate Theory of Pain

A

Touch stimuli suppress the transmission of pain messages through the spinal cord gates in the dorsal horn

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15
Q

Sensory Axons

- Anoxia Induced Increase in Latency Period

A

Anoxia increases latency because:

  • Switch from myelinated Adelta fibres to unmyelinated C fibres which have a slower conduction velocity
  • Switch from Adelta fibres to C fibres, where Adelta fibres are mechanically activated, while C fibres are activated by the diffusion of chemicals into the fibre endings
  • Conduction speed may be decreased by ischaemia
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16
Q

Sensory Axons

- Anoxia Effects

A
  • Loss of various sensations
  • Hyperalgesia
  • Increase in latency
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17
Q

Somatic Afferents

A

Pseudo-unipolar cells travel to the spinal cord.

Cell body in the dorsal root ganglion of the spinal nerve

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18
Q

Modality Separation

- 2 Pathways

A
  • Dorsal column-mediated lemniscal system

- Anteriolateral system

19
Q

Dorsal-Column Mediated Lemniscal System

  • Fibres
  • Information
A

Aα and Aβ fibres

  • Touch
  • Proprioception
20
Q

Anteriolateral System

  • Fibres
  • Information
A

Adelta and C fibres

  • Pain
  • Temperature
21
Q

Dorsal-Column Mediated Lemniscal System

- 2 Routes

A
  • Spinal nerves (body)

- Cranial nerves (head)

22
Q

Dorsal-Column Mediated Lemniscal System

- Spinal Nerve Pathway

A
  1. Cell bodies in the dorsal root ganglion
  2. Dorsal column
  3. Dorsal column nuclei
  4. Decussate
  5. Bypasses trigeminal nucleus
  6. Thalamus
  7. Primary somatosensory cortex (SI) or secondary somatosensory cortex (SII)
23
Q

Dorsal-Column Mediated Lemniscal System

- Cranial Nerve Pathway

A
  1. Cranial nerve V (trigeminal)

2. Joins medial lemniscus tract

24
Q

Dorsal-Column Mediated Lemniscal System

- Spinal Cord

A

Neurones either:

  • Synapse in dorsal horn then pass to the dorsal column nuclei
  • Travel directly to dorsal column nuclei
25
Q

Dorsal-Column Mediated Lemniscal System

- Dorsal Column

A
  • Fasciculus cuneate above mid-thoracic level

- Fasciculus gracilis below mid-thoracic level

26
Q

Dorsal-Column Mediated Lemniscal System

- Dorsal Column Nuclei

A
  • Cuneate nucleus

- Gracile nucleus

27
Q

Dorsal-Column Mediated Lemniscal System

- Somatotopic Organisation

A

Cells in the dorsal column nuclei are somatotopically organised:

  • Leg neurones = medial
  • Arm neurones = lateral

Maintained at all levels of the pathway

28
Q

Anterolateral System

- 2 Routes

A
  • Spinal nerves (body)

- Cranial nerves (head)

29
Q

Anterolateral System

- Spinal Nerve Pathway

A
  1. Cell bodies in the dorsal root ganglion
  2. Laminae 1 and V-VII of the dorsal horn
  3. Decussation
  4. Antero-lateral system

Either:

  • Peripaqueductal grey (PAG) and reticular formation then thalamus then anterior cingulate cortex (ACC) or insula
  • Thalamus then primary somatosensory cortex
30
Q

Anterolateral System

- Cranial Nerve Pathway

A
  1. Cranial nerve V (trigeminal)
  2. Trigeminal nucleus
  3. Decussate
  4. Join anterograde-lateral system
31
Q

Anterolateral System

- Spinal Cord

A

Neurones branch and give rise to branches that ascend and descend a few mm in Lissauer’s tract

Synapse in laminae I and V-VII in the dorsal horn, then give rise to either:

  • Direct pathway for nociceptors in primates
  • Indirect pathway for nociceptive and tactile cutaneous inputs, which activates cells in the deep dorsal horn
32
Q

Anterolateral System

- Lesions

A

Reduce pain sensations from the contralateral side of the body.

However this is temporary

33
Q

Trigeminal neuralgia

A

Also known as tic douloureux

Syndrome where gentle stroking of the face or mouth provokes a stabbing pain

Example of allodynia

34
Q

Spinal Cord Sectioning

- Hemisection

A

Brown-Sequard syndrome

  • Loss of dorsal column mediated lemniscal system, causing loss of ipsilateral touch and proprioception below the level of the lesion
  • Loss of anterolateral system, causing loss of contralateral pain and temperature sensation below the level of the lesion
35
Q

Spinal Cord Sectioning

- Cavitation Loss

A 
Syringomyelic syndrome (syringomyelia)
- Disrupts decussating fibres of the anterolateral system, causing bilateral loss of pain and temperature sensation below the level of the lesion. This is distributed in a cape, as loss is in the upper limbs and trunk
36
Q

Spinal Cord Sectioning

- Cavitation Loss Cause

A

Central cyst

37
Q

Spinal Cord Sectioning

- Posterior Column Lesions

A

Posterior column syndrome

  • Disrupts dorsal column mediated lemniscal syndrome causing bilateral loss of touch and proprioception below the level of the lesion
  • Loss of proprioceptive feedback, causing characteristic stamping gait
38
Q

Spinal Cord Sectioning

- Posterior Column Lesion Causes

A
  • Tabes dorsalis

- Tertiary syphyllis

39
Q

Spinal Cord Sectioning

- Complete Transection

A

Impairment of all sensory modalities below the level of the transection

40
Q

Lateral Inhibition

- Principle

A

Allows amplification of differences in the output of neighbouring neurones and effectively enhances somatosensory contrast

41
Q

Lateral Inhibition

- Mechanism

A

excitatory centre cells synapse directly in the dorsal column nuclei

Inhibitory surround cells have an excitatory synapse in the dorsal column nuclei which excites an inhibitory interneurone, that synapses with the excitatory cell innervated by the excitatory central cell

42
Q

Unmasking

A

Blocking a normal input to the CNS may be followed by immediate unmasking of previously ineffective inputs.

This is an example of gate control, where the failure of one input unmasks the presence of inputs which are ordinarily suppressed by inhibitory mechanisms

43
Q

Thalamus

- Role

A

Signal tuning through adaptive signal processing using descending information delivered by cortical neurones returning to the thalamus

44
Q

Thalamus

- Nuclei

A

Ventral medial nucleus (VMPO) is a specific nucleus for pain and temperature sensation

  • Inhibited by warming of the contralateral body
  • Excited by cooling the contralateral body

NAB (37 decks)

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