6b. Primary Afferents to Thalamus Flashcards
Sensory Axon Types
- Aα and Aβ
- Adelta fibres
- C fibres
Sensory Axon Types
- Aα and Aβ Fibre Characteristics
- Large
- Myelinated
- 40-80m/s conduction velocity
Sensory Axon Types
- Aβ Fibre Sensations
- Touch
- Proprioception
Pain is not elicited by stimulation of the Aβ fibres alone
Sensory Axon Types
- Adelta Fibre Characteristics
- Intermediate
- Myelinated
- 5-30m/s conduction velocity
Sensory Axon Types
- Adelta Fibre Sensations
- Cold
- Stabbing pain
Sensory Axon Types
- C Fibre Characteristics
- Small
- Unmyelinated
- 0.5-2m/s conduction velocity
Sensory Axon Types
- C Fibre Sensations
- Warmth
- Itching
- Burning pain
Sensory Axon
- Mild Anoxia
Affects larger Aβ fibres first, so the following sensations are lost:
- Touch
- Proprioception
Sensory Axon
- Prolonged Anoxia
Affects both Aβ and Adeta fibres, so the following sensations are lost:
- Touch
- Proprioception
- Initial sharp pain
- Cold
Sensory Axon
- Mild Anaesthetic
Affects C fibres, so the following sensations are lost:
- Burning pain
- Itch
- Warmth
Sensory Axon
- Prolonged Anaesthetic
Affects C fibres and Adelta fibres, so the following sensations are lost:
- Burning pain
- Itch
- Warmth
- Cold
- Stabbing pain
Sensory Axon
- Diabetes Mellitus nad Multiple Sclerosis
Myelin sheath of large diameter neurones degenerates resulting in slow conduction or failure of impulse transmission
Sensory Axon
- Anoxia Induced Hyperalgesia
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
Gate Theory of Pain
Touch stimuli suppress the transmission of pain messages through the spinal cord gates in the dorsal horn
Sensory Axons
- Anoxia Induced Increase in Latency Period
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
Sensory Axons
- Anoxia Effects
- Loss of various sensations
- Hyperalgesia
- Increase in latency
Somatic Afferents
Pseudo-unipolar cells travel to the spinal cord.
Cell body in the dorsal root ganglion of the spinal nerve
Modality Separation
- 2 Pathways
- Dorsal column-mediated lemniscal system
- Anteriolateral system
Dorsal-Column Mediated Lemniscal System
- Fibres
- Information
Aα and Aβ fibres
- Touch
- Proprioception
Anteriolateral System
- Fibres
- Information
Adelta and C fibres
- Pain
- Temperature
Dorsal-Column Mediated Lemniscal System
- 2 Routes
- Spinal nerves (body)
- Cranial nerves (head)
Dorsal-Column Mediated Lemniscal System
- Spinal Nerve Pathway
- Cell bodies in the dorsal root ganglion
- Dorsal column
- Dorsal column nuclei
- Decussate
- Bypasses trigeminal nucleus
- Thalamus
- Primary somatosensory cortex (SI) or secondary somatosensory cortex (SII)
Dorsal-Column Mediated Lemniscal System
- Cranial Nerve Pathway
- Cranial nerve V (trigeminal)
2. Joins medial lemniscus tract
Dorsal-Column Mediated Lemniscal System
- Spinal Cord
Neurones either:
- Synapse in dorsal horn then pass to the dorsal column nuclei
- Travel directly to dorsal column nuclei
Dorsal-Column Mediated Lemniscal System
- Dorsal Column
- Fasciculus cuneate above mid-thoracic level
- Fasciculus gracilis below mid-thoracic level
Dorsal-Column Mediated Lemniscal System
- Dorsal Column Nuclei
- Cuneate nucleus
- Gracile nucleus
Dorsal-Column Mediated Lemniscal System
- Somatotopic Organisation
Cells in the dorsal column nuclei are somatotopically organised:
- Leg neurones = medial
- Arm neurones = lateral
Maintained at all levels of the pathway
Anterolateral System
- 2 Routes
- Spinal nerves (body)
- Cranial nerves (head)
Anterolateral System
- Spinal Nerve Pathway
- Cell bodies in the dorsal root ganglion
- Laminae 1 and V-VII of the dorsal horn
- Decussation
- Antero-lateral system
Either:
- Peripaqueductal grey (PAG) and reticular formation then thalamus then anterior cingulate cortex (ACC) or insula
- Thalamus then primary somatosensory cortex
Anterolateral System
- Cranial Nerve Pathway
- Cranial nerve V (trigeminal)
- Trigeminal nucleus
- Decussate
- Join anterograde-lateral system
Anterolateral System
- Spinal Cord
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
Anterolateral System
- Lesions
Reduce pain sensations from the contralateral side of the body.
However this is temporary
Trigeminal neuralgia
Also known as tic douloureux
Syndrome where gentle stroking of the face or mouth provokes a stabbing pain
Example of allodynia
Spinal Cord Sectioning
- Hemisection
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
Spinal Cord Sectioning
- Cavitation Loss
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
Spinal Cord Sectioning
- Cavitation Loss Cause
Central cyst
Spinal Cord Sectioning
- Posterior Column Lesions
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
Spinal Cord Sectioning
- Posterior Column Lesion Causes
- Tabes dorsalis
- Tertiary syphyllis
Spinal Cord Sectioning
- Complete Transection
Impairment of all sensory modalities below the level of the transection
Lateral Inhibition
- Principle
Allows amplification of differences in the output of neighbouring neurones and effectively enhances somatosensory contrast
Lateral Inhibition
- Mechanism
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
Unmasking
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
Thalamus
- Role
Signal tuning through adaptive signal processing using descending information delivered by cortical neurones returning to the thalamus
Thalamus
- Nuclei
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
