Lecture 22: Somatosensory Processing 2

Question 1

Describe the role that the thalamus plays in somatosensory processing

Answer 1

• Relay station : DCML and spinothalamic tracts (sensory information) -> 1° somatosensory cortex
• Ventroposteriolateral nucleus (VPL) - where 2° axons synapse onto
• Maps - different fibres end in different regions based on info they project

Question 2

What are the somatosensory cortices

Answer 2

Primary somatosensory cortex (S1)
Secondary somatosensory cortex (S2)
Posterior parietal and superior temporal cortex

Question 3

Describe the primary somatosensory cortex (S1)

Answer 3

Major site of where axons from thalamus terminate

Somatotopically arranged (homunculus)

Consists of Brodmann areas 3a, 3b, 1, 2 (somatic sensory cortex)

Different areas -> receive different information, project to different brain regions - vibration & constant touch mapped in different S1 regions

Arranged in 6 layers

Question 4

Describe the information received in different areas of the primary somatosensory cortex

Answer 4

Brodmann’s area 1 - rapidly adapting receptors of skin (vibration)

Brodmann’s area 3a - slowly (constant touch) and rapidly adapting receptors of skin

Layer 4 - sensory information from thalamus

Layer 2 & 3 - info to contralateral S1 (input coordination), posterior parietal cortex & motor cortex (planning & movement)

Layer 5 - info to basal ganglia, brain stem & spinal cord (coordination of response to stimuli by controlling parts of motor system)

Question 5

Describe the different S1 areas that project to different brain regions to integrate information

Answer 5

Areas 3a, 3b, 1, 2 -> 2° somatosenory cortex -> amygdala & hippocampus (learning, emotional response)

Area 2 -> parietal areas 5, 7 & somatosensory association cortex -> motor & premotor cortical areas (motor response to stimuli)

• Area 3a -> Area 2
• Area 3b -> Area 2, Area 1

Question 6

Describe the 2° somtosensory cortex (S2)

Answer 6

Integration with other sensory information involvement
Project to a number of areas
- S1
- contralateral S2 [coordinating response]
- association cortex [visuomotor system - associate between different stimuli types]
- pre motor cortex (motor planning)
- amygdala & hippocampus

Question 7

Describe the the somatosensory association cortex

Answer 7

Posterior parietal & superior temporal association cortex
Integration of different sensory information -> better understanding of the world
- Involved in higher order processing
- receive input from other areas

Projects to motor cortices - initiate voluntary movement

Question 8

What is somatosensory plasticity

Answer 8

Cortical representation changes that take place after injury or experience

Rapid, transient - unmasking of connections or synaptic efficacy upregulation. Or else cortical map return to normal

Structural plasticity - permanent change in cortical representation, stable enlargement of cortical representation that occurs over time

Question 9

Overlapping projection zones to allow change in fields

Answer 9

• injury or training -> loss of inhibition
  
  • projections can project into other fields but are inhibited
• injury - neuronal inhibition loss -> other body parts take over other areas of brain. ~ rapid
• dependent synaptic strengthening (consolidation) takes longer -> larger representation in cortex
• input loss : permanent réorganisation of cerebral cortex -> sprouting of other axons
• phantom sensation: arm stimulated but brain perceives it as hand stimulation

Question 10

Describe cortical plasticity during learning using Braille as an example

Answer 10

Very detailed information extracted from fingertips -> cortical representation enlargement of reading finger

However, lesions in S1 no cause loss of Braille ability -> S1 area change no correlate with Braille ability

Cross modal plasticity - deafferanted area used for highest demanding task

Blind individuals - occipital lobe activated -> somatosensory system recruit it to learn Braille. -> occipital lobe damage -> disrupt Braille 7 letter recognition

Question 11

What are the 4 somatosensory injury catergories

Answer 11

Peripheral nerve
Spinal cord
Thalamus
Cortex

Question 12

Peripheral nerve injury

Answer 12

Include one or more peripheral nerves
Affect different fibres in the nerve
Areas above/below nerve unaffected

Question 13

Incomplete spinal cord injury
- posterior cord syndrome (PCS)
- anterior cord syndrome (ACS)
- central cord syndrome (CCS)

Answer 13

1. PCS - damage to posterior spinal cord portion / posterior spinal arteries interruption -> fine touch & vibration affected but not crude touch, pain or temperature
2. ACS - damage to anterior spinal cord portion / anterior spinal artery blood supply - pain, temperature, crude touch affected (Corticospinal tract & spinothalamic tract) but not fine touch & vibration
3. CCS - proprioception & vibration sense loss below level of injury (think of somatotopic arrangement).
   - inverse paraplegia : motor impairment in arms and hand and to lesser extent legs

Question 14

Thalamus injury

Answer 14

Damage to VPN -> Lose all somatosensory modalities -> small (usually more than one area affected)

Question 15

Complex deficits in cortex
- Define
- 3 examples

Answer 15

Parietal damage in associate areas
Due to damage in more than one somatosensory area

Hermineglect - deficit in attention and sense to contralateral body side
Agraphesthesia (~ association area damage) - unable to recognize cutaneous sensation orientation
Somatosensory agnosia - cannot identify object without visual input