Sensory System

Question 1

Define modalities and list them

Answer 1

• Modalities - different types of sensation
• Exist due to variety of receptors
• Spinothalamic system modalities - temperature, pain, pressure/crude touch
• Dorsal column modalities - vibration, proprioception, fine touch, two point discrimination
• Some sensations are due to multiple modalities
  
  • Eg. Stickiness, orgasm

Question 2

State the effect of different degrees of activation of primary sensory neurones

Answer 2

• The stronger the receptor is activated, the faster the train of action potentials through the neurone
• Likewise, minimal activation of receptor sets up slower train of action potentials

Question 3

Describe examples of units having different frequency of activation of primary sensory neurones

Answer 3

• Rapidly adapting units
  
  • Initially fires quick action potentials but slows
  • Good at detecting changes
  • Eg. Mechanoreceptors on skin initially notice sitting on chair but then forget
• Slowly adapting units
  
  • Action potential will not change frequency over time
  • Eg. Pain receptors - don’t want to lose pain sensation
    - Pain does not go away easily

Question 4

Describe the concept of receptive field

Answer 4

• Multiple receptors of same type converge to primary sensory neurone
  
  • Each supply a region of skin within the dermatome
  • If region of skin is big, then low acuity
  • If region of skin is small, then high acuity
• Overlap of receptive fields mean sensation felt in multiple dermatomes

Question 5

Describe primary, secondary and tertiary sensory neurones, including location of cell body and axon

Answer 5

• Primary sensory neurone
  
  • Has cell body in dorsal root ganglion
  • Has receptor attached
  • Axon runs ipsilateral with cell body
• Secondary sensory neurone
  
  • Has cell body in dorsal horn or medulla
  • Decussates - crosses to contralateral side
• Tertiary sensory neurone
  
  • Has cell body in thalamus
  • Projects to primary sensory cortex

Question 6

Describe the different arrangement of fibres from the lower and upper body in the 2 sensory pathways

Answer 6

• In the dorsal column pathway, the lower body maps to the medial portion of the tract
• In the spinothalamic tract, the lower body maps to the lateral/superficial parts of the tract

Question 7

Describe the structure of the dorsal column pathway

Answer 7

• 1˚ sensory neurone arising from the lower body run medial to the upper body neurones
• 1˚ sensory neurones run as the dorsal column pathway to the medulla, where it synapses with its 2˚ sensory neurones
  
  • 1˚ sensory neurones from the lower body synapse at the gracile nucleus in the medulla
  • 1˚ sensory neurones from the upper body synapse at the cuneate nucleus in the medulla
• 2˚ sensory neurones run as the medial lemniscus pathway to the thalamus where it synapses with the 3˚ sensory neurones
  
  • Once fibres reach the thalamus, it can be consciously perceived
• 3˚ sensory neurones synapse with the cortex
  - Upper limb represented laterally while lower limb represented medially

Question 8

Describe the structure of the spinothalamic pathway

Answer 8

• 1˚ sensory neurones synapse with their 2˚ sensory neurone at the dorsal horn of the corresponding spinal cord level
• 2˚ sensory neurones decussate at the same level at the ventral white commissure and then ascend up the spinal cord
  
  • 2˚ sensory neurones arising from the lower body run lateral to the upper body neurones
• 2˚ sensory neurones synapse with their 3˚ sensory neurone within the thalamus
• 3˚ sensory neurones synapse at the cortex
  
  • Upper limb represented laterally while lower limb represented medially

Question 9

Describe sensory loss in spinal cord lesions using Brown-Sequard syndrome

Answer 9

• Brown-Sequard syndrome - destruction of one half of spinal cord segment
  
  • Destruction commonly due to trauma or ischaemia
• Leads to unilateral destruction of dorsal horn, ventral horn, grey matter, white matter pathways, dorsal and ventral roots
• Ipsilateral complete segmental anaesthesia affecting a single dermatome - due to destruction of dorsal root and dorsal horn
  
  • Loss of all sensory modalities at specific dermatome
• Ipsilateral loss of dorsal column modalities below the lesion (as decussation takes place in medulla)
• Contralateral loss of spinothalamic modalities below the lesion (as decussation takes place immediately)

Question 10

Why will a patient with dorsal column lesion lose their balance if they stand up and close their eyes

Answer 10

• Equilibrium relies on having at least 2 out of 3 inputs to the brain - vision, proprioception and input from the vestibular system

Question 11

State how pain is felt

Answer 11

C fibres carry pain signal which are excitatory and synapses in the dorsal horn - switches on the spinothalamic tract

Question 12

Describe how pain can be inhibited

Answer 12

• A fibres carry impulses from mechanoreceptors in the skin
  
  • Excites inhibitory enkephalinergic interneurones which inhibit 2nd order neurone of C fibres
  • Explains why rubbing a painful area helps relieve pain - mechanoreceptors are activated
  • Heat and analgesics also inhibit interneurones of C fibres
• Descending inputs from the brain can also activate inhibitory interneurones
  - Psychologically inhibit pain