Session 4: Sensory Pathways

Question 1

LO22 List and define the major somatosensory modalities.

Answer 1

1. Touch 2. Pain 3. Propioception 4. Temperature

Question 2

LO 23 Define sensory receptors.

Give the 3 types of sensory receptors

Answer 2

“Sensory receptors are transducers that convert energy from the environment into neuronal action potentials”

Subcategorised into:

• Thermoreceptors
• Mechanoreceptors (5 types)
• Nociceptors

Question 3

Give and describe the 3 types of sensory receptors.

Answer 3

1. Thermoreceptors:
   
   • Αδ- and C- fibres
   • Free nerve endings
   • Transient receptor potential (TPR) ion channels
   • 4 heat activated: TRPV1-4
   • 2 cold activated: TRPM8, TRPA1
2. Mechanoreceptors (5 types)
   
   • Meissner’s corpuscle:fine discriminative touch, low frequency vibration
   • Merkel Cells:light touch and superficial pressure
   • Pacinian corpuscle: deep pressure, high frequency vibration and tickling
   • Ruffini endings: continuous pressure or touch and stretch
3. Nociceptors

Question 4

LO 23 Define stimulus threshold.

Answer 4

a threshold is the point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold)

Question 5

LO 23 Define stimulus intensity.

Answer 5

increased stimulus strength and duration = increased neurotransmitter release = greater intensity

Question 6

LO 23 Adaptation: Distinguish between tonic and phasic receptors.

Answer 6

Tonic receptors:

• Detect continuous stimulus strength
• Continue to transmit impulses to the brain as long the stimulus is present.
• Keeps brain constantly infomred
• eg Merkel cells: slowly adapting to allow superficial pressure and fine touch to be perceived
• DO NOT ADAPT/ ADAPT SLOWLY

Phasic receptors:

• detect a change in stimulus strength
• transmit an impulse at the start and end of the stimulus eg when a change is taking place
• eg pacinian receptor: sudden pressure excites receptor, transmits signal again when pressure is released
• ADAPT QUICKLY

Question 7

LO 23 Define receptive fields. Distinguish between small and large receptive fields.

Answer 7

the region on the skin which causes activation of a single sensory neuron when activated

• Small receptive fields: allow for detection of fine detail over a small area, precise
  
  • eg fingers have many densely packed mechanoreceptors with small receptive fields
• Large receptive fields: allow the cell to detect changes over a wider area (less precise perception)

Question 8

LO 23 Define 2 point discrimination

Answer 8

• Minimum distance at which two points are perceived as separate
• Related to size of receptive field

Question 9

LO 23 Define lateral inhibition.

Answer 9

• a receptive field can overlap with another receptive field
• difficult to distinguish between 2 stimulus locations
• lateral inhibition prevents the overlap of receptive fields
• facilitates pinpoint accuracy in localisation of the stimulus
• mediated by inhibitory interneurons within dorsal horn of spinal cord
• facilitates enhanced sensory perception (discrimination)

Question 10

Somatosensory dermatomes

Answer 10

each spinal nerve has a specific dermatome on the skin

Question 11

What are the two main types dorsal horn neurons divide to?

Answer 11

• Those with axons that project to the brain (projection neurons)
• Those with axons that remain in the spinal cord (interneurons)

Question 12

LO 25 Name the 2 ascending/sensory pathways:

Answer 12

1. Spinothalamic Tract:
   
   • Lateral spinothalamic tract: pain and temp
   • Anterior spinothalamic tract: crude touch
2. Dorsal Column: fine touch, vibration and propioception

details of pathway in notes

Question 13

LO 27 Explain the physiological function of nociceptors.

• Give the function of Ad and C fibers.
• Describe spinal cord nociceptive processing.
• Name the 2 pain pathways.

Answer 13

• Ad fibers mediate sharp, intense or first pain
  
  • Type 1: noxious mechanical
  • Type 2: noxious heat
• C-fibers: mediate dull, aching or second pain
  
  • noxious thermal, mechanical and chemical stimuli (polymodal)

Spinal cord nociceptive processing: Glutamate (excitatory) is relased from sensory afferents in response to acute orpersistent noxious stimuli

2 pain pathways:

1. lateral spinothalamic tract: SENSORY component
2. Spinoreticular tract: EMOTIONAL component

Question 14

LO 28 What is gate control theory?

Explain how nociceptive input can be gated by peripheral and central mechanisms

Answer 14

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

Descending Control pathways:

• Periaqueductal Grey (PAG)
• Facilitation and inhibition of nociceptive processing in the dorsal horn
• Monoamines: serotonin and noreadrenaline