sensory systems

Question 1

Explain the concept of sensory transduction

Answer 1

• All sensory receptors transduce their adequate stimulus into a depolarisation, the receptor general potential
• The size of the potential encodes the intensity of the signal
• The receptor potential then evokes firing of action potentials for long distance transmission
• The frequency of the action potentials encodes the intensity of the stimulus
• Receptor field encodes the location of the stimulus
• Gives information on the modality (type of sensory cell stimulated), intensity and location

Question 2

Describe the major pathways by which information from the body about pain and temperature reaches consciousness

Answer 2

• Thermoreceptors and nociceptive fibres (Aδ and C)

• Synapse in the dorsal horn
• The second order fibres crosses over the midline in the spinal cord
• They project up through the contralateral spinothalamic (anterolateral) tract to the reticular formation, thalamus and cortex

Question 3

Describe the major pathway by which information from the body about touch and limb position reaches consciousness

Answer 3

• Mechanoreceptor fibres (Aα and Aβ)

• Synapse in the dorsal horn
• Synapse in the cuneate and gracile nuclei
• The 2nd order fibres cross over the midline in the brainstem (desiccation of the pyramids) and project into the reticular formation, thalamus and cortex

Question 4

How can tissue damage lead to pain?

Answer 4

When a tissue is damaged, bradykinin is released which stimulate VR1 which leads to depolarisation and then excitation (pain)

Question 5

Explain why pain originating from the viscera can often result in sensation being referred to a somatic structure from the same dermatomes

Answer 5

• The reason for this is convergence
- Saves on neurons
- Reduces acuity
• Non-specific ascending pathways are the pathways taken

Question 6

Explain the gate control hypothesis for pain modulation

Answer 6

• Segmental controls
- Activity in Aα/Aβ fibres activates inhibitory interneurons
- Inhibitory interneurons release opioid peptides (endorphins) that inhibit transmitter release from the Aδ/C fibres and “close the gate”
• Descending controls
- The same inhibitory interneurons are also activated by descending pathways from PAG and NRM, hence also “closing the gate”

Question 7

Explain the actions of non-steroidal anti-inflammatory drugs

Answer 7

• Prostaglandins sensitise nociceptors to bradykinin

- NSAIDs inhibit cyclooxygenase from converting arachidonic acids to prostaglandins

Question 8

Explain the actions of local anaesthetic

Answer 8

Blocks Na+ action potential and therefor all axonal transmissions

Question 9

Explain the actions of transcutaneous electric nerve stimulation

Answer 9

Electrical stimulation of the skin acts as an analgesia although we are not entirely sure why, maybe the gate control mechanism

Question 10

Explain the actions of opioids (e.g. morphine)

Answer 10

• Reduces the sensitivity of nociceptors to bradykinin
• Blocks transmitter release in the dorsal root
• Actives descending inhibitory pathways