Sensory Systems (Including Physiology of Pain)

Question 1

true or false:

all sensory receptors transduce their adequate stimulus into a depolarisation, the receptor potential

Answer 1

true

Question 2

the size of the receptor potential encodes the what of a stimulus?

Answer 2

intensity

Question 3

where are action potentials transmitted to?

Answer 3

CNS by axons

Question 4

cutaneous sensation is mediated by 3 types of primary afferent fibres, what are they?

Answer 4

Aβ = large myelinated (30-70m/s) touch, pressure, vibration

Aδ = small myelinated (5-30m/s) cold, “fast” pain, pressure

C = unmyelinated fibres (0.5-2m/s) warmth, “slow” pain

Question 5

what is proprioception mediated by?

Answer 5

2 types of primary afferent fibres

Aα & Aβ eg muscle spindles, golgi tendon organs etc

Question 6

what are the mechanoreceptive fibres?

Answer 6

Aα & Aβ

Question 7

what do the mechanoreceptive fibres do?

Answer 7

project straight up through ipsilateral dorsal columns

synapse in cuneate & gracile nuclei

the 2nd order fibres cross over midline (decussate) in the brain stem & project to reticular formation, thalamus and cortex

Question 8

what are the thermoreceptive and nociceptive fibres?

Answer 8

Aδ & C

Question 9

what do the thermoreceptive and nociceptive fibres do?

Answer 9

synapse in the dorsal horn

the 2nd order fibres cross over the midline in the spinal cord

project up through the contralateral spinothalamic (anterolateral) tract to reticular formation, thalamus and cortex

Question 10

what does damage to dorsal columns cause?

Answer 10

causes loss of touch, vibration, proprioception below lesion on ipsilateral side

Question 11

what does damage to anterolateral quadrant cause?

Answer 11

causes loss of nociceptive & temperature sensation below lesion on contralateral side

Question 12

where is the ultimate termination of the postcentral gyrus?

Answer 12

somatosensory cortex

Question 13

what produces the sensory homunculus?

Answer 13

postcentral gyrus

Question 14

how are sensory pathways processed?

Answer 14

adaptation
convergence
lateral inhibition

Question 15

how does adaptation process sensory pathways?

Answer 15

gradual decrease over time in the responsiveness of the sensory system to a constant stimulus

Question 16

how does convergence process sensory pathways

Answer 16

saves on neurones
but reduces acuity
may underlie referred pain

Question 17

how does lateral inhibition help to process sensory pathways?

Answer 17

activation of one sensory input causes synaptic inhibition of its neighbours
gives better definition of boundaries
cleans up sensory information

Question 18

what are all the different types of pain?

Answer 18

sharp, stabbing vs diffuse, throbbing pain
fast (initial) pain vs slow (delayed) pain
acute vs chronic pain
visceral pain
referred pain
phantom limb pain

Question 19

what is signal transduction in nociceptors activated by?

Answer 19

low pH, heat (via ASIC, TRPV1 etc)

local chemical mediators (eg bradykinin, histamine, prostaglandins)

Question 20

what is analgesia?

Answer 20

Non-steroidal anti-inflammatory drugs (NSAIDs)

Question 21

why are NSAIDS analgesic?

Answer 21

because they inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins

Question 22

how does analgesia work as a local anaesthetic?

Answer 22

block Na+ action potential and therefore all axonal transmission

Question 23

what do opiates do to nociceptors?

Answer 23

reduce sensitivity

Question 24

what are the functions of opiates?

Answer 24

block transmitter release in dorsal horn (hence epidural administration)
activate descending inhibitory pathways