Sensory Systems (Including Physiology of Pain)

Question 1

sensory receptors respond to stimulus over a specific area, what is the area called?

Answer 1

receptive field

Question 2

what do sensory receptors transduce their adequate stimulus into?

Answer 2

a depolarisation, the receptor (generator) potential

Question 3

what encodes the intensity of the stimulus

Answer 3

size of receptor potential

then frequency of action potentials

Question 4

what does the receptor potential evoke?

Answer 4

firing of action potentials for long distance transmission

Question 5

what encodes the location of the stimulus?

Answer 5

receptive field

Question 6

what is proportional to duration of a series of action potentials?

Answer 6

stimulus duration

Question 7

what determines acuity?

Answer 7

Density of innervation, and size of receptive fields

Question 8

what are 3 types of primary afferent fibres that mediate cutaneous sensation?

Answer 8

Aβ
Aδ
C

Question 9

describe Aβ fibres

Answer 9

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

Question 10

describe Aδ fibres

Answer 10

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

Question 11

describe C fibres

Answer 11

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

Question 12

what are 2 types of primary afferent fibres that mediate proprioception & mechanoreception?

Answer 12

Aα & Aβ

Question 13

what are 2 types of primary afferent fibres that mediate thermoreception & nociception ?

Answer 13

Aδ & C

Question 14

describe the pathway of mechanoreceptive fibres

Answer 14

project straight up through ipsilateral dorsal columns
synapse in cuneate & gracile nuclei
the 2nd order fibres cross over midline in the brain stem

Question 15

describe the pathway of thermoreceptive & nociceptive fibres

Answer 15

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

Question 16

where do the Primary afferent fibres project in the brain?

Answer 16

reticular formation, thalamus and cortex

Question 17

what is affected when there is damage to dorsal columns?

Answer 17

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

Question 18

what is affected when there is damage to anterolateral quadrant?

Answer 18

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

Question 19

where is the ultimate termination of sensory information?

Answer 19

sensory homunculus

in the somatosensory cortex (S1) of the postcentral gyrus

Question 20

what is neural/sensory adaption?

Answer 20

change over time in the responsiveness of the sensory system to a constant stimulus
ie action potentials stop firing after putting on clothes

Question 21

what is the difference between fast and slow adaptation?

Answer 21

Fast adaptation occurs immediately after stimulus presentation i.e., within 100s of milliseconds.
Slow adaptive processes that take minutes-days

Question 22

what is neural convergence?

Answer 22

One postsynaptic cell receives convergent input from a number of different presynaptic cells (saves on neurones but reduces acuity, may underlie referred pain)

Question 23

describe lateral inhibition

Answer 23

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

Question 24

what is chronic and phantom limb pain due to?

Answer 24

neural plasticity

Question 25

what are nociceptors activated by?

Answer 25

low pH, heat (via ASIC, TRPV1 etc)

local chemical mediators (eg bradykinin etc)

Question 26

what are the two ways the body inhibits pain?

Answer 26

Segmental controls

Descending controls

Question 27

what activates the inhibitory interneurones in segmental controls?

Answer 27

activity in Aα/β fibres

Question 28

describe how Inhibitory interneurones inhibit pain

Answer 28

release opioid peptides (endorphins) that inhibit transmitter release from Aδ/C fibres, hence “closing the gate”

Question 29

what activates the Inhibitory interneurones in decending controls?

Answer 29

descending pathways from brain:

Peri-aqueductal grey matter (PAG) Nucleus raphe magnus (NRM)

Question 30

how do Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit pain?

Answer 30

prostaglandins sensitise nociceptors to bradykinin

NSAIDS block prostaglandins so bradykinin signal stays small

Question 31

how are NSAIDS anti-inflammatory?

Answer 31

they inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins

Question 32

how do Local anaesthetics inhibit pain?

Answer 32

block Na+ action potential and therefore all axonal transmission
(why area affected is numb)

Question 33

how do Trans cutaneous electric nerve stimulation (TENS) inhibit pain?

Answer 33

increases activity in Aα/β fibres (segmental control activated)

Question 34

how do Opiates (eg morphine)

inhibit pain?

Answer 34

reduce sensitivity of nociceptors
block transmitter release in dorsal horn (hence epidural administration)
activate descending inhibitory pathways