Sensory systems (including physiology of pain)

Question 1

what are different types of sensory information associated with

Answer 1

specific receptor types responding to a specific sensory modality

Question 2

give some examples of specific receptors

Answer 2

mechanoreceptors, chemoreceptors, thermoreceptors, nociceptors, proprioceptors

Question 3

what is the receptive field of a receptor

Answer 3

the area in which they respond to a stimulus

Question 4

what are the different types of receptor structures

Answer 4

can be free nerve endings e.g. nociceptors

can be complex structures e.g. pacinian corpuscle, meissners corpuscle

Question 5

list some receptor structures and what stimuli they respond to

Answer 5

1. tactile meissners corpuscle - light tough
2. tactile merkles corpuscle - touch
3. free nerve ending - pain
4. lamellated pacinian corpuscle - deep pressure
5. ruffini corpuscle - warmth

Question 6

what do all sensory receptors do the their adequate stimulus

Answer 6

transduce it into a depolarisation (the receptor generator potential)

Question 7

what determines the size of the receptor generator potential

Answer 7

the intensity of stimulus

Question 8

what does the receptor generator potential evoke

Answer 8

firing of APs for long distance transmission

Question 9

what determines the frequency and duration of APs fired

Answer 9

the intensity of stimulus

porportionally

Question 10

what does the receptive field encode

Answer 10

the location of the action potential

Question 11

what determines neurotransmitter release

Answer 11

the pattern of action potentials arriving at the axon terminal

Question 12

briefly summaries the steps in signal transduction

Answer 12

1. stimulus
2. transduction site = receptro generator potential
3. trigger zone - receptor potential integrated
4. myelinated axon - firing of APs - frequency and duration based on stimulus
5. axon terminal - neurotransmitter release in response to AP

Question 13

what determines acuity

Answer 13

density of innervation and size of receptive fields

Question 14

what transmits action potentials to the CNS

Answer 14

axons

Question 15

what 2 types of primary afferent fibres mediate proprioception

Answer 15

mechanoreceptive:

Aα & Aβ

eg muscle spindles, golgi tendon organs etc

Question 16

what 3 types of primary afferent fibres mediate cutaneous sensation

Answer 16

thermoreceptive and nociceptive:

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 17

where do all the primary afferent fibres enter the spinal cord

Answer 17

at the dorsal root ganglion

or cranial ganglia for the head

Question 18

what do Abeta fibres mediate

Answer 18

touch pressure, vibration

Question 19

what do Adelta fibres mediate

Answer 19

cold, fast pain, pressure

Question 20

what do C fibres mediate

Answer 20

warmth, slow pain

Question 21

what is the route of transmission of Aa, Ab fibres

Answer 21

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 22

what is the route of transmission of Adelta and C fibres

Answer 22

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 23

what does the different pathways for transmission of sensory information explain

Answer 23

explains consequences of different spinal cord injuries

Question 24

what does damage to the dorsal column cause

Answer 24

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

Question 25

what does damage to the anterolateral quadrant cause

Answer 25

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

Question 26

where is the ultimate termination of the sensory fibres pathway

Answer 26

somatosensory corted (S1) of the post central gyrus

Question 27

what produces the sensory homunculus

Answer 27

extent of representation - related to the density of receptors in each location

Question 28

what are the three processes in sensory pathways

Answer 28

adaptation
covergence
lateral inhibition

Question 29

what is adaptation

Answer 29

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

Question 30

what is convergence

Answer 30

when non-specific ascending pathways combine e.g. touch and temperature responses so that there is one larger group of info arriving in the brain

reduces acuity and may underlie referred pain

Question 31

what is lateral inhibition

Answer 31

when activation of one sensory input causes synaptic inhibition of tis neighbours

gives better definition of boundaries - cleans up sensory information

enhances the perception of the stimulus

Question 32

list some examples of different types of pain

Answer 32

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 33

what activate signal transduction in nociceptors

Answer 33

low pH, heat

local chemical mediators (e.g. bradykinin, histamine, prostaglandins)

Question 34

how does pressure alleviate pain

Answer 34

gate control theory:

• pressure activates Aa/b fibres
• activate inhibitory interneurones
• interneurons release opioid peptides (endorphins)
• endorphins inhibit transmitter relase from Adelta/C fibres
  = CLOSE THE GATE OF PAIN

happens at the segmental level

Question 35

how does the gate control theory work in the descending controls

Answer 35

same inhibitory interneurones are also cite by descending pathways from the peri-aqueductal grey matter and nuclus raphe magnus - also close the gate

activated by opiates

Question 36

what do prostaglandins do to nociceptors

Answer 36

sensitise them to bradykinin

Question 37

how do NSAIDS work as an analgesic

Answer 37

they inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins

they are also anti-inflammatory so work well with pain associated with inflammation

Question 38

how does local anaesthetics block pain

Answer 38

block Na+ action potential and therefore all axonal transmission

Question 39

how do opiates e.g. morphine work as an analgesic

Answer 39

reduce sensitivity to nociceptors

block transmitter release from the dorsal horn (hence epidural administration)

activate descending inhibitory pathways

Question 40

what is TENS

Answer 40

trans cutaneous electric nerve stimulation