PHYS: Somatic Sensations (inc. Pain and Temperature)

Question 1

exteroception, proprioception, interoception

Answer 1

• exteroception: info abt external world (mechanoreceptors, thermoreceptors, nociceptors) - type of somatic sense
• proprioception (kinaesthesis): info abt position of body in space (type of somatic sense)
• interoception: info re: homeostasis e.g. BP, hunger, thirst (type of visceral sense)

Question 2

3 ways to activate a mechanoreceptor

Answer 2

• stretch
• tethered: proteins act as elastic strings, press on ion channels and cause them to open
• indirect gated

Question 3

how is a mechanoreceptor activated?

Answer 3

• translation: stimulus causes physical change in receptor shape > change in membrane potential
• transduction: spike region generates AP if stimulus is large enough
• transmission: conducting region sends message to brain via AB sensory fibres

Question 4

how does the location of a skin mechanoreceptor determine its function?

Answer 4

• located deeper (pacinian + ruffini) = bigger receptor field size = more crude/delocalised touch and vibration
• superficial (merkel + meissner’s) = smaller size = finer touch

Question 5

meissner’s corpuscle:
- structure
- location
- function
- adaptivity

Answer 5

• barrel-shaped, encapsulated, connected to AB sensory fibres
• dermal papillae (non-hairy/glabarous skin)
• fine/light touch
• rapidly adapting (FAI)

Question 6

pacinian corpuscle:
- structure
- location
- function
- adaptivity

Answer 6

• onion ring, encapsulated, connected to AB sensory fibres, large RF
• deep dermis (hairy + non-hairy skin)
• deep pressure + vibration
• VERY rapidly adapting (FAII)

Question 7

merkel’s discs:
- structure
- location
- function
- adaptivity

Answer 7

• small receptive field, unencapsulated, connected to AB sensory fibres
• upper dermis
• low frequency vibration (Braille), indentation, pressure, texture, VERY FINE DETAIL
• slow adapting (SAI)

Question 8

ruffini endings:
- structure
- location
- function
- adaptivity

Answer 8

• large receptive fields, encapsulated, connected to AB sensory fibres
• dermis
• detect stretch/tension
• slow adapting (SAII)

Question 9

describe fast vs slow adapting receptors, including their alternative names

Answer 9

• fast (phasic): only fire APs @ onset and offset of stimulus - good for detecting changes
• slow (tonic): fire as long as the stimulus is present - good for sustained stimuli

Question 10

types of fast/slow adapting receptors

Answer 10

• FA I (meissners - small RF): only respond @ offset and onset
• FA II (pacinian - large RF): fire during quick changes
• SA I (merkel’s - small RF): fire APs throughout duration of stimulus
• SA II (ruffini - large RF): more sparse APs throughout stimulus

Question 11

3 types of proprioceptors

Answer 11

• cutaneous mechanoreceptors (Ruffini endings)
• joint receptors (Ruffini endings + pacinian corpuscle)
• muscle spindles + Golgi tendon organs

Question 12

muscle spindles
- structure
- afferent fibres
- efferent fibres

Answer 12

• encapsulated proprioceptive structures containing intrafusal muscle fibres, parallel w/ muscle fibres = muscle stretch causes receptor stretch
• afferent: stretch detected by type 1a (Aa = fast adapting) and type II (AB = slow adapting) sensory fibres
• efferent: initiate stretch reflex (muscle contraction) to maintain posture and coordination via gamma motor neurons - e.g. patellar jerk

Question 13

Golgi tendon organs
- location
- afferent fibres
- efferent fibres

Answer 13

• attached to tendons
• afferent: type 1B sensory fibres detect muscle stretch (type of Aa fibre)
• efferent: alpha motor neurons initiate INVERSE stretch reflex (muscle relaxation) to prevent injury and overstretching

Question 14

why does pain exist (short term and long term)?

Answer 14

• short-term pain = withdrawal from source prevents further damage
• also a social signal to alert others to danger
• long-term pain = promotes behaviours e.g. sleep and inactivity = recovery

Question 15

what are nociceptors?
- what do they detect?
- where are they located?

Answer 15

• FREE nerve endings
• polymodal: can detect thermal, mechanical, chemical stimuli that are painful
• located everywhere except CNS

Question 16

4 types of sensory neuron fibres (lvl of myelination + function)

Answer 16

• Aa: heavily myelinated (proprioception + motor)
• AB: myelinated (touch, pressure, vibration)
• Adelta: little myelinated (sharp/fast pain, temperature)
• C: unmyelinated (temperature, slow pain, itch)

Question 17

how is pain felt as ‘ouch’ vs ‘groan’

Answer 17

• Adelta fibres are myelinated so signals are quicker = quick ouch sensation, readily localised b/c all afferent fibres reach the thalamus and somatosensory cortex (NEOSPINOTHALAMIC - SYNAPSE LAMINA I)
• C fibres are unmyelinated so signals are slower = groan sensation, more poorly localised b/c fibres end in lower brain regions (PALEOSPINOTHALAMIC - SYNAPSE LAMINA II AND III)

Question 18

2 types of A delta receptors

Answer 18

• mechanical: mediate fast pain response to mechanical stimuli e.g. pin prick
• thermal: mediate fast pain response to noxious temps, mostly cold
  (both have high threshold b/c only respond to painful stimuli)

Question 19

4 types of C-fibres

Answer 19

• nociceptive (peptidergic and non-peptidergic)
• silent
• itch (e.g. histamine)
• non-nociceptor

Question 20

silent C-fibres

Answer 20

• sensitive to heat but not mechanical stimuli - can be unsilenced by chemical stimuli e.g. capsaicin

Question 21

non-nociceptor C-fibres

Answer 21

• respond to pleasurable touch and cooling stimuli

Question 22

2 divisions of the spinothalamic tract

Answer 22

• anterior: crude touch, pressure, itch
• lateral: pain, temp, sexual sensations

Question 23

spinothalamic tract pathway

Answer 23

• 1st order sensory neurons synapse w/ 2nd order neurons in the dorsal horn
• 2nd order neurons decussate contralaterally in spinal cord and synapse w/ 3rd order neurons in thalamus (ventral posterior nucleus) > somatosensory cortex

Question 24

sensory pathway from face and head (trigeminal pain/temp pathway)

Answer 24

• 1st order neurons: three branches (V1, V2 and V3) of trigeminal nerve > synapse w/ 2nd order neurons in spinal trigeminal nucleus
• 2nd order neurons: decussate in trigeminal lemniscus (brainstem) and synapse in ventral posteromedial nucleus of the thalamus
• 3rd order neurons: travel via the internal capsule to the primary somatosensory cortex within postcentral gyrus

Question 25

how does sensitisation work during tissue injury?

Answer 25

• nociceptors adapt in response to multiple components of the ‘inflammatory soup’
• increased chance of activation and electrical activity > hyperalgesia/allodynia

Question 26

• primary hyperalgesia
• secondary hyperalgesia
• allodynia

Answer 26

• primary: pain hypersensitivity in immediate region of injury
• secondary: pain hypersensitivity in surrounding area of injury
• allodynia: non-noxious stimuli is perceived as painful e.g. warm water on a sunburn

Question 27

gate control theory

Answer 27

• AB (touch) fibres can inhibit C and A-delta fibres (nociceptive) b/c AB fibres are myelinated = quicker
• i.e. this is why pain is reduced when you rub the area

Question 28

how do descending pathways inhibit synaptic transmission in the dorsal horn? (pain modulation)

Answer 28

• descending inhibitory neuron releases enkephalin > acts on u-opioid receptors on 1st order neuron coming into dorsal horn
• reduces release of excitatory neurotransmitters e.g. glutamate to slow pain transmission
• originates from PAG

Question 29

why does chronic pain (>3 months) occur

Answer 29

• in response to persistent injury, nervous system has increased plasticity > enhanced pain signalling, hypersensitivity and reduction in threshold
• no physiological purpose unlike acute pain

Question 30

types of pain

Answer 30

• nociceptive: visceral or somatic (deep or superficial)
• neuropathic: central or peripheral
• inflammatory: tissue inflammation or hypersensitivity

Question 31

congenital insensitivity vs erythromelalgia

Answer 31

• insensitivity: loss of nav1.7 function
• erythromelalgia: gain of nav1.7 function (blockage of blood vessels in lower extremities > hyperinflammation)

Question 32

examples of neuropathic pain

Answer 32

• complex regional pain syndrome: develops after injury, surgery, AMI, starts in a limb
• post-herpetic neuralgia: viral origin e.g. shingles/HIV
• back/spinal injury
• diabetic neuropathy

Question 33

which drug blocks NMDA receptor activity?

Answer 33

• ketamine

Question 34

which neurotransmitter is responsible for sensitisation in the dorsal horn?

Answer 34

• glutamate
• also microglial activation > release BDNF > shift in gradient of pain transmission neurons > more excitability

Question 35

phantom limb pain

Answer 35

• following amputation, nerves that would normally innervate the missing limb can cause pain
• can arise for peripheral, spinal, central mechanisms

Question 36

referred pain and what might it be caused by?

Answer 36

• when pain in visceral structures radiates to cutaneous areas and sensation arises from body surface
• may be due to 1st order neurons converging on 2nd order neurons of the same spinal cord region > dorsal horn neurons become hypersensitive > interpreted as coming from the skin

Question 37

thalamic pain

Answer 37

• caused by vascular injury to thalamic neurons (responsible for sensory processing)
• damaged neurons can initiate excruciating pain signals w/o nociceptive stimulation > referred to contralateral side
• unresponsive to conventional analgesics so anticonvulsants have to be used

Question 38

DCML pathway

Answer 38

• 1st order – dorsal root ganglion to medulla (decussate here)
• 2nd order – medulla to thalamus
• 3rd order – thalamus to primary somatosensory cortex