somatosensation II

Question 1

what do the DCML and STT do?

Answer 1

they cross the midline at different sites

• nociceptive afferents (STT) cross at the bottom
• mechanoreceptive afferents (DCML) cross higher up

Question 2

2 major central pathways of the somatosensory system

Answer 2

Dorsal column-medial leminiscal system

Spinothalamic tract

Question 3

explain the dual aspect model of pain?

Answer 3

1. sensory - discriminative
   - location, intensity, duration, quality
2. affective motivational
   - unpleasantness, effects on arousal, mood and behaviour

Question 4

wha are nociceptors?

Answer 4

neurons specialised for detecting painful stimuli

Question 5

nociceptors vs non-nociceptive neuron activation

Answer 5

graded increase in skin temperature increases the firing rate of non-nociceptive neurones.

at temperatures that are noxious (<45 degrees), nociceptor will respond and continue to respond.

Question 6

TRPV 1 receptor is involved in?

Answer 6

the transduction of noxious heat

Question 7

which fibres contain the TRPV 1 receptor?

Answer 7

A-delta and C fibres

Question 8

what binds to the TRPV 1 receptor?

Answer 8

capsaicin

• ion channels, Ca2+/Na+ open to heat and some chemicals
• lipid soluble, bypassing the normal thermal activation, binds to the inner surface and activates the receptor
• hence burning heat when eating a chilli

Question 9

what role of A-delta and C fibres play in pain?

Answer 9

A-delta is first pain, C fibres are second pain

Question 10

explain the maintenance of nociceptor activity after injury?

Answer 10

even after the stimulus is removed, we still feel pain. this is due to maintained c-fibre firing

• following injury, we get an inflammatory response (sub p, cytokines, prostaglandins)
• these directly depolarise c fibre terminals
• substance p released from local axonal branches

Question 11

what is hyperalgesia?

Answer 11

increased sensitivity to pain

Question 12

what is allodynia?

Answer 12

a normally inoxious stimulus elicits pain

Question 13

describe the anterolateral system?

Answer 13

dorsal horn interneurons, located deep in the dorsal layer

• recieve input from A-delta and C fibres
• axons cross and ascend in anterolateral white matter
• receive convergent inputs from afferents

Question 14

referred pain?

Answer 14

angina is pain in the heart

• the afferents from the damaged cardiac muscle /converge synapse onto the same dorsal horn interneurons (in the cervical segment of the spinal cord) as the afferents in the shoulder, hence why pain is felt in the shoulder too
• visceral pain is rarely felt in the right place

Question 15

do STT and DCML axons converse onto the same thalamic neurones?

Answer 15

no - the pathways are parallel

STT projects to primary somatosensory cortex via the thalamus - S1 necessary for pain localisation

Question 16

2 systems in central pain processing?

Answer 16

lateral system
-somatosensory cortex
medial system
-anterior cingulate

Question 17

pain treatment?

Answer 17

NSAIDs
Opiate drugs
Tricyclic antidepressants

Question 18

pain dissociated from tissue damage?

Answer 18

phantom limb pain is pain in a non-existant limb

central pain

Question 19

chronic pain

Answer 19

chronic pain is pain that occurs for over 3 months - alarmingly high prevalence

Question 20

neuropathic pain

Answer 20

nerve damage from prior injury

Question 21

what can can anterior cingulotomy be used for?

Answer 21

lesion to disconnect the anterior cingulate cortex on both sides, last resort for pain that won’t go away

Question 22

DCML white matter tract

Answer 22

thick fibres, wider diameter
a-beta and a-alpha afferent fibres, touch, vibration, tactile, proprioception (red)
-large fibres, large diameter, myelinated, fast conduction

Central portion of primary afferent axons enters the dorsal horn and ascends in white matter columns
They terminate in the medulla (dorsal column pathway)
Cuneate nucleus received a-beta from upper part of the body
Gracile nucleus received a-alpha from the lower part of the body
Axons of second order neurones (receive info from the primary afferents) cross the midline to the medial leminiscus, they then go to the midrain and terminate at the specific nucelus complex of the thalamus – ventral posterior nucleus
The axons of thalamic neurons project to the cerebral cortex – specifically to the primary somatosensory cortex

Question 23

STT white matter tract

Answer 23

thin fibres, pain, itch, crude touch, coarse, temperature
a-delta and C fibres (blue)
small diameter, thinly myelinated/unmyelinated, medium/slow conducting

Located in superficial and deep layers of dorsal horn
Synaptic input from C- and A-δ fibres
Axons cross and ascend in anterolateral white matter
Some are multi-modal (receive convergent nociceptive and non-nociceptive inputs)
Some receive convergent input from visceral afferents
STT projects to primary somatosensory cortex via the thalamus