somatosensation II Flashcards
what do the DCML and STT do?
they cross the midline at different sites
- nociceptive afferents (STT) cross at the bottom
- mechanoreceptive afferents (DCML) cross higher up
2 major central pathways of the somatosensory system
Dorsal column-medial leminiscal system
Spinothalamic tract
explain the dual aspect model of pain?
- sensory - discriminative
- location, intensity, duration, quality - affective motivational
- unpleasantness, effects on arousal, mood and behaviour
wha are nociceptors?
neurons specialised for detecting painful stimuli
nociceptors vs non-nociceptive neuron activation
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.
TRPV 1 receptor is involved in?
the transduction of noxious heat
which fibres contain the TRPV 1 receptor?
A-delta and C fibres
what binds to the TRPV 1 receptor?
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
what role of A-delta and C fibres play in pain?
A-delta is first pain, C fibres are second pain
explain the maintenance of nociceptor activity after injury?
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
what is hyperalgesia?
increased sensitivity to pain
what is allodynia?
a normally inoxious stimulus elicits pain
describe the anterolateral system?
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
referred pain?
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
do STT and DCML axons converse onto the same thalamic neurones?
no - the pathways are parallel
STT projects to primary somatosensory cortex via the thalamus - S1 necessary for pain localisation
2 systems in central pain processing?
lateral system
-somatosensory cortex
medial system
-anterior cingulate
pain treatment?
NSAIDs
Opiate drugs
Tricyclic antidepressants
pain dissociated from tissue damage?
phantom limb pain is pain in a non-existant limb
central pain
chronic pain
chronic pain is pain that occurs for over 3 months - alarmingly high prevalence
neuropathic pain
nerve damage from prior injury
what can can anterior cingulotomy be used for?
lesion to disconnect the anterior cingulate cortex on both sides, last resort for pain that won’t go away
DCML white matter tract
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
STT white matter tract
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
