Flashcards in Somatosensory System Deck (21):
A physical feeling from stimulation of a sense organ
How are the senses classified?
Special = CN: hearing, taste, sight, smell
General = 1) somatic (body wall, skin, mucosa) 2) visceral
What are the modalities of the senses?
Somatic = pain, temp, pressure, stretch, vibration, joint position, 2-point discrimination
Each modality has diff qualities associated with it (temp: hot, cold)
How are the modalities determined?
By the type of receptor being activated
How is the quality of the modality determined?
How the receptor is activated
Describe the basic layout of the somatosensory system
Primary sensory neurone – 1st order sensory neurone in the dorsal root ganglion – enters spinal cord
(Properties of the sensory neurone and the receptor determine the modality)
Synapses with 2nd order neurone
2nd order neurone sends axon decussates the midline and up to the thalamus
Synapses with 3rd order neurone in the thalamus
3rd order neurone projects to the cortex
Give some examples of cutaneous receptors in the skin
Outline how the generator potential is produced
Mechanoreceptors get physical deformation = open ion channels = Na rushes in
Small deformation = small Na (and vis-versa)
If deformation is large enough = large Na influx = AP generated
Your perception of the strength of the stimulus is proportional to what?
The frequency of APs along the first order neurone
How are the receptors from which the generator potential is produced classed?
Tonic receptors – slowly adapting (pain) = AP frequency remain firing at a very high rate
Phasic receptors – rapidly adapting (pressure) = AP frequency slow down though adaption
The larger the receptive field = poorer acuity
Acuity is inversely proportional to the size of the receptive field
Lateral inhibition increase acuity
What clinical response occurs from the overlapping of primary receptive fields?
Means dermatome boundaries are not clear lines
Explain lateral inhibition
Associated with every 1st order neurone are inhibitory neurones
Excitatory AP stimulate the inhibitory neurones to send inhibitory signals to the neighbouring axons
Localising the epicentre of sensation
Where are the locations of the somatosensory neurones?
1st order = dorsal root ganglion
2nd order = grey matter in the cord, brainstem- decussates
3rd order = thalamus (projects to cortex)
Outline the dorsal-column medial lemniscus pathway
Light touch, 2-point discrimination, vibration, joint position sense
Medial to lateral = sacral, lumbar, thoracic, cervical = to stop crossing over
Lumbar 1st order synapse in medulla grey mater - gracile nucleus 2nd order dips down decussates, synapse in thalamus with 3rd order neurone - project medially
Cervical 1st order synapse in medulla grey mater - cuneate nucleus 2nd order dips down decussates, synapse in thalamus with 3rd order neurone - project laterally
White mater pathway created by 2nd order neurones = medial lemniscus
What are the internal arcuate fibres?
Axons of second-order sensory neurones that compose the gracile and cuneate nuclei of the medulla oblongata
Which nucleus handles touch, vibration, 2-point discrimination, joint position from T6 downwards?
Outline the spinothalamic pathway
Pain, crude touch, temp
Medial to lateral = cervical, thoracic, lumbar, sacral
Lumbar 1st order neurone synapses at entry level, dorsal horn 2nd order neurone decussates, anterior-lateral spinothalamic tract, to thalamus, synapse with 3rd order neurone – project medially
Cervical 1st order neurone synapses at entry level, dorsal horn 2nd order neurone decussates, anterior-lateral spinothalamic tract, to thalamus, synapse with 3rd order neurone – project laterally
Why does rubbing a painful area alleviate pain?
Activation of mechanoreceptors = activation of A-beta neurones that project into the dorsal horn = exciting inhibitory enkephalinergic interneurones = inhibits 2nd order pain neurone = switching off pain transmission
Which fibres carry pain?
C fibres – slow conducting