3. Somatosensory system Flashcards
what is the difference between somatic and visceral sensation?
somatic is conscious, visceral isn’t
what modalities are involved in the spinothalamic system?
temperature
pain
pressure/crude touch
what modalities are involved in the doral column-medial lemniscus system?
vibration
proprioception
fine touch
two point discrimination
how is sensory information transmitted?
receptor -> primary sensory neurones/dorsal root ganglia neurones (primary sensory neurones have their cell bodies in the dorsal root ganglia) /primary afferent which causes generation of action potentials (if strong receptor activation = high frequency of APs in primary sensory neurones and weak = low)
they collect information froma single dermatome along their peripheral axon and project onto spinal cord along their central axon
what are the two types of adapting receptors?
rapidly adapting receptors eg: mechanoreceptors respond best to changes in strength of stimulus - not aware of clothes on skin
slowly adapting receptors eg: nociceptors change their frequency of firing very little after initial stimulus
what else do single primary neurones supply and what is the consequence of this?
supply a given area of skin - which is its receptive field. if an area of skin is supplied by sensory neurones with relatively large receptive fields - low sensory acuity and vice versa
there is some overlap of receptive fields of primary sensory neurones from adjacent dermatomes
how does the somatosensory system pathway connect?
- receptor > 1st order sensory neurones have their cell bodies in the dorsal root ganglia. their central axon projects ipsilaterally to the cell body and projects onto 2nd order neurones
- 2nd order neurones have their cell bodies in medulla/dorsal horn and decussate and project onto 3rd order neurone
- 3rd order neurones have their cell bodies in thalamus and project onto primary sensory cortex in the post central gyri
what pattern of sensory information is present at what level?
at level of spinal nerves and spinal cord = dermatomal organisation
at level of thalamus and above = homoncular pattern
what is the pathway through the dorsal column system?
- 1st order neurones from T7 and below ascend through the gracile fasiculus to the gracile nucleus in the medulla and those from T6 and above ascend through the cuneate fasiculus to the cunneate nucleus in the medulla
- the 2nd order neurones project to contralateral thalamus in the medial lemiscus
- the 3rd order neurones recieving information from lower part of body project to medial portion of PSC and those from upper body to lateral of PSC
what is the pathway through the spinothalamic system?
- 1st order neurones project onto 2nd order neurones on the ipsilateral spinal cord dorsal horn in the segment at which they enter the cord through at dorsal root
- the axons of 2nd order neurones decussate in the ventral white commissure of the cord and form the spinothalamic tract that projects to the thalamus
- the 3rd order neurones receiving information from more inferior parts of the body project tp medial part of PSC and superior - lateral part
what is the difference between the topographical organisation of the two tracts?
for spinothalamic, axons from lower parts of body run most lateraly and those from superior body segments are added medially due to decussation of 2nd order at level of entry first order
what is brown sequard syndrome?
spinal cord hemisection in which one half of a single cord segment - due to trauma or ischaemia
causing ipsilateral loss anasthesia affecting single dermatome (due to destruction of dorsal root and horn)
ipsilateral loss of dorsal column modalities below destroyed segment
contralateral loss of spinothalamic modalities at and below the destroyed segment (due to early decussation) - the level can be up to a couple of segments lower due to ascent of some primary afferents in lissauer’s tract
