Session 4 - Somatosensory system Flashcards
what is sensation split into?
What is a modality?
a unit of sensation that is detected by a distinct receptor type
What are the 2 different systems of modalities?
- spinothalamic system
- Dorsal column-medial lemniscus system
what are the different modalities in the spinothalamic system?
- temperature - thermoreceptors
- Pain - nociceptors
- Pressure/crude touch - mechanoreceptors
- These are all needed for survival
What are the different modalities of the DC-ML system?
- vibration - mechanoreceptor
- Proproception/joint position sense/kinaesthetic sense (knowledge of where limbs are in space, important for locomotion)
- Fine touch - mechanoreceptor
- 2 point discrimination - mechanoreceptor (resolving 2 points)
Which parts of the body have high sensory resolution and which has poor?
- tips of finger have high
- Elbow has poor
Describe primary sensory neurones
- takes info from the receptor along a spinal nerve
- Cell body in dorsal root ganglion
- Project ipsilaterally into cord on 2nd order neurone
Describe secondary sensory neurones in the ST system
- their cell body is in the dorsal horn or medulla (depending on the tract)
- They DECUSSATE
- Project onto 3rd order neurones
Describe 3rd order neurones in the STT
- cell body in thalamus
- Project onto primary sensory cortex at post central gyrus via the internal capsule (2 way tract for the transmission of info to and from the cerebral cortex)
what is an analogue signal?
its related to ion flux during the generator potential
what is a digital signal?
the frequency of APs in the primary sensory cortex
what does a strong receptor activation cause?
high frequency of action potentials in the primary sensory neurone
what does a weak receptor activation cause?
a low frequency of action potentials in the primary sensory neurone
what are the 2 different types of sensory receptors?
Rapidly adapting receptors:
- they emit a high frequency of APs but then the rate of APs slow down over rapidly after the initial stimulus
- Eg. Mechanoreceptors
- Eg. Sitting on a chair and not being aware of clothes on our skin
- They respond best to changes in strength of stimulation
Slowly adapting receptors
- frequency of action potentials doesn’t change, forcing you to fix the problem causing the pain.
- Eg. Nociceptor and tooth ache
- The frequency of the firing of the APs changed very little after the initial stimulus
what is a receptive field?
a single primary sensory neurone supplies a given area of skin.
Describe the sensory acuity of an area of skin that’s supplied by sensory neurones with large receptive field
it will have low sensory acuity, meaning it would have poor 2 point discrimination where the 2 points will have to be far apart to be distinguished
Describe the sensory acuity of an area of skin that’s supplied by sensory neurones with a relatively small receptive field
it’ll have a high sensory acuity - so it will have great 2 point discrimination where two points could be very close together to be distinguished.
What area is an example of an area of skin that has a low acuity
the back
What is an area of skin that has high acuity?
the skin of the fingertip
Why can dermatome have fuzzy boundaries?
Because the receptive fields of primary sensory neurones from adjacent dermatomes can overlap
What is the autonomous zone?
Areas of the skin where there is no overlaping receptive Fields
what is this somatotrophy, topographical representation?
- the idea that for every point on the surface of the body, an equivalent point can be identified along the sensory pathway.
- Adjacent body regions are supplied by adjacent regions of the brain/sensory system (w some exceptions).
- This way of organisation of the pathways minimises the amount of wiring required to transmit sensory information from. Less axon and myelin are used compared to a disordered trajectory.
What is the difference with the motor system?
its similar but runs in reverse
What happens to information in the spinothalamic system as we move upward through the neuraxis?
it becomes reorganised, such that at the level of the spinal nerves and spinal cord we have a dermatomal organisation, but at the levels of the thalamus and above we have a homuncular pattern
What are the gracile nucleus and cuneate nucleus? (W/relation to the first order neurones) in the DCML
- both contain where the first order neurones synapse onto the 2nd order neurones
- The gracile nucleus is a nucleus in the medulla that collects information form the spinal cord level T7 downwards (lower body)
- The first order neurones of the DCML system that are from spinal cord level T7 or below ascend through the gracile fasciculus to the gracile nucleus in the medulla.
- The first order neurones from T6 and above (upper body), ascend through the the cuneate fasciculus to the cuneate nucleus in the medulla.
What it’s the medial lemniscus?
the pathways connecting the nuclei with the thalamus, running through the brain stem
Second order neurones in the DCML ?
- 2nd order neurones in the gracile nucleus project to the contralateral thalamus in the medial lemniscus.
- Neurones in the cuneate nucleus projects to the contralateral thalamus in the medial lemniscus.
- The second order neurones decussate.
3rd order neurones in the DCML?
- Thalamic neurones receiving info form the lower part of the body, via the gracile nucleus, project to the MEDIAL part of the primary sensory cortex.
- Thalamic neurones receiving info from the upper half of the body, via the cuneate nucleus project to the LATERAL part of the primary sensory cortex.
What is the topographical organisation of the dorsal column?
- axons from the lower parts of the body run most MEDIALLY
- Axons from progressively superior body segments are added LATERALLY to the dorsal columns
- Higher up the cord you go, the neurones get added onto the lateral aspect to the dorsal columns.
What is the difference between the DCML and the spinothalamic tract with regards to the synapsing of the 1st sensory neurone to the second?
the first order neurone in the DCML ascends before synapsing with the 2nd order neurone but in the spinothalamic system it synapses around the same level that it enters the spinal cord.
What happens in the DCML system as you go up the cord?
the neurones get added onto the lateral aspect of the dorsal columns. The axons are added laterally to the pathway.
What would a central cord lesion affect first? (in the DCML)
it would affect the lumbar region first and then affect the cervical region as it grows
What happens to the axons as you go up the body in the spinothalamic tract?
they get added on medially, to the medial aspect for the spinothalamic tract
What is the spinothalamic tract?
the tract that connects the spinal cord to the thalamus - the 2nd order neurone travels up the cord.
What does the spinothalamic tract project into?
the thalamus
What is the ventral white commisure?
- where the 2nd order neurone axons decussate and then they go onto form the spinothalamic tract?
- It’s a point of vulnerability because it can easily get squashed (by a central cord lesion?)
Where do the third order neurones project into in the spinothalamic tract?
- the thalamic neurones that receive info from more of the inferior parts of the body project to the medial part of the primary sensory cortex.
- The thalamic neurones that receive info from more superior parts of the body, project to the lateral part of the primary sensory cortex/
What is the topographical organisation of the spinothalamic tract?
- The axons from the lower part of the body run most laterally.
- Axons from progressively superior body segment are added medially onto the tract.
- (Opposite for dorsal column, due to the decussation of the spinothalamic tract 2nd order neurones at the level of entry of the first order neurones)
what is brown sequard syndrome?
a complete cord hemisection causing destruction of one lateral half of a single cord segment, resulting from trauma or ischaemia.
which structures are destroyed in this syndrome? (BSS)
- the dorsal horn
- The ventrail horn
- All other cord grey matter
- All white matter pathways
- Dorsal and ventral roots
What does this syndrome lead to?
- ipsilateral complete segmental anaesthesia affecting a single dermatome (due to destruction of dorsal root and dorsal horn)???????
- Ipsilateral loss of dorsal column modalities
- Contralateral loss of spinothalamic modalities below the lesion
- Damage to descending corticospinal tracts, ascending dorsal column tracts, spinothalamic tract
What else does this syndrome cause?
- paralysis and loss of proprioception on the same side of lesion
- Loss of pain and temperature sensation on the opposite side of the leison
What is Lissauer’s tract?
- first order neurones of the spinothalamic tract can ascend a couple of segments in this tract before synapsing in the dorsal horn
- This explains why the sensory level for pain and temperature can be a couple of segments lower
what do A fibres carry?
they carry impulses from mechanoreceptors in the skin
What do C fibres carry?
they carry pain
which fibres carry impulses from mechanoreceptors in the skin?
A fibres
which fibres carry pain impulse?
C fibres
describe how rubbing a painful area can relieve the pain to an extent?
- mechanoreceptors detect the rubbing movement
- they are conveyed along AB(beta fibres) which is a type of primary afferent.
- AB fibres have their cell bodies in the dorsal root ganglion
- many of these A fibres don’t project onto a second order neuron
- instead they stimulate a particular type of excitatory enkephalinergic interneurone which can inhibit the second order sensory neurone in the pain pathway
- this then inhibits the pain
Explain what happens in situations where people suffer extreme pain and can’t feel it
- eg in hypnosis or extreme trauma
- there’s a descending modulation which travels down the spinal cord from the brain
- this inhibits the pain pathway and switches off transmission at the level of the spinal cord.
What happens in hypnosis in relation to pain?
- hypnosis can activate cortical neurones that project down to the midbrain.
- these are excitatory neurones so they stimulate a second order neurone which sits in the midbrain.
- this second order neurone sits in the periaqueductal grey which is a region that’s important for pain transmission
- the neurones in this area then project down into the more caudal portions of the brainstem, particularly the medulla - where they stimulate another population of neurones
- the neurones sit in the nuclear raphe magnus (a large nucleus near the midline) and they descend down in the cord where they then have an inhibitory effect upon the 2nd order sensory neurones in the spinothalamic system (pain transmission)
