Session 4 - Somatosensory system

Question 1

what is sensation split into?

Question 2

What is a modality?

Answer 2

a unit of sensation that is detected by a distinct receptor type

Question 3

What are the 2 different systems of modalities?

Answer 3

• spinothalamic system
• Dorsal column-medial lemniscus system

Question 4

what are the different modalities in the spinothalamic system?

Answer 4

• temperature - thermoreceptors
• Pain - nociceptors
• Pressure/crude touch - mechanoreceptors
• These are all needed for survival

Question 5

What are the different modalities of the DC-ML system?

Answer 5

• 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)

Question 6

Which parts of the body have high sensory resolution and which has poor?

Answer 6

• tips of finger have high
• Elbow has poor

Question 7

Describe primary sensory neurones

Answer 7

• takes info from the receptor along a spinal nerve
• Cell body in dorsal root ganglion
• Project ipsilaterally into cord on 2nd order neurone

Question 8

Describe secondary sensory neurones in the ST system

Answer 8

• their cell body is in the dorsal horn or medulla (depending on the tract)
• They DECUSSATE
• Project onto 3rd order neurones

Question 9

Describe 3rd order neurones in the STT

Answer 9

• 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)

Question 10

what is an analogue signal?

Answer 10

its related to ion flux during the generator potential

Question 11

what is a digital signal?

Answer 11

the frequency of APs in the primary sensory cortex

Question 12

what does a strong receptor activation cause?

Answer 12

high frequency of action potentials in the primary sensory neurone

Question 13

what does a weak receptor activation cause?

Answer 13

a low frequency of action potentials in the primary sensory neurone

Question 14

what are the 2 different types of sensory receptors?

Answer 14

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

Question 15

what is a receptive field?

Answer 15

a single primary sensory neurone supplies a given area of skin.

Question 16

Describe the sensory acuity of an area of skin that’s supplied by sensory neurones with large receptive field

Answer 16

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

Question 17

Describe the sensory acuity of an area of skin that’s supplied by sensory neurones with a relatively small receptive field

Answer 17

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.

Question 18

What area is an example of an area of skin that has a low acuity

Answer 18

the back

Question 19

What is an area of skin that has high acuity?

Answer 19

the skin of the fingertip

Question 20

Why can dermatome have fuzzy boundaries?

Answer 20

Because the receptive fields of primary sensory neurones from adjacent dermatomes can overlap

Question 21

What is the autonomous zone?

Answer 21

Areas of the skin where there is no overlaping receptive Fields

Question 22

what is this somatotrophy, topographical representation?

Answer 22

• 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.

Question 23

What is the difference with the motor system?

Answer 23

its similar but runs in reverse

Question 24

What happens to information in the spinothalamic system as we move upward through the neuraxis?

Answer 24

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

Question 25

What are the gracile nucleus and cuneate nucleus? (W/relation to the first order neurones) in the DCML

Answer 25

- 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.

Question 26

What it’s the medial lemniscus?

Answer 26

the pathways connecting the nuclei with the thalamus, running through the brain stem

Question 27

Second order neurones in the DCML ?

Answer 27

- 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.

Question 28

3rd order neurones in the DCML?

Answer 28

- 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.

Question 29

What is the topographical organisation of the dorsal column?

Answer 29

- 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.

Question 30

What is the difference between the DCML and the spinothalamic tract with regards to the synapsing of the 1st sensory neurone to the second?

Answer 30

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.

Question 31

What happens in the DCML system as you go up the cord?

Answer 31

the neurones get added onto the lateral aspect of the dorsal columns. The axons are added laterally to the pathway.

Question 32

What would a central cord lesion affect first? (in the DCML)

Answer 32

it would affect the lumbar region first and then affect the cervical region as it grows

Question 33

What happens to the axons as you go up the body in the spinothalamic tract?

Answer 33

they get added on medially, to the medial aspect for the spinothalamic tract

Question 34

What is the spinothalamic tract?

Answer 34

the tract that connects the spinal cord to the thalamus - the 2nd order neurone travels up the cord.

Question 35

What does the spinothalamic tract project into?

Answer 35

the thalamus

Question 36

What is the ventral white commisure?

Answer 36

- 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?)

Question 37

Where do the third order neurones project into in the spinothalamic tract?

Answer 37

- 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/

Question 38

What is the topographical organisation of the spinothalamic tract?

Answer 38

- 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)

Question 39

what is brown sequard syndrome?

Answer 39

a complete cord hemisection causing destruction of one lateral half of a single cord segment, resulting from trauma or ischaemia.

Question 40

which structures are destroyed in this syndrome? (BSS)

Answer 40

- the dorsal horn - The ventrail horn - All other cord grey matter - All white matter pathways - Dorsal and ventral roots

Question 41

What does this syndrome lead to?

Answer 41

- 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

Question 42

What else does this syndrome cause?

Answer 42

- paralysis and loss of proprioception on the same side of lesion - Loss of pain and temperature sensation on the opposite side of the leison

Question 43

What is Lissauer’s tract?

Answer 43

- 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

Question 44

what do A fibres carry?

Answer 44

they carry impulses from mechanoreceptors in the skin

Question 45

What do C fibres carry?

Answer 45

they carry pain

Question 46

which fibres carry impulses from mechanoreceptors in the skin?

Answer 46

A fibres

Question 47

which fibres carry pain impulse?

Answer 47

C fibres

Question 48

describe how rubbing a painful area can relieve the pain to an extent?

Answer 48

- 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**

Question 49

Explain what happens in situations where people suffer extreme pain and can’t feel it

Answer 49

- 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.

Question 50

What happens in hypnosis in relation to pain?

Answer 50

- 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)