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Flashcards in Somatosensory system Deck (19)
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1
Q

What two systems make up somatic sensation?

A

Spinothalamic

Dorsal column medial lemniscus

2
Q

What sensations are detected by the spinothalamic system and through which receptors?

A

Temperature - thermoreceptors
Pain - nociceptors
Pressure / crude touch - mechanoreceptors

3
Q

What sensations are detected by the DCML system?

A

Vibration
Proprioception
Two-point discrimination
Fine touch

4
Q

Where is the cell body of primary sensory neurons found, and where do they project to?

A

Cell body in the dorsal root ganglion.

Project to the spinal cord.

5
Q

How are different strength stimuli encoded in action potentials?

A

Greater stimuli e.g. pressure leads to greater receptor activation, causing a high frequency of action potentials in the primary sensory neuron.

The opposite is true for lesser stimuli.

6
Q

Describe how rapidly adapting receptors change in response to constant stimuli.

A

The frequency of firing reduces rapidly after the initial stimulus, unless it changes. This is why you don’t notice the clothes on your skin.

7
Q

Give an example where it is beneficial to have slowly adapting receptors, and explain how they work.

A

In pain it is useful to not be able to adapt to the stimulus, as this means that you are more likely to protect the painful area which ultimately promotes healing. Slowly adapting receptors change their frequency of firing very little after the initial stimulus so you never really get used to pain.

8
Q

If an area of skin is supplied by sensory neurons with large receptive fields what is its sensory acuity like?

A

Low sensory acuity / poor 2-point discrimination.

9
Q

Why can dermatomes have ‘fuzzy’ boundaries?

A

Receptive fields of primary sensory neurons from adjacent dermatomes overlap.

10
Q

Describe how first order neurons send signals to the primary sensory cortex.

A
  1. First order neurons have cell bodies in the DRG. Central axon projects ipsilaterally onto second order neurons.
  2. Second order neurons have cell bodies in spinal cord dorsal horn or medulla. They decussate before projecting onto third order neurons.
  3. Third order neurons have cell bodies in the thalamus. They relay the signal to the primary sensory cortex (post central gyrus)
11
Q

Describe the topographical layout of the brain in basic detail.

A

For every point on the surface of the body, there is an equivalent point in the sensory and motor cortex.
Gives a ‘homuncular pattern’ e.g. hands represented more than the legs…

12
Q

In the DCML system, detail how first order neurons from the lower body (T7 and below) reach the primary sensory cortex.

A
  1. Axons of 1st order neurons ascend ipsilaterally through the gracile fasciculus to the gracile nucleus in the medulla.
  2. Gracile nucleus projects to the contralateral thalamus in the medial lemniscus.
  3. Contralateral thalamus projects to the medial part of the primary sensory cortex.
13
Q

In the DCML system, detail how first order neurons from the upper body (T6 and above) reach the primary sensory cortex.

A
  1. Axons of 1st order neurons ascend ipsilaterally through the cuneate fasciculus to the cuneate nucleus in the medulla.
  2. Cuneate nucleus projects to the contralateral thalamus in the medial lemniscus.
  3. Contralateral thalamus projects to the lateral part of the primary sensory cortex.
14
Q

In the DCML system where do the axons of lower and progressively superior body segments run?

A

Lower parts of the body runs most medially, progressively superior body segments are added laterally to the dorsal columns.

15
Q

In the STT, how do first order neurons project up to the primary sensory cortex for both upper and lower body segments?

A
  1. First order neurons project onto second order neurons in the ipsilateral spinal cord dorsal horn (generally in the segment at which they enter - exception Lissauer’s tract).
  2. Second order neurons decussate in the ‘ventral white commisure’, then form the STT which projects to the thalamus.
  3. From the thalamus, axons from inferior body parts project to the medial primary sensory cortex, and superior axons project to the lateral primary sensory cortex.
16
Q

In the STT system where do the axons of lower and progressively superior body segments run?

A

This is opposite to DCML.
Axons from lower body parts run laterally (and project medially from thalamus)
Axons from more superior body parts are added medially (and project laterally from the thalamus)

17
Q

Describe the lesion in Brown-Sequard syndrome

A

Complete cord hemisection (destruction of one lateral half of a single cord segment)

18
Q

What signs are seen in Brown-Sequard syndrome?

A

Ipsilateral =
a. Complete segmental anaesthesia affecting a single dermatome.
b. Loss of DCML modalities below the destroyed segment
Contralateral =
a. Loss of STT modalities at and below the detroyed segment (can be up to 2 segments lower due to Lissauers tract)

19
Q

How is the activation of encephalinergic inhibitory interneurons involved in the descending inhibition of pain?

A

These interneurons can be activated by incoming impulses from mechanoreceptors (rubbing the sore area relieves the pain)
(They can also be activated by descending inputs from higher centres such as PAG and nucleus raphe magnus)
High adrenaline can also inhibit the sensation of pain.