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Clinical neuroscience > Somatosensory system > Flashcards

Somatosensory system Flashcards

1
Q

What classifications can sensation be split into?

A
  • General sensation, referring to body wall and viscera
  • This is subdivided into visceral sensation and somatic sensation
  • Special sensation - refers to vision, hearing, balance, taste and smell
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2
Q

What are the divisions of somatic sensation?

A
  • Spinothalamic system
  • Dorsal column-medial lemniscus system
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3
Q

What is a modality?

A
  • A unit of sensation
  • Relies on a distinct receptor type
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4
Q

What modalities is the spinothalamic system responsible for?

A
  • Temperature (thermoreceptors)
  • Pain (nociceptors)
  • Pressure/crude touch (mechanoreceptors)
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5
Q

What modalities is the dorsal column-medial lemniscus system responsible for?

A
  • Vibration (mechanoreceptors)
  • Proprioception/joint position sense/kinaesthetic sense (variety of receptors such as muscle spindles and Golgi tendon organs)
  • Fine touch (mechanoreceptors)
  • Two point discrimination (mechanoreceptors)
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6
Q

What is the role of primary sensory neurones?

A
  • These receive information from receptors and are responsible for the initial encoding of sensory information
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7
Q

Where do primary sensory neurones receive their information from?

A
  • A single receptor type
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8
Q

Where are primary sensory neurones located in the body?

A
  • Cell body is found in dorsal route ganglion
  • Collect information from a single dermatome along their peripheral axon
  • Primary sensory neurones project into the spinal cord along their central axon
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9
Q

How is the strength of receptor activation conveyed?

A
  • It is converted from an analogue signal to a digital signal
  • Digital signal is equal to the frequency of action potentials in the primary sensory neurones
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10
Q

What does strong receptor activation lead to?

A
  • High frequency of action potentials in the primary sensory neurone
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11
Q

What does weak receptor activation lead to?

A
  • Low frequency of action potentials in the primary sensory neurone
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12
Q

What is meant by a rapidly adapting receptor?

A
  • These respond best to changes in strength of stimulation (e.g. mechanoreceptors)
  • Their frequency of firing diminishes rapidly after the initial stimulus
  • E.g. this is why we are not aware of the clothes on our skin
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13
Q

What is meant by a slowly adapting receptor?

A
  • These receptors change their frequency of firing very little after the initial stimulus
  • This explains why pain can be so persistent and we don’t get used to experiencing pain
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14
Q

What is a receptive field?

A
  • A given area of skin supplied by a single primary sensory neurone
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15
Q

What results in an area of low sensory acuity?

A
  • When an area of skin is supplied by sensory neurones with relatively large receptive fields
  • Area will have poor two-point discrimination
  • E.g. skin of back
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16
Q

What results in an area of high sensory acuity?

A
  • When an area of skin is supplied by sensory neurones with relatively small receptive fields
  • Area will have great two-point discrimination
  • E.g. skin of fingertip
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17
Q

Why do dermatomes have ‘fuzzy’ boundaries?

A
  • Due to overlap of receptive fields of primary sensory neurones
18
Q

Outline the role of first order sensory neurones and where they project to

A
  • Have their cell bodies in the dorsal root ganglion
  • Communicate with a receptor
  • Their central axon projects ipsilateral to the cell body
  • Project onto second order neurones
19
Q

Outline the role of second order sensory neurones and where they project to

A
  • Have their cell bodies in the spinal cord dorsal horn or medulla
  • Decussate
  • Project onto third order neurones
20
Q

Outline the role of third order sensory neurones and where they project to

A
  • Have their cell bodies in the thalamus
  • Project to the primary sensory cortex (post-central gyrus)
21
Q

What is the principle of somatotopy?

A
  • 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 tend to map to adjacent regions of the sensory system
  • This applies to dermatomal organisation and the homunculus
22
Q

Why is the sensory cortex organised in a somatotopic fashion?

A
  • This way of organising the pathways minimises the amount of wiring required to transmit sensory information
23
Q

What happens to modalities at the level of the sensory homunculus?

A
  • They all converge
24
Q

Outline the organisation of first order neurones of the DCML system

A
  • Ascend ipsilaterally through dorsal columns of the spinal cord
25
Q

How are the lower body first order neurones of the DCML system organised?

A
  • Those from the lower body (T7 and below) ascend through the gracile fasciculus to the gracile nucleus in the medulla
26
Q

How are the upper body first order neurones of the DCML system organised?

A
  • Those from the upper body (T6 and above) ascend through the cuneate fasciculus to the cuneate nucleus in the medulla
27
Q

Outline the organisation of the second order neurones of the DCML system

A
  • Neurones in the gracile nucleus project to the contralateral thalamus in the medial lemniscus
  • Neurones in the cuneate nucleus project to the contralateral thalamus in the medial lemniscus
28
Q

Outline the organisation of the third order neurones of the DCML system?

A
  • Thalamic neurones receive information ultimately from the lower half of the body and project to the medial part of the primary sensory cortex
  • Thalamic neurones receive information ultimately from the upper half of the body and project to the lateral part of the primary sensory cortex
29
Q

Outline the topographical organisation of the dorsal columns

A
  • Axons from lower parts of the body run most medially
  • Axons from progressively superior body segments are added laterally to the dorsal columns
30
Q

Outline the organisation of the first order neurones of the spinothalamic tract

A
  • They project to the ipsilateral dorsal cord
  • Onto second order neurones in the ipsilateral spinal cord dorsal horn
  • In the segment at which they enter the cord through the dorsal root
31
Q

Outline the organisation of the second order neurones of the spinothalamic tract

A
  • Their cell bodies are in the dorsal horn
  • Axons decussate in the ventral white commissure of the cord
  • Then form the spinothalamic tract
  • The spinothalamic tract projects to the thalamus
32
Q

Where do the tracts of the DCML decussate?

A
  • In the brain
33
Q

Outline the organisation of the third order neurones of the spinothalamic tract

A
  • Thalamic neurones receiving information from more inferior parts of the body project to the medial part of the primary sensory cortex
  • Thalamic neurones receiving information from more superior parts of the body project to the lateral part of the primary sensory cortex
34
Q

Outline the topographical organisation of the spinothalamic tract

A
  • Axons from the lower parts of the body run most laterally/superficially
  • Axons from progressively superior body segments are added medially/deeper onto the spinothalamic tract
  • Due to the decussation of STT second order neurones at the level of entry of the first order neurones
35
Q

What is Brown Sequard syndrome?

A
  • A complete cord hemisection causing destruction of one lateral half of a single cord segment
36
Q

What does Brown Sequard syndrome result from?

A
  • Trauma or ischaemia
37
Q

Which structures are completely destroyed unilaterally due to Brown Sequard syndrome?

A
  • The dorsal horn
  • The ventral horn
  • All other grey matter
  • All white matter pathways
  • Dorsal and ventral roots
38
Q

What are the signs of Brown Sequard syndrome?

A
  • Ipsilateral complete segmental anaesthesia affecting a single dermatome due to destruction of dorsal root and dorsal horn
  • Ipsilateral loss of dorsal column modalities below the destroyed segment
  • Contralateral loss of spinothalamic modalities at and below the destroyed segment
39
Q

Why does Brown Sequard syndrome cause ipsilateral loss of dorsal column modalities below the destroyed segment?

A
  • Because the dorsal columns decussate in the brain
40
Q

Why does Brown Sequard syndrome cause contralateral loss of spinothalamic modalities at and below the destroyed segment?

A
  • Because the spinothalamic tracts decussate at their level of entry into the spinal cord
41
Q

Why does rubbing a sore area relieve pain?

A
  • Second order neurones of the spinothalamic system dealing with pain receive nociceptive primary afferents as well as inhibitory interneurons which contain encephalin
  • You can activate the inhibitory interneurons by stimulating incoming impulses from mechanoreceptors
42
Q

What else can activate encephalinergic interneurones?

A
  • Descending inputs from higher centres such as the periaqueductal grey matter or the nucleus raphe magnus

Clinical neuroscience (19 decks)

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