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Clinical Neuroscience > The Somatosensory System > Flashcards

The Somatosensory System Flashcards

1
Q

Compare the 2 classes of sensation

A

General;

  • Referring to body wall and viscera
  • Somatic (Conscious) and Visceral (Unconscious)

Special;
- Vision, Hearing, Smell, Taste, Balance

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2
Q

What are sensory modalities?

A

Different forms of sensory experience which exist due to the presence of a variety of receptors

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3
Q

List the sensory modalities carried in the Spinothalamic System, as well as their corresponding receptor types

A
  • Temperature (Thermoreceptors)
  • Pain (Nociceptors)
  • Pressure/ crude touch (Mechanoreceptors)
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4
Q

List the sensory modalities carried in the Dorsal Column-Medial Lemniscus System, as well as their corresponding receptor types

A
  • Vibration (Mechanoreceptors)
  • Proprioception (Muscle spindles, Golgi tendon organs etc)
  • Fine touch (Mechanoreceptors)
  • Two point discrimination (Mechanoreceptors)
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5
Q

Which neurones receive information from receptors?

A

Primary sensory/ First Order/ Pseudounipolar neurones

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6
Q

The strength of receptor activation is converted from an Analogue to a Digital signal.

Compare the AP frequency in 1st Order neurones in strong and weak receptor activation

A

Strong activation: High frequency of APs

Weak activation: Low frequency of APs

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7
Q

Compare the 2 types of 1st Order neurones

A

Rapidly adapting;

  • AP frequency decreases rapidly after initial stimulation
  • Respond best to changes in strength of stimulation

Slowly adapting;
- AP frequency changes very little after initial stimulus

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8
Q

Give examples of Rapidly and Slowly adapting receptors

A

Rapidly;

  • Mechanoreceptors
  • This is why you’re not aware of clothes on skin, or after sitting on a chair for a long time you get used to it

Slowly;

  • Nociceptors
  • This is why pain can be persistent
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9
Q

What is a receptive field?

A

An area of skin supplied by a single 1st Order neurone

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10
Q

How many types of receptor is one 1st Order neurone connected to?

A

One

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11
Q

Describe the Sensory Acuity of an area of skin supplied by sensory neurones with LARGE receptive fields

Give an example

A
  • Low sensory acuity (Poor two point discrimination where 2 points must be far apart to be distinguished)
  • Skin of the back
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12
Q

Describe the Sensory Acuity of an area of skin supplied by sensory neurones with SMALL receptive fields

Give an example

A
  • High sensory acuity (Good two point discrimination where 2 points could be very close together and be distinguished)
  • Skin of fingertip
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13
Q

What’s the clinical significance of the overlap of receptive fields of 1st Order neurones?

A

Dermatomes can have ‘fuzzy’ boundaries, hence we test the centre of dermatomes rather than the edges/ boundaries

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14
Q

The somatosensory system is made up of a chain of 3 neurones.

List these 3

A
  • 1st order neurone (In PNS, other than synapse)
  • 2nd order neurone (In CNS)
  • 3rd order neurone (In CNS)
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15
Q

Describe 1st Order neurones in the pathway of the Somatosensory system

A
  • Communicate with a receptor
  • Cell bodies are in the Dorsal Root Ganglion
  • Synapse with IPSILATERAL 2nd order neurone in the CNS
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16
Q

Describe 2nd Order neurones in the pathway of the Somatosensory system

A
  • Cell bodies are in the Spinal Cord Dorsal Horn or the Medulla
  • Decussate
  • Project onto 3rd order neurones
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17
Q

Describe 3rd Order neurones in the pathway of the Somatosensory system

A
  • Cell bodies are in the Thalamus

- Project to Primary Sensory Cortex

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18
Q

What is Somatotopy/ Topographical representation?

A

The idea that:

  • Every point on the surface of the body has an
  • Equivalent point along the sensory pathway
19
Q

With some exceptions, what can generally be said about adjacent body regions with regards to Somatotopy?

A

Adjacent body regions map to adjacent regions of the sensory system

(Minimises the amount of ‘wiring’ needed to transmit sensory information)

20
Q

How does the pattern of information organisation change as we move from the Spinal Cord to the Thalamus

A

At spinal cord level: Dermatomal pattern

At thalamus level: Homuncular pattern

21
Q

At what level do all the sensory modalities converge?

A

At the level of the sensory homunculus

22
Q

Which sensory modalities are carried in the Dorsal Column-Medial Lemniscus (DCML) System?

A
  • Fine touch
  • Proprioception
  • Vibration
  • Two point discrimination
23
Q

Describe the pathways of 1st order neurones in the Dorsal Column-Medial Lemniscus (DCML) System

A

Axons ascend ipsilaterally through Dorsal Columns of spinal cord;

  • If from Lower Body (T7 and below), ascend through Gracile Fasciculus to synapse with Gracile Nucleus in Medulla
  • If from Upper Body (T6 and above), ascend through Cuneate Fasciculus to synapse with Cuneate Nucleus in Medulla
24
Q

Describe the pathways of 2nd order neurones in the Dorsal Column-Medial Lemniscus (DCML) System

A

Neurons from Gracile and Cuneate nuclei;

  • Decussate
  • Project to the Contralateral Thalamus via the Medial Lemniscus
25
Q

What is the Medial Lemniscus?

A

The pathway connecting the Gracile and Cuneate nuclei to the contralateral Thalamus

(Contains 2nd order neurones)

26
Q

Describe the pathways of 3rd order neurones in the Dorsal Column-Medial Lemniscus (DCML) System

A
  • Thalamic neurones receiving information from Gracile nucleus project to the Medial Primary Sensory Cortex
  • Thalamic neurones receiving impulses from Cuneate Nucleus project to the Lateral Primary Sensory Cortex
27
Q

Describe the topographical organisation of the dorsal columns

What is the clinical significance of this?

A
  • Axons from lower body run most medially
  • Axons from progressively superior body segments are added laterally
  • Lower body segments are more at risk from Central cord lesions that grow outwards
28
Q

Which sensory modalities are carried in the Spinothalamic System/ Spinothalamic Tract/ Anterolateral System?

A
  • Pain
  • Temperature
  • Pressure/ crude touch
29
Q

Describe the pathways of 1st order neurones in the Spinothalamic System

(Cell bodies in Dorsal Root Ganglion as expected)

A

Axons project to Ipsilateral dorsal cord, but ascend contralaterally

  • Synapse onto ipsilateral 2nd order neurones in the dorsal horn, at the level they enter the segment
30
Q

Describe the pathways of 2nd order neurones in the Spinothalamic System

A
  • Cell bodies in ipsilateral Dorsal Horn
  • Axons decussate in Ventral White Commissure to form Spinothalamic Tract
  • Spinothalamic tract projects to the Thalamus
31
Q

Describe the pathways of 3rd order neurones in the Spinothalamic System

A
  • Thalamic neurones receiving information from Inferior body, project to Medial Primary Sensory Cortex
  • Thalamic neurones receiving information from Superior body, project to Lateral Primary Sensory Cortex
32
Q

Describe topographical organisation of the Spinothalamic Tract

What is the clinical significance of this?

A
  • Axons from lower body run most laterally
  • Axons from progressively superior body segments are added medially
  • Upper body segments are more at risk from Central cord lesions that grow outwards
33
Q

What causes the opposite topographical organisation between the DCML System and STT?

A

In the STT, 2nd order neurones decussate at the level of entry of 1st order neurones

34
Q

What is Brown-Sequard Syndrome?

A

The clinical syndrome that occurs when half of the spinal cord is damaged (Right or left)

(A complete cord hemisection)

35
Q

List 5 structures that would be destroyed unilaterally in a complete cord hemisection

A
  • Dorsal + ventral roots
  • Dorsal horn
  • Ventral horn
  • Grey matter
  • White matter pathways
36
Q

List 3 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 sensory modalities below the destroyed segment
  • Contralateral loss of Spinothalamic Tract sensory modalities below the destroyed segment
37
Q

With regards to pain, what are the 2 inputs to the 2nd order neurones of the STT?

A
  • From C Fibres/ Excitatory primary afferents from Nociceptors
  • Inhibitory interneurons
38
Q

What neurotransmitter is used by the inhibitory interneurons to inhibit the 2nd order neurones of the STT?

A

Encephalin, which is an endorphin

Encephalin is a peptide neurotransmitter

39
Q

List 2 causes of activation of the Encephalinergic interneurons that inhibit the 2nd order neurones of the STT

A
  • Incoming impulses from mechanoreceptors

- Descending inputs from higher centres such as Periaqueductal grey matter or the Nucleus Raphe Magnus

40
Q

Describe in detail why rubbing a sore area relieves pain

A
  • Rubbing the sore area activates mechanoreceptors
  • Impulses travel via A-Beta fibres (1st order neurones) to stimulate Encephalinergic Interneurones
  • These interneurons inhibit the 2nd order neurones of the STT

Thus, reduced frequency of pain impulses sent to Primary Sensory Cortex

41
Q

Describe how hypnosis can produce descending inputs to alleviate pain?

A
  1. Activation of cortical neurones which descend and synapse in the Peri-aqueductal Grey Matter, stimulating Peri-aqueductal Grey Neurones
  2. PA Grey Neurones descend and synapse in Nucleus Raphe Magnus, stimulating NRM neurones
  3. NRM neurones descend and stimulate Encephalinergic Interneurones
  4. These interneurones inhibit the 2nd order neurones of the STT

Thus, reduced frequency of pain impulses sent to Primary Sensory Cortex

42
Q

The Nucleus Raphe Magnus is part of the Reticular Formation

What is the Reticular Formation?

A

A set of interconnected nuclei that are located throughout the brainstem

43
Q

Describe Lissauers Tract

A
  • Allows 1st order neurones to bypass a lesion, thereby presentation of symptoms at different neural levels

Clinical Neuroscience (19 decks)

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