Session 3: Somatosensory

Question 1

What are the classification of the sensation?

Answer 1

• General sensation, referring to the body wall and viscera (including parietal layer of serous membranes and mucosa of pharynx, nasal cavity and anus)
• Special sensation, referring the special senses of vision, hearing, balance, taste and smell

Question 2

What is general sensation divided into?

Answer 2

• Somatic sensation

- Visceral sensation

Question 3

Which sensations are controlled by the spinothalmic system?

Answer 3

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

Question 4

Which sensation are controlled by dorsal column-medial lemniscus system?

Answer 4

• Vibration (mechanoreceptors)
• Proprioception, or joint position sense, or kinaesthetic sense (detected by a variety of receptors such as muscle spindles and Golgi tendon organs)
• Fine touch/Light touch (mechanoreceptors)
• Two point discrimination (mechanoreceptors

Question 5

How do primary sensory neurone receive information?

Answer 5

-Each individual primary Nerone receives input from a single receptor type

Question 6

Where are primary sensory neurone cell bodies found?

Answer 6

-Dorsal root ganglion

Question 7

Where do dorsal root ganglions receive information?

Answer 7

-Collect information from a single dermatome along their peripheral axon

Question 8

How do primary sensory neurone communicate with spinal cord?

Answer 8

• Primary sensory neurone project to spinal cord along their central axon
• Strength of receptor activation is converted to analogue signal to a digital signal
• Strong receptor activation causes high frequency of action potentials in the primary sensory neurone
• Weak receptor activation causes a low frequency of action potentials in the primary sensory neurone

Question 9

What are rapidly adapting receptors?

Answer 9

• Respond best to changes in strength of stimulation. However, their frequency of firing diminishes rapidly after the initial stimulus (i.e. they rapidly adapt).
• Adaptation of these receptors explains why you are not aware of your clothes on your skin

Question 10

What are slowly adapting receptors?

Answer 10

• Change their frequency of firing very little after the initial stimulus.
• This explains why pain can be so persistent, and you never really get ‘used to’ having pain

Question 11

What is a receptive field?

Answer 11

Single primary sensory neurone supplies a given area of skin (it’s receptive field)

Question 12

What happens if an area of skin is supplied by sensory neurones with relatively large receptive fields?

Answer 12

• Area will have low sensory acuity
• Poor two-point discrimination where two points would need to be far apart to be distinguished
• Skin on the back has relatively low acuity

Question 13

What happens if an area of skin is supplied by sensory neurones with relatively small receptive fields?

Answer 13

• This area will have high sensory acuity
• Great two-point discrimination where two points could be very close together to be distinguished).
• The skin of the fingertip has relatively high acuity

Question 14

What is the effect of overlap in receptive fields?

Answer 14

-The overlap of receptive fields of primary sensory neurones from adjacent dermatomes is one of the reasons why dermatomes can have ‘fuzzy’ boundaries

Question 15

What are first order neurones?

Answer 15

• First order sensory neurones
• 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

Question 16

What are second order neurones?

Answer 16

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

Question 17

What are third order neurones?

Answer 17

• Have their cell bodies in the thalamus

- Project to the primary sensory cortex (postcentral gyrus)

Question 18

What are the features of the first order neurones of the dorsal column medial lemniscus pathway?

Answer 18

• Those from the lower body (T7 and below) ascend through the gracile fasciculus to the gracile nucleus in the medulla)
• Those from the upper half of the body (T6 and above) ascend through the cuneate fasciculus to the cuneate nucleus in the medulla

Question 19

What are the features of the second order neurones of the dorsal column medial lemniscus pathway?

Answer 19

• 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

Question 20

What are the features of the third order neurones of the dorsal column medial lemniscus pathway?

Answer 20

• Thalamic neurones receiving information ultimately from the lower half of the body (via gracile nucleus) project to the medial part of the primary sensory cortex
• Thalamic neurones receiving information ultimately from the upper half of the body (via cuneate nucleus) project to the lateral part of the primary sensory cortex

Question 21

What is the topographical organisation of the dorsal columns?

Answer 21

• Axons from the lower parts of the body run most medially

- Axons from progressively superior body segments are added laterally to the dorsal columns

Question 22

Which side of the body is supplied by the spinothalmic tract?

Answer 22

Axons of first order neurones project to the ipsilateral dorsal cord, but the spinothalamic tract supplies the contralateral half of the body

Question 23

What are the features of the first order neurones of the spinothalamic tract?

Answer 23

-They project onto second order neurones in the ipsilateral spinal cord dorsal horn in the segment at which they enter the cord through the dorsal root (generally)

Question 24

What are the features of the second order neurones of the spinothalamic tract?

Answer 24

• Their cell bodies are in the dorsal horn
• Their axons decussate in the ventral white commissure of the cord and then go on to form the spinothalamic tract
• The spinothalamic tract projects to the thalamus

Question 25

What are the features of third order neurones of the spinothalamic tract?

Answer 25

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

Question 26

What is the topographical organisation of the spinothalmic tract?

Answer 26

• 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
• This is the opposite of the situation for the dorsal columns, and is due to the decussation of the STT second order neurones at the level of entry of the first order neurones

Question 27

Which structures are destroyed completely unilaterally?

Answer 27

• The dorsal horn
• The ventral horn
• All other cord grey matter
• All white matter pathways
• Dorsal and ventral roots

Question 28

What are the signs of Brown-Sequard syndrome?

Answer 28

• 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

Question 29

How is pain modulated?

Answer 29

• Second order neurones of the spinothalamic system dealing with pain receive nociceptive primary afferents as well as inhibitory interneurones which contain the endorphin encephalin
• These encephalinergic interneurones can be activated by incoming impulses from mechanoreceptors (hence explaining why rubbing a sore area relieves the pain)
• Additionally, these encephalinergic interneurones can also be activated by descending inputs from higher centres such as the periaqueductal grey matter or the nucleus raphe magnus